Detached shop conversion

Hi folks. Here's a quick update with some of the plans I'll be submitting to the city with my permits. Turns out it's pretty fun drawing this stuff (breaking out my Adobe Illustrator skills from my graphic design days). Any comments or suggestions? I still need to make the floorplan drawing (just a big rectangle really), HVAC details, and electrical.

Nobody likes my diagrams, boo... :cry: But hopefully the city likes them, I should hear in a couple of days about my permit application. I built a new door jamb for the exterior door yesterday, converting it to out swing and repairing some damage to the door. For those who have said hanging a door is not for the faint of heart, you are correct! That was a real bear considering it's just a rectangular frame. Oiy! But it's smooth as silk now and the basic seal is tight! Secondary seal coming soon...

The 2X4 supports @ 24 inch centers on the top side of the truss chord, what is that doing and why are there so many of them?

xSpace wrote:

The 2X4 supports @ 24 inch centers on the top side of the truss chord, what is that doing and why are there so many of them?

Those are to support the OSB/drywall layers attached between the trusses (middle leaf in my unfortunately 3 leaf ceiling/roof). Same design as Onpurposeproductions' build here. 24" O.C. might be more than required, you think...? I figured one doesn't normally build framing at more than 24", so it would be hard for the city to argue with that spacing. Speaking of which, I just got the call from the permit guy at the city. They approved my design! He said he'd have some minor comments but it looks great. Woohoo!!!!! :yahoo:

Alright, let me ask you a question. Wouldn't it be just as labor intensive to actually mass up the roof area, basically treating it like a basement floor mass up, as opposed to actually MAKING the three leaf, that here-to-fore- did not exist? I'm gonna ask some questions to some folks and get their opinion on which is the more advantageous...because I see a roof system as an outer leaf and an inner decoupled ceiling system as an inner leaf...so ...anyway, I'll be back in a few days.

A very good point Brien. I went back and forth a lot trying to decide whether to beef up the underside of the roof or build the middle leaf. Labor/material wise it's close to a wash. My sticking point was attic ventilation - there are bird blocks between every other truss for roof ventilation, and 3 roof vents at the peak. That adds up to a big ol' hole in the roof! I couldn't figure out a reasonably simple (inexpensive and not excessively labor intensive) way to keep the required roof ventilation while creating a sealed MAM system. I toyed with the idea of using silencers connected to the bird blocks, but just didn't see that I'd be able to get enough isolation with all of those holes. Though another member of this forum did indicate that silenced bird blocks worked out for him. Basically I figure that the isolation I lose with the 3 leaf roof is less than I'd lose through the bird blocks and roof vents. (in fact, don't those ventilation holes reduce some of the 3 leaf affect?) One of those "live with it if you have to" things. Definitely interested in any further commentary about this, as I haven't started building the ceiling yet!

If you don't mind I'd like to put my 2 cents worth in here.... As far as the 2 versus 3 leaf systems go - this is always a challenge when it comes to roof assemblies - either the roof has to breathe or it has to have one heck of a good vapor retardant on the inner face. It is very difficult with a trussed roof to come up with any effective method of blocking the passage of vapor - so we end up with a 3 leaf system. This is pretty normal under these conditions. The trick then becomes how to effectively add enough mass in our design to overcome the "dreaded" 3 leaf system we're now constructing. I have a couple of comment here though that have nothing to do with the 3 leaf issue...... First - you can lose all of the 2x4 backers you have running perpendicular to the trusses..... they are redundant at best. You're correct that you really want to stay within that 24" span you spoke of - however you already have that with the trusses themselves - simply reverse the plywood/drywall at the top of the bottom chord and let the furring carry the load...... That's quite a bit off added weight you have on the bottom chord of the truss that you can lose...... 2nd - I do have a bit of concern with the load you're adding to the bottom chord of that truss....... the fact that your building department has said this looks fine does not mean it is safe..... they do not do a structural analysis of anything - and they are not responsible should anything happen down the road. If I were you I would invest a couple of hundred dollars and have this blessed by a local structural engineer....... I am curious how you are handling the lateral bracing of the inner structure..... what are you doing to brace this inner room? I noticed your firestop detail - and would point out that this is an acoustic short from the outside to inside of your building. Rod

Thanks for weighing in Rod! Your 2 cents are worth at least 4. So your opinion is that the 1x2 furring strip will be sufficient to hold up the drywall/OSB for the middle leaf? Screwed or nailed into the truss? And how do I get the leaf to stay up there while I attach the furring? You say reverse the OSB/drywall in the middle leaf. So your opinion is that the drywall should be ABOVE the wood? Top to bottom: drywall, OSB, furring? Would you prefer plywood to OSB? As to the load on the bottom chord, I did run this design by 2 local structural engineers. Both agreed that it should be fine, but didn't actually come out to measure and calculate the load. The second indicated that since these are engineered trusses they should meet the local standards and would should carry the load, and flat out told me I'd be wasting my money hiring her unless the city told me I needed to get an engineering evaluation. Basically she said "rather than pay me $400 to come calculate the load, go to permitting FIRST, and they'll tell you whether or not you need an engineer, and probably save you the cash". She was familiar with the guy at the city who approves the plans, and when I spoke to him he knew her as well. So I felt pretty comfortable with the truss capabilty...but now you have me worried again. :shock: Your thoughts...? Lateral bracing is provided by RSIC DC04 brackets attached to top support of the inner stud walls and the upper 2x4s on the outer stud assembly (not in the drawings). This is already built and is very sturdy. I initially had a few short pieces of 2x4 bracing the walls while I waited for the brackets to arrive. It was amazing when I removed them - whack one with a hammer and it was LOUD. When the last piece lost contact with both stud assemblies the volume of the hammering was reduced tremendously! I was surprised at how much the whole assembly amplified the sound of the hammer! Some firestop detail is also not in the drawing for clarity. I plan to stop the drywall short of the outer studs by 1/4" and fill with backer rod/caulk or firestop foam. So no short there... -Peter

Thank you Rod (I woke him up last night with questions about this "type" of project:) So let me continue where I left off...or maybe more of a clarification. "and 3 roof vents at the peak. That adds up to a big ol' hole in the roof!" Right, and this next bit assumes that you have continuous soffit vents and continuous ridge venting...no gable venting since this would be a short circuit in the convection process. So my thinking here is that the mass would be applied to the truss chords/joists with no coupled leaf assembly/3 leaf: The big ol' hole in the roof is far away, and that is a good thing since only mids and highs are going to be able to get into this area and those frequency ranges are easily taken care of. And the air space is large, even at the smallest point, it may be as large as 12 inches, depending on, well everything. The point is, all the mass on one side, you could just as easily invert the above materials in the picture. Wouldn't this, TL wise, and all labor intensiveness considered, be a method to eliminate the 3 leaf which is never good, still allow the roof to breath, add a higher TL rating to the assembly, satisfy code/fire rating considerations, and generally just be a good thing? I hear a yes out there...wait, was that me?

Cool, thanks for roping in Rod. Seems like this might be a fairly common issue, one which I havent found a well documented solution for. Maybe we can take care of that here. :D It's an interesting idea you have here. I'm quite uncertain what the effect of doing it your way Brien, as opposed to my 3 leaf approach. One worry is the coupling of all this mass to the trusses, seems like that would transmit a lot of the vibration to the rest of the structure. So it's your belief that since it's the same total mass without the 3rd leaf TL would be improved?

If all the mass is applied either top or bottom...what does that matter? "So it's your belief that since it's the same total mass without the 3rd leaf TL would be improved?" What does a third leaf do but reduce the TL at LF?

"I'm quite uncertain what the effect of doing it your way Brien, as opposed to my 3 leaf approach." I am certain that you guys really do not know what the effect TL wise will be of doing it the way you are doing it, will be. That is why I ask questions... When I did my room, I used the method I showed above in the picture...now here is what we do not have. The math on the difference between the two rooms of similar size and roof pitch and the two opposing methods.

"I'm quite uncertain what the effect of doing it your way Brien, as opposed to my 3 leaf approach." Bad comma use there, I should rephrase that. What I meant is that I'm quite uncertain which would work better, not that I have any certainty about either approach. :) Sure the 3rd leaf reduces TL, just a question of whether the reduction is greater than what's gained by the decoupling and MAM assembly. I make no claim on understanding the physics well enough to predict. Did the mucho mass on a single leaf work out well for you! Was your build in a similar situation? I suppose I could build it one way, measure, the tear it up and build the other. Or not. ;)

And these are the questions that need answers!!! I WANNA KNOW!!! :mrgreen:

In theory, if you have no choice but to go with three leaves, it makes more sense to put the largest amount of mass on the middle leaf, if you can, rather than spread it around evenly. And if the third leaf (the roof deck itself in this case) is a large distance away from the middle leaf, then there shouldn't be too much of an problem. If you can give me your best guesses as to the following, then I can calculate the theoretical resonances for you: Surface density of the roof deck (outer leaf) Surface density of the middle leaf Surface density of the room ceiling (inner leaf) Distance between outer leaf and middle leaf Distance between middle leaf and inner leaf Of course, the numbers will just be ballpark, since there are so many uncertainties, and they will only be the resonant frequencies, not the actual degree of isolation you can expect. But that might help you to feel a bit happier about what you are doing! (Or not, as the case may be!) - Stuart -

Stuart - Bit of guesswork here, as I'm not certain of the thickness of OSB on the roof. Beat guess: Outer leaf: 2 lbs s.f. (possibly less, 1.6?) Middle leaf: 4.5 lbs s.f. Inner leaf: 5 lbs s.f. Middle/outer leaf distance: 6" at edge, 3' at peak (do you average that?) Inner/middle leaf distance: 6"

OK, very rough estimate of the MSMSM resonance for your situation: f+ = 28 Hz f- = 20 Hz So it should isolate starting at about 27 Hz, isolate reasonably at around 39 Hz, and isolate well from 59 Hz up. That's without insulation in the cavities. If you could fill both cavities with insulation (hard to do, and also check on possible fire code restrictions with that), then theoretically it would start isolating at 20 Hz, isolate reasonably at 28 Hz, and well at 42 Hz. This is very much "ROUGH ESTIMATE"! There are so many variables and unknowns that accuracy is out the window, and even the equations I'm using assume equal mass on all leaves, and equal air gaps, which of course isn't the case here. But it does give you a rough idea of how that might perform, and it seems that your isolation should start at around 40 Hz or so, which isn't bad. As a comparison, if you were to put the same total mass on only two leaves, with the same separation of 12" (6+6), then it would be about half an octave better. In other words, it would start isolating at 15 Hz, and isolate well at 31. Of course, all of the above is theoretical, assuming perfect mass, perfect springs, perfect seals, perfect damping, standard air temperature and pressure, etc. Actual millage may vary. Void where prohibited by law. Caveat emptor. (And all the other usual disclaimers.) - Stuart -

What about installing as single layers of mass?

Thanks for the calcs Stuart. Interesting to note, but still doesn't really answer the nagging question: Would all mass on inner leaf perhaps be a better isolator than the 3 leaf construction in this example? I'd also love to hear from Rod or anyone else about whether: - I can really ditch the perpendicular 2x4 supports for the middle leaf (presuming that I build it) and rely on the 1x2 furring strip to hold up the drywall/OSB for the middle leaf? - Screwed or nailed into the truss? - And if so how might I get the leaf to stay up there while I attach the furring?

Still hoping for answers to the above questions, perhaps some studio porn will motivate those in the know. ;) Pics or it didn't happen, right? Here's the exterior of the building. The demo junk in the carport has since been hauled away...it was nearly 2 tons. Whew! Got a decent price on the drywall (102 sheets!) from the local contractor supply, $8.something a sheet with free delivery. He gave me the backer rod for $40, said it was laying around forever, no one wanted it. I also got my hat channel and caulk from him. I'm most of the way through my second case of caulk and starting to look like Popeye. Here's a view of the interior framing with 1 layer of drywall in the bays. Temporary lateral bracing... DC-04 iso clips for lateral bracing (thanks for the tip TomVan!). These are screwed to the header of the inner and outer studs, after which I removed the temporary 2x4 braces. Endless caulk and backer rod. I'm finished with the caulking of the outer beef up layer, ready to proceed with some electrical and ceiling work. My lovely wife did about a third of the backer rod stuffing before she got too frustrated and had to be kicked off the jobsite! After doing that much I have an efficient system using a 3" putty knife to kind of roll it back into the cracks. Caulking started very neat and got sloppy soon thereafter. I only have so much patience doing clean work that will never be seen! :shot: