Brick garage with corrugated plastic roof - leaf options??

Hello all. I am based in the UK and am sending this question as a basic preliminary in case an answer is applicable without any more detail and drawings etc. Please be aware that I can and will provide much more detail as I go along and also please be aware that I have been reading through various VERY useful threads on this site for many years now and am well aware of the MASS > space > MASS theory including the vast pool of info relating to builds etc - the main bookmarks I will be using for my main references in this build are the old "shed to studio" thread with all associated spinoffs etc. I would like to paint a VERY rough picture of my situation in order to hopefully get a very quick kick in the right direction for my initial problem. OK - detached house with ATTACHED garage. 3 x garage walls are breeze/cinder block, with 4th wall being the house wall (so therefore must be brick, as this garage was an add-on). Solid concrete floor throughout (dimensions are 5.80m x 3.30m x "oops" - sorry, I have not measured height yet). Wooden joists/beams/rafters that run 1.00m apart across the width of the room and tied into the wall on both sides - I have not measured these joists/beams/rafters, but they must be a good 12" x 2" - and these beams are at a pitch running away from the house creating a natural runoff above. Now, the first issue and the reason for this quick step one question. There is no ceiling and what constitutes the roof is simply a single layer of the cheapest, thinnest, flimsy old corrugated plastic sheeting as is used on carports and lean-to's. Furthermore, this sheeting is very old, there are quite a few holes and it needs to come off. So, to the point. Although I have not measured the exact distance from floor to the bottom of the wooden joists overhead, I can see in my mind's eye that there will NOT be enough space for me to install a MASS > space > MASS ceiling and then simply throw a new and obviously better, waterproof roof on the top. So, my initial thoughts are to take the plastic sheets off and secure (how ever many?) layers of (suggestions?) directly on top of the roof joists/beams/rafters, sealed with a layer of some form of waterproofing, and then a NEW plastic corrugated roof on top of that to deal with the bulk of the weather missiles if you know what I mean. Anything that got through the plastic sheets would be minimal and would land on the sealed outer layer of (suggestions?) roof which is pitched for runoff - and I would check the plastic roof every few weeks, it being very easy to replace a damaged sheet if necessary. My thinking is that this gives me an initial outer leaf, constituting concrete floor, 1 x brick wall, 3 x breeze/cinder block walls and a (suggestions?) roof --- I would then plug all of the internal gaps where the walls and "new roof" meet with whatever is best suited (initial thoughts would be concrete applied by trowel, or caulk etc). Building framed, single sided, double layer plasterboard walls inside of this outer shell with a "puck-raised" wooden floor and a double layered ceiling above (2 x compromising options for this - to follow) would theoretically give me the room within a room, MASS > space > MASS result ---- unless I have got this all wrong and if there is a flaw to my thinking. Regarding the "inner" ceiling (as just mentioned in paragraph above) - for the sake of retaining the fairly limited height, I would have to devise a fairly complicated system of new joists that would have to be tied into the framed wall structures, and that sit "inside" the existing joists so that they present a new surface for ceiling boards an inch or so below the existing joists --- OR I would have to basically fit the inner ceiling to RC suspended from the bottom of the EXISTING joists - which would have whatever external roof is chosen/suggested fixed to the top, so would be sacrificing decoupling I suppose? Because of these various permutations, what to do with the initial external roof to replace the plastic sheets is a critical choice and step for me to take as I would like to do this first - unless my entire train of thought is wrong from the onset. I will update more accurate dimensions etc as soon as I get home tonight - I just wanted to get this online as soon as possible in case this external roof thing is a simple solution issue. Please help?? Thanks Steve

I have been reading through various VERY useful threads on this site for many years now and am well aware of the MASS > space > MASS

Spring, not space. The "S" in MSM means "Spring". It comes form the principle of physics on which it is based: A mass suspended from another mass by a spring will have a certain resonant frequency, governed by the properties of the system.

Although I have not measured the exact distance from floor to the bottom of the wooden joists overhead, I can see in my mind's eye that there will NOT be enough space for me to install a MASS > space > MASS ceiling and then simply throw a new and obviously better, waterproof roof on the top.

Did you look at page 77 of Rod's book? If you do something like that, you don't have to lose more than an inch or two of height. In other words, your final inner-leaf celing can be just a couple of inches lower than the bottom of the existing joists. I started out with a very similar situation: The plastic garbage is now gone, replaced with a full 18mm plywood deck, and asphalt shingles. Works great!

Building framed, single sided, double layer plasterboard walls inside of this outer shell with a "puck-raised" wooden floor

:shock: You want to float your floor???? Are you SURE about that???? FLOATING FLOORS: viewtopic.php?f=2&t=8173 Apart from anything else, a floating floor will eat heavily into your height, and you say that you don't have much to start with. Better to go stick with your existing concrete floor, which is just fine acoustically.

Regarding the "inner" ceiling (as just mentioned in paragraph above) - for the sake of retaining the fairly limited height, I would have to devise a fairly complicated system of new joists that would have to be tied into the framed wall structures, and that sit "inside" the existing joists so that they present a new surface for ceiling boards an inch or so below the existing joists

Well, it does not have to be complex at all! Very simple, in fact: your new joists will just sit on top of your new inner-leaf framing. Period. Nothing complicated about that!

OR I would have to basically fit the inner ceiling to RC suspended from the bottom of the EXISTING joists - which would have whatever external roof is chosen/suggested fixed to the top, so would be sacrificing decoupling I suppose?

Well, if you use or resilient channel, or alternatively clips plus hat channel, then you won't sacrifice much in terms of isolation. It won't be quite as good as a fully decoupled ceiling, but it would still work. But since you didn't say anything at all about sound levels, or even the purpose of your room, or even what you have in mind, roughly, it's hard to say if that would be enough or not.

I just wanted to get this online as soon as possible in case this external roof thing is a simple solution issue.

Simple solution: Rip off the plastic and dump it. replace it with plywood, roofing felt over that, asphalt shingles over that. Done! Then either hang your inner leaf below the joists with RC or with clips plus hat channel. Done! OR if you do need better isolation, the skip the RC / clips / hat channel, and just do a normal inner-leaf ceiling supported on the normal inner-leaf walls. Done! It doesn't have to be complicated. It just has to be done right. - Stuart -

Gotcha ... thanks for the reply. OK - for a bit more detail on requirements - I work mostly alone, primarily as a vocalist, although I do play drums, bass, keys and a bit of guitar. So their is NOT typically going to be 2 hour sessions of full band rehearsals going on 3 times a week. I do, however, have a few tasty guitar amps that will be pumped from time to time for tracking purposes - and although I do most of my composition on V-Drums, I also own an acoustic drumkit which may, from time to time, be set up for tracking. I will also, of course, be utilising the odd session player - so being a single room, there will be occassions when levels are pushed --- and this needs to be taken into consideration alongside the fact that I have a standard dayjob, so all of the work in this space will be in the evenings and on the weekends. For the most part - the space will be used for general composition/production/mixing. I use Mackie HR-824's with a pair of NS-10's for monitoring and in terms of the rest of the gear it is all analogue (large frame console, 9 x 10U/16U racks and lots of synthesizers. So in terms of desired goal - I suppose most people like myself would like as much of our thumping kick drum samples to be contained without upsetting the neighbours as possible - and without expecting to be pushing the Mackie's flatout, there will be times when you want to lift the faders for a few minutes at 11.00pm without getting a bad rep in the neighbourhood. BTW - you did provide me with some useful info a few months back when we were first taking a look at the possibility of renting this house (viewtopic.php?f=2&t=13714) and I thank you again for that. We have now moved in and due to a better look at kitchen space etc, it has been decided that I can use the entire garage as opposed to splitting the studio into two seperate spaces at each end of the house. I don't want to get into too many details just yet - but there is one small problem that I have discovered and that is that there is a municipal inspection plate ("drainage") that sits pretty much directly under the double external garage doors and this plate extends about 20cm into the garage. As inconvenient as this is, I would have to have done some serious battoning up of the double doors anyway, so I have decided to build a wall JUST inside of the inspection plate. A new VERY basic drawing is as follows: I cannot brick up any of the doors and windows, so will have to batten the door to the rear and the window to the rear up with a few layers of board bolted against neoprene or similar (unless any better solutions?). The door leading to the kitchen would also have to be replaced with a solid door of sorts - BUT, although these are all as important as any other decision, my first and foremost concern is getting the old roof off and a new roof on, followed by sealing up the gaps between roof and existing walls. I would then seal off the lower end of the garage at the inspection plate side with a framed wall and implement some sort of battening to close up the rear door and window. So - if the outer roof IS going to be the outer leaf as such, is a single layer of plywood, roofing felt and shingles enough, or would a double layer of boards (of whatever kind) be more beneficial? Also, as you have directed me to the link covering floated floors, I would quite happily NOT "float" my floor on top of the concrete. I would, however, want to put down a layer of polystyrene for heat insulation and then some tongue and groove flooring on top. There is no issue with this - or with this flanking the inner walls is there? And regarding the walls (considering there will not be a floated floor as such) - do they sit directly onto the concrete floor or will I find these options in the other threads. I spent a LOT of time on this forum about 6 - 7 years ago and it took me AGES to trawl through everything - so I am sure there is a lot more here now, but it takes a very long time to find the right threads and the ones I have always had bookmarked from years ago were the ones with a lot of detail and profile diagrams pertaining to leaf constructions, layers, caulking etc etc - so there may well be a lot more threads with very similar FAQ's and I will look for them as I am not too lazy to search - just wanting to get my INITIAL plan of action clear in my head - ie. "X" number of layers of "X" for my new roof, "X" number of layers of "X" for my wall to seal off the double garage doors, perhaps a few tips for battening the smaller door and window off, whether or not the walls will be set straight on the concrete or not, whether or not my insulated floor needs more careful planning and then the ceiling - which you have already given me the two basic options on already. OK - sorry about the rambling - and thanks for taking the time to comment. I know everybody is busy and there are lots of threads out there, but I will post some actual pictures as soon as we are about to start the outer roof story - at the moment the garage is packed full of stuff from the previous house and I am hoping to clear that out this weekend so that the roof can go up next week. Cheers for now. Steve

Hey Steve, thanks for the link to the old thread! I recall a bit better now. Based on your description of the stuff that you want to track in there, it looks like you'll be producing levels over 100 dB, heading for 115 and even more. You might want to check your local municipal regulations, to see what they say about allowable noise levels so that you can get an idea of how much isolation you should be planning for. But my guess is that you are going to need some fairly serious isolation.

9 x 10U/16U racks and lots of synthesizers.

Wow! That's a lot of gear! Somewhere between 2 and 3 full 40U racks! So two things you will definitely need to plan for well in advance: electrical work to supply juice to all that stuff, and enough HVAC to deal with all the heat. Even if those racks are only half full, that's still a fair amount of power and heat that you'll have to take care of. Not to mention noise: there must be quite a few cooling fans in all that stuff, meaning lots of annoying whining, humming and air movement, so you might want to consider a "mini machine room" somewhere in your plans. Personally, I find it hard to mix well with too much of that kind of noise going on around me.

I don't want to get into too many details just yet - but there is one small problem that I have discovered and that is that there is a municipal inspection plate ("drainage") that sits pretty much directly under the double external garage doors and this plate extends about 20cm into the garage.

You and me both! I have exactly the same problem in my garage, where my studio is going in, except that mine is right in the middle of the space where the live room will be. I still haven't figured out how to deal with that, but I fear there may be a jack hammer and some serious plumbing in my future... At least you have a work-around.

As inconvenient as this is, I would have to have done some serious battoning up of the double doors anyway, so I have decided to build a wall JUST inside of the inspection plate.

Can you seal the existing garage doors in some way, and beef them up with additional mass? You really will need that for the kind of isolation you are talking about, I reckon. The door by itself probably isn't anywhere near enough to be an effective outer leaf.

A new VERY basic drawing is as follows:

SketchUp!!!!! It is well worth the learning curve, believe me!

I cannot brick up any of the doors and windows, so will have to batten the door to the rear and the window to the rear up with a few layers of board bolted against neoprene or similar (unless any better solutions?).

Somewhere here on the forum are a few designs for dealing with that. Try searching for things like "window plug" or suchlike. That's the basic idea.

So - if the outer roof IS going to be the outer leaf as such, is a single layer of plywood, roofing felt and shingles enough, or would a double layer of boards (of whatever kind) be more beneficial?

The more the merrier! You are going to need lots of mass up there, so if doing multiple layers of ply, MDF, etc would be good. You could also consider beefing the roof deck from below, between the joists, using drywall (cheap mass). HOWEVER! Before deciding on that you are going to need a structural engineer to take a look at what you have and tell you how much mas you can hang off it. If you can't get enough mass on those joists, he'll be able to tell you what you need to do so that you CAN get enough mass up there. But yeah, you are going to need mass up there. Something equivalent to at least two layers of 5/8 drywall, more if you can afford it. Dums and bass amps are LOUD!

I would, however, want to put down a layer of polystyrene for heat insulation and then some tongue and groove flooring on top. There is no issue with this - or with this flanking the inner walls is there?

It sounds like you are talking about "laminate flooring"? Yeah, that will work just fine. You might have to level your cement first, though, as laminate is fairly unforgiving about an uneven base below it. It needs to be pretty damn flat and smooth. Not hard to do, but important to consider.

And regarding the walls (considering there will not be a floated floor as such) - do they sit directly onto the concrete floor or will I find these options in the other threads.

Yup! direct on the concrete. You might want to put neoprene, rubber, or such like underneath the framing, but that's more for getting a good tight seal, rather than for floating the walls. Yes, you will get some flanking out trough the floor, but probably not enough to be a large concern. Worst case, you could rip out the existing concrete floor and pour a new slab with its own foundations just inside of, and NOT TOUCHING the old foundations, and stand your inner leaf on that. Take a look at Sharward's thread to see how to do that. ( viewtopic.php?f=10&t=2921 ). Expensive, but it will give you the best possible isolation.

"X" number of layers of "X" for my new roof, "X" number of layers of "X" for my wall to seal off the double garage doors,

It should be pretty much the same all around: For example, if you are going to use 3 + 3 layers of 5/8" drywall (or the equivalent thereof) on your roof / ceiling MSM, then it makes no sense to only put 2 + 1 layers of 1/2" drywall on the MSM system for the garage door wall. All the walls should also be 3 + 3 layers of 5/8" or equivalent in that case. If not, then your isolation will only be as good as your weakest link. So if your garage door turns out to be your weakest link, and you have no way of beefing that up, then it makes little sense to do multiple layers on the roof, since the sound will just leak out through the door anyway, no matter how good the roof is.

but I will post some actual pictures as soon as we are about to start the outer roof story -

And the SkethUp model! Don't forget that. Accurate, and detailed.... :)

so that the roof can go up next week.

Whoa!!!! :shock: I wouldn't commit to doing anything with the roof until you have your COMPLETE plan in place!!! Don't back yourself into a corner from which there is no escape... :!: - Stuart -

Hey Stuart. My main mission for now will be sketchup. I work in IT, so this leads to the situation where you hate spending anymore time than you have to in front of a PC, but I would like to approach this build properly, so I will give it a go tonight. A few updates ... heaven knows where I envisaged the thickness of the existing joists, but they measure just under 7" x 2" so I guess that must be classified as 6x2. Also, the floor to bottom of joist height is 2.40m at house side and 2.30m at the opposite end. One point concerning the wall that I intend putting up at the double door end to seal off the inspection plate (and the double doors themselves) - I do intend building a second wall inside of this as well, so the double doors and the exterior wall they are fitted to will not be the outer leaf as such - the first wall that goes up to seal the area off will be the outer leaf. This is, of course, as long as the exterior wall on that side and the double doors will not become a THIRD leaf - I would be thinking not due to the fact that the doors do not close to any form of seal?? Sketchup, here we come. Speak soon. Steven

This is, of course, as long as the exterior wall on that side and the double doors will not become a THIRD leaf - I would be thinking not due to the fact that the doors do not close to any form of seal??

Actually, the garage door itself probably WILL still be a third leaf! It doesn't have to be sealed to act like a leaf. As long as it is a reasonably large surface with appreciable mass, separated by an air gap, then it will act as a leaf. The door fits that definition, I think. The sealing only affects how efficiently it acts as an isolator, but doesn't affect the actual MSM resonance itself much. Air still has stiffness and is still a spring, even if it is not sealed. So I reckon it will be a third leaf. Unfortunately... Sorry! Wish I had good news for ya, but I don't...

Sketchup, here we come.

:yahoo: :) :!: - Stuart -

Hello again. OK - for a start, I have been chased all over work like a cat amongst a pack of wolves and have not had the time to learn Sketchup properly. I am busy with the tutorials etc, and will be trying to have a go at a proper mock-up of the room etc as soon as I can. In the meantime, however, I thought I would add a few photographs that I took this evening, as I would like to do some prep work (planning, costing and maybe even getting the work done) on the existing shell as per my original query. OK - so here are my main problems in terms of a starting point of the existing room before I even begin to plan the "inner room". So, on to photo #1. This is of the wall that is basically the exterior face of the house itself onto which this garage is built. Once the new roof board/s are up there, there should be no "major" problems --- at least compared to the next three walls!Now for photo #2. This is of the opposite wall furthest from the house as such. The problem here for me is that there is a length of 3x2 that sits on top of the wall and the roof is fastened to this. That will create a 3" weak spot compared to the rest of the wall as such.Onto photo #3. This is of the first perpendicular wall at the opposite end to the main garage double doors. The obvious problem here is the substantial gap between the top of the wall and the top of the "pitched" joists.And then photo #4. The side at the main double garage doors. Same substantial gap as previous photo, plus no lintel, so nothing to build mass onto up there.So - with my main issue here being to get that plastic rubbish off and to put a new external roof on to give me a decent outer leaf as a starting point, I have a few obvious problems to tackle. My take on this (bearing in mind I am thinking off the top of my head as opposed to feeding off any existing threads etc) would be to get that roof off when the weather is showing signs of a few clear days and then do the following: 1. On the side opposite the main house wall, where the beam/batten sits flush on top of the wall, would be to fit a temporary "stop board" and to fill in the recess with concrete - once set, that would finish off the existing wall all the way up to any new roof (not talking about a hermetic seal at this point). 2. On the short side opposite the main double doors, to build up with brickwork as close as possible to top of joists and then to do the same "stop board" theory as above to fill in with concrete so as to finish off at the same pitch as the joist. 3. The big one --- possibly look at what is entailed to get a reinforced lintel above the door frame (would obviously remove the doors and frame for this and then refit underneath the lintel once in place. With lintel in place, apply same theory as previous wall - build up as high as possible and finish with concrete. This would give me an all around (reasonably) flush lip upon which to fix the exterior roof boards --- yes/no? There are two ingredients to throw in here. Firstly, underneath these terribly fitted double doors is the dreaded Municipal inspection plate as per the following photo.This needs to be accessible for the Council by law, so, as per where I ended up in an earlier post in this thread, I would have been tempted to build a double (staggered) stud wall just inside this plate, thereby losing about 30cm of internal length - but Stuart did point out that this will breach the 3rd leaf theories. So I should therefore bulk up this door to add mass to it as an outer leaf and then build just a single wall inside of the plate --- but what about the fact that that entire wall on that side is currently just a 53cm column on the one side, a 45cm column on the other and poorly fitted double garage door with nothing above it as such. Is there no reasonable chance that this barrier will not be effective as a problematic leaf --- and that I could therefore NOT worry about the whole lintel thing on that side etc. I would then address points 1 and 2 as above (if that approach is deemed suitable) and when building the double (staggered) wall on the double door side, I would take the outside leaf of this framed wall all the way up to the underside of the new roof to effectively become the new outer leaf wall on that side. Lastly, (apologies for the extremely long post) - a recap on what to put up for the exterior roof. OSB or Plywood? The exisiting roof joists are quite far apart (4ft - or 1.20 metres) - so I will probably go for my INNER ceiling being on a frame that rests on top of the INNER framed walls rather than hanging off the underside of the joists on RC. So should I go with a single layer of exterior boards on top and then add a couple of layers of wallboard from the underside between the joists to add mass --- or put a double layer on the top/exterior as well. I know the more might be the better, but I do not have an open-ended budget as I will be doing this from month to month, so if it is one and the same, it may be more "affordable" for me to rip off what's there, build up the "gaps" on top of the walls, SINGLE layer on the outside (with felt and ashpalt shingles) and then I can do the underside adding of a few layers of mass as I go along - with at least a reasonable shell in place. Thanks for any help. Steve

I think you are going to have to take venting into consideration for your new roof. Air should flow from the low side up towards the house side where it would exit through something like this: http://www.cor-a-vent.com/pdf/R2W-INST.pdf As far as the inspection plate. One solution would be to remove the garage doors completely and build a new exterior wall that is inset so the plate is completely on the outside of the structure..

"OSB or Plywood? " Either. What you do not want to put on this roof is shingles. The manufacture will not cover anything less than 2 inch rise in a 12 inch run. And for good reason. Water will dam up under the tabs, especially if you get much snow and or freezing water and rot the roof in a few seasons. A roll roofing or torch down is suggested in these cases or another comparable sheet type covering.

Thanks for the tips. Re: ventilation - to be honest, I have obviously missed any references to this before and did not realise that it was an issue (although it does make logic sense). What about the fact that this entire outer leaf (assuming I were to theoretically seal it all up as per my post) would then have an all-around air gap as such (ie. the inner framed room, which would not be tied to the outer leaf anywhere, would have a gap all the way around including up to and under the exterior roof). Since I would then be bringing HVAC in from the outside, is it not an option to split the send/return feeds of the HVAC and have it move air around this gap? If I am being ridiculous here and have missed any thread or references to ventilating a ceiling, then please feel free to kick me in the right direction and I will research accordingly. I thought the general concensus was to achieve as close to an airtight seal as is possible - or is this mainly in reference to the space inside the inner room? Re: shingles - they will have to be scratched from my options then as I have a 4" drop over the total width of the garage, that being about 11 foot. OK - so, aside from ventilation and also aside from the big decision around the double door and inspection plate problem, is my "basic idea" of bulking up above the existing walls, a SINGLE layer of OSB/ PLY on top, sealed with some heavy duty roof roll and a couple of layers of additional mass (gypsum) on the underside between the joists an acceptable overview in terms of prepping the existing shell to a workable state? Also, should I be putting anything (plastic sheet) between the exterior OSB/PLY and the roofing felt, or am I looking at the felt doing the entire damp/weather-proofing? I am considering putting a lightweight frame on TOP of the roofing felt and fitting the same cheap plastic corrugated "carport" roofing sheets on top of this. The vast majority of rain/snow etc would then run off these plastic sheets and only the tiny minority that gets underneath (or through - in the case of future holes) would hit the roofing felt. Speak soon and thanks again. Steve

Probably best if I mention here, that since posting my previous entry this morning, I have now searched for and found a few threads relating to ceiling ventilation, heat, mould, mildew etc. I have not found a comprehensive thread to follow that may apply to my scenario, but am still looking. I suppose my situation is pretty much the same as a basement type build - in terms of "outer roof" leaf being literally on the topside of the ceiling joists with the "inner ceiling" leaf pretty much directly on the bottom of these joists (in terms of distance) - and therefore the same fairly confined ceiling space that needs to breathe. So can anyone remember offhand if this is covered in one of the comprehensive basement threads? I know, I know - use the search function --- just trying to sneak in as much reading as I can whilst at work and currently juggling 16 open threads on this forum - and therefore forgetting each time where I was and on which thread etc. :roll: Steve

turnitup wrote:

is my "basic idea" of bulking up above the existing walls, a SINGLE layer of OSB/ PLY on top, sealed with some heavy duty roof roll and a couple of layers of additional mass (gypsum) on the underside between the joists an acceptable overview in terms of prepping the existing shell to a workable state?

If you make your roof deck part of your outer leaf then it won't be airtight due to venting. You could add your outer leaf mass on the underside of your rafters. You are going to need additional framing up there regardless.

Right - so at this point it wouldn't really make much difference what I put on top as a new EXTERNAL roof, as it will purely act as a weather/moisture barrier. I could therefore rip off the old corrugated plastic sheets, throw up a layer of OSB covered by roofing felt, ensure that there is a ventilation concept of sorts under there and then create my 2 x areas of mass beneath that. This is unless I were to look into some form of "baffled" ventilation, or whatever other proper term/s are used for a ventilated and sealed roof. SO - if I were to do the probably easier option of just getting a nice waterproof roof up there with standard ventilation, and then 2 centres of mass separated by an air gap beneath that as a "double mass ceiling" --- are there any rules (construction rules) that dictate how far this "weather barrier" external roof needs to be from the first of the ceiling masses --- and how much space there should be between these ceiling masses? I ask this, because I would now be faced with the dreaded ceiling height issues - so if feasible, could I fit 2-3 layers of board in between the joists from the underside, secured to cleats a few inches up from the bottom of the joists, so that the last layer sits flush with the bottom of the joists as such (or maybe even an inch or so ABOVE the bottom edge of the joist) - and then suspend the lower ceiling on joists resting on top of the inner wall frames. Here is a basic JPEG of this OK - so the existing joists are 6x2 and I would like to be suspending the new joists resting on the inner wall frames so that they actually sit "inside" the existing joists as per the picture. My 4 x questions (as marked on the drawing) are: 1. What is the minimum gap that I should leave here. 2. Considering this void has to ventilate, do I need to put insulation AND/OR some form of plastic damp shield (barrier matting) in this area as well and does this insulation then affect the "free air" space that needs to be in there to allow ventilation. 3. What is the minimum gap I can have here. 4. If I were to have this lower frame assembly sitting inside out, thereby placing the two lowermost layers an inch or so beneath the existing joists - how would that affect the minimum gap above this etc. My last question is concerning my concrete floor. At what point do you have to consider ventilation (as such) there as well? What I mean by this is that I was intending to simply place polystyrene down on top of plastic sheeting and then tongue and groove type boards on top of that - so no space for air or any possible moisture to be dried out as such. Do I need to research this in more depth as well? Thanks for all the help guys - I know a lot of these questions are covered in other threads, but I am just trying to get the outside shell direction settled --- the framing/caulking/detailed construction details that are listed all over this forum will be applied with as few questions as possible once I get started in the right direction. Steve

Last question before I go ahead and start building - can anyone comment on what the difference would be if I were to make my current joist environment an airtight "non-ventilated" ceiling with 2 x OSB on top, sealed all around (including on top of the walls as per my photo's) and then 2 x 5/8" gypsum either on RC on the bottomside of the joists - OR on a new set of joists that rest on top of the wall frames --- and then lay another 6x2 frame on top of this existing roof structure, with (whatever) form of exterior sheathing and allow a ventilated system to operate in the space between this new roof and the existing structure. I mean - for all intents and purposes, does this new roof not then become the "roof/attic/ventilation-criteria" part of a standard build?

turnitup wrote:

Last question before I go ahead and start building - can anyone comment on what the difference would be if I were to make my current joist environment an airtight "non-ventilated" ceiling with 2 x OSB on top, sealed all around (including on top of the walls as per my photo's) and then 2 x 5/8" gypsum either on RC on the bottomside of the joists - OR on a new set of joists that rest on top of the wall frames --- and then lay another 6x2 frame on top of this existing roof structure, with (whatever) form of exterior sheathing and allow a ventilated system to operate in the space between this new roof and the existing structure. I mean - for all intents and purposes, does this new roof not then become the "roof/attic/ventilation-criteria" part of a standard build?

Ventilation has to be part of the structure. It is better developed as what is typical in this building procedure in your country. The one thing I can say is that if you build an enclosed roof cavity this can backfire on you. Let me say this again, Ventilation has to be part of the structure. So never-mind the perceived leakage that happens as a by-product of an adequately ventilated roof system. The spring is still there. Either way, RC or a ceiling on the newly framed walls the air tract will or should exist. You cannot alter the basic flow of air to a structure for the simple want of better isolation. If you are in want of a hard and fast answer I will say this. Do not deprive the building from the air it needs. I cannot tell you how to construct this project as you reside in a country far away from the procedures that I am familiar with.

Thanks X-Space --- for replying to both of my threads. I have one last question for overall clarification on this - trying not to flog the proverbial horse here, but we have some clear weather ahead here in London --- it has been pretty much a miserable and wet 2 or 3 months on the trot up until now, so I would dearly like to make a start on the outer roof before the heavens open up again. Now, to my question --- assuming I were to fit a beefed up outer deck on top of my existing joists as an outer leaf and that my inner ceiling (on seperate joists resting on wall frames) was right underneath the existing joists --- that would create an air space (chamber/attic) of around 6" - 7". I quite like the ventilation concept as linked by Stunbutt earlier in this thread (thanks) - and although they specify a considerably greater pitch than what I have, I may look into that product and perhaps try to include a few fans to move the air if necessary --- BUT --- once I have added insulation, will it still be possible/feasible to get some sort of ventilation going on in there - I mean, will air even be able to flow efficiently in there - or is it irrelevant how much airspace there is, as long as there is an allowance for air to move through the space from bottom and to be able to escape at the top? Thanks for comments so far. Steve

Think I may have found the answer to this on a UK site that deals with garage conversions - as in, converting garages to liveable spaces etc. They mention in two seperate areas on their site that building regulations state a minimum gap of 50mm between insulation and outer deck (in a cold roof scenario). This will be achievable for my purposes - now I just need to settle on a method and perhaps query my low pitch for a workable solution with this same company --- might be worth me getting them in for some consulting advice. I will update here with any info from them in case it helps anyone else. Steve

Right - been in touch with a consulting firm, building regulations and tech support of specific insualtion suppliers and my solution is going to be to go the "warm roof" approach. This is a legal (in the UK) way to negate the need for ventilation in a ceiling and in my case that is the way I will be choosing to go to save on ceiling height and to make my 2 x ceiling leaves a less complicated design. It would also allow me to then build up my walls as per my pictures earlier on in this thread and seal up the entire existing shell up to the underside of the roof deck as a sealed outer leaf. Once the inner room has been built, I apparently have a 0% risk of condensation (according to the consultants running the recommended materials and layers through whatever calculation software they use) in the void between the outer roof structure and my inner ceiling structure. This is a massive result for me in terms of finding a direction to start working towards. :jammin: