New build wooden garden mixing studio, England, UK

[garethmetcalf]

Hi Everyone,
Overview
Having ruled out converting a room in my house, I’m planning a studio in the back garden. This is a ground up build so I have the time to get the design right.

Location:
In a corner of my back garden, next to two fences so it has to be within the UK permitted development limitations, meaning the highest point can only be 2.5m. This is for me with very limited client use so no concerns over needing toilets or other facilities in this room. I do want a window for some daylight, though. Surrounded by neighbours but nearest house is 10m (other than my own).

Map of studio.png

Usage:
Predominately mixing, with some vocal and guitar tracking. I use Mackie HR824 which will be on stands made from building blocks.
Internal acoustics to be decided once structure built, but working on the assumption that as a minimum I’ll have superchunks in each wall corner, plus first reflection points and treatment across the back wall.

Basic construction:
• Concrete Slab
• Single pitch roof (highest point 2.5m) – outside to inside:
o EPDM rubber membrane: 2mm
o cement bonded particle board: 12mm
o cement bonded particle board: 12mm
o 2 x 8” joists (out of phase with)…
o 2 x 8” joists
o Vapour barrier
o plasterboard (Gyproc Plank): 19mm
o Green glue
o plasterboard (Gyproc Soundbloc): 12mm

• Double wall two leaf timber stud construction – outside to inside:
o Siberian birch cladding: 21mm thick
o breather membrane
o 1 layer OSB: 18mm thick
o 2”x 4” studs (with RWA45 filling the studs)
o 1” air gab (between studs)
o 2”x 4” studs (with RWA45 filling the studs)
o vapour barrier
o plasterboard (Gyproc Plank): 19mm
o Green glue
o plasterboard (Gyproc Soundbloc): 12mm

• Double door to outside
• Window to outside(following guidance in Rod’s book)

I will be employing a builder for ground clearance and construction of the concrete slab. I will be building the rest myself.

Air:
No need for Air Con as the UK is cold. However I will be putting two ducts: air in and air out, both with home made silencers in the wall cavity.

Internal dimensions :
Option 1 (according to University of Salford best room ratios, 1 : 1.4 : 3.49)
• Height = 2100mm
• Width = 3318mm
• Length = 3969mm

design 1.png

Option 2 (more useful rectangular shape – still looks ok on Amroc calculator)
• Height = 2100mm
• Width = 3200mm
• Length = 4300mm

design 2.png

Questions:
• Are the Option 2 dimensions OK?
• The two roof layers have to be close at one end to get the minimum slope required (2.13cm per 1m length). To do this I will have to offset the joists for outside and inside roof, and at the closest the joist for the outside leaf will be 5cm from the back of the plasterboard for the inside leaf. Is this OK or am I better dropping the internal height to less than 2.1m?
• Given the above, I have specified cement bonded particle board for the outside roof leaf. Is this required or would OSB be effective enough?
• Do I need an additional layer in the outside wall leaf, or would the one layer of OSB combined with the Siberian birch cladding be similar mass to my internal leaf?
• Have I got the vapour barrier in the correct place in the walls?

As always, thanks for the excellent help and advice.
Gareth

[Soundman2020]

Predominately mixing, with some vocal and guitar tracking. I use Mackie HR824 which will be on stands made from building blocks.

Ave you considered flush-mounting your speakers? If you want the best possible acoustics in your room, that's the way to go. It eliminates all of the ugly artifacts associated with the front wall, due to having the speakers in the room.

Internal acoustics to be decided once structure built,

Maybe you meant "treatment to be decided...."? The actual basic acoustics of the room are set by the physical dimensions, long before the structure is actually built.

...
o cement bonded particle board: 12mm
o 2 x 8” joists (out of phase with)…
o 2 x 8” joists
o Vapour barrier
o plasterboard (Gyproc Plank): 19mm
o Green glue
o plasterboard (Gyproc Soundbloc): 12mm
...

I'm assuming that your second set of joists will rest only in the inner-leaf walls? You didn't specify that in your description, but I'm assuming that will be the case?
Also, are you certain you need 2x8 joists to span the width of the room? At just 3.2m, I suspect that 2x6 would do the job.

...
o plasterboard (Gyproc Plank): 19mm
o Green glue
o plasterboard (Gyproc Soundbloc): 12mm
...

Why the two different types and thicknesses of drywall? There's no real advantage to having different materials. Yes, there's a slight improvement due to the impedance mismatch, and the differing coincidence dips, but that is lost again due to the lower mass. It's better to go with two layers of the thickest, highest density material.

No need for Air Con ...

I beg to differ! There most definitely is a need! How else would you control the humidity??? The average RH in Derby is pretty close to 90%. How would you get that down to where it needs to be for a studio, at around 40%, if you don't have HVAC? Many musical instruments, some mics, and even some gear needs controlled humidity.

HVAC is not an optional luxury in a studio: it is a basic necessity. You need HVAC roughly as much as you need speakers...

However I will be putting two ducts: air in and air out, both with home made silencers in the wall cavity.

Unless you have a very large cavity, there's not enough room to fit in silencers. Did you do the math for figuring out your air flow yet? You need to move a lot of air at very low speeds. That implies large diameter ducts, and even larger cross section for the silencer boxes. You only have 8" of cavity depth. My thinnest ever silencer box worked out to just under 11", and that still doesn't consider the air gap you'd need on either side. Try putting your silencer boxes in the roof cavity, or if they won't fit there, then put them outside the outer-leaf and inside the inner-leaf.

... according to University of Salford best room ratios, 1 : 1.4 : 3.49...

I think you have a type in there! If it was 1 : 1.4 : 3.49 with a 2.1 m ceiling, it would have to be 739cm long...

Besides, 2.1m is very low for the ceiling. I would make a serious attempt to get more height out of it.

Option 2 (more useful rectangular shape – still looks ok on Amroc calculator)

Looks much better to me!

• Are the Option 2 dimensions OK?

For the room itself, yes, but your interior geometry needs fixing (speaker/listening position).

• The two roof layers have to be close at one end to get the minimum slope required (2.13cm per 1m length). To do this I will have to offset the joists for outside and inside roof, and at the closest the joist for the outside leaf will be 5cm from the back of the plasterboard for the inside leaf. Is this OK or am I better dropping the internal height to less than 2.1m?

That would be OK from the point of view of the MSM system, but not OK from the point of view of overall room acoustics. 2.1m is very low for the ceiling. To put that in perspective, that's roughly the height of a typical doorway...

• Given the above, I have specified cement bonded particle board for the outside roof leaf. Is this required or would OSB be effective enough?

Have you considered doing a flat roof?

• Do I need an additional layer in the outside wall leaf, or would the one layer of OSB combined with the Siberian birch cladding be similar mass to my internal leaf?

Is the classing absolutely solid, and 21mm thick throughout? No gaps, holes, slots, grooves, etc? If so, then you should be OK, but if this is done as a series of planks that overlap, then that would not be so good.

• Have I got the vapour barrier in the correct place in the walls?

It looks like it, for your climate. The interior of the studio will always be warmer than the outside, and the vapour barrier should go on the warmer wall surface, to prevent condensation, so that's correct. But do check your local code, and with local contractors, to find out what the normal practice is where you live.

- Stuart -

[garethmetcalf]

Hi Stuart

Thank you very much for your quick and helpful replies. I have commented below on your questions.

Ave you considered flush-mounting your speakers? If you want the best possible acoustics in your room, that's the way to go. It eliminates all of the ugly artifacts associated with the front wall, due to having the speakers in the room

I've started trying to read up on this but struggling to find the information I need to do this properly. It's not something I've ever done before.

Internal acoustics to be decided once structure built,Maybe you meant "treatment to be decided...."? The actual basic acoustics of the room are set by the physical dimensions, long before the structure is actually built.

Correct, sorry, my bad writing. I meant internal treatment to be decided. Regarding the physical dimensions, I figured a rectangular room with good/OK ratios would be the best compromise between construction simplicity and basic acoustics.

I'm assuming that your second set of joists will rest only in the inner-leaf walls? You didn't specify that in your description, but I'm assuming that will be the case?
Also, are you certain you need 2x8 joists to span the width of the room? At just 3.2m, I suspect that 2x6 would do the job.

Sorry, again you're quite right. The internal roof will be supported on the inner leaf walls.
Regarding the size of the joists, I have read conflicting information. If I could use 2x6 that would be great as it would give either a slightly larger air gap, or perhaps more usefully a slightly higher ceiling.

Why the two different types and thicknesses of drywall? There's no real advantage to having different materials. Yes, there's a slight improvement due to the impedance mismatch, and the differing coincidence dips, but that is lost again due to the lower mass. It's better to go with two layers of the thickest, highest density material.

Great, I'll go with two layers of the 19mm plasterboard.

HVAC is not an optional luxury in a studio: it is a basic necessity. You need HVAC roughly as much as you need speakers...

Ah. OK, this will require some reading up then. I've no idea where to start in specifying AC.

Did you do the math for figuring out your air flow yet? You need to move a lot of air at very low speeds. That implies large diameter ducts, and even larger cross section for the silencer boxes. You only have 8" of cavity depth. My thinnest ever silencer box worked out to just under 11", and that still doesn't consider the air gap you'd need on either side. Try putting your silencer boxes in the roof cavity, or if they won't fit there, then put them outside the outer-leaf and inside the inner-leaf.

No, I haven't done any maths on this. Have you a link to a reference on this so I can calculate?
I hadn't thought about putting the silencers on the extremities of the leaves - I could do this (perhaps hidden inside a bass trap?)

I think you have a type in there! If it was 1 : 1.4 : 3.49 with a 2.1 m ceiling, it would have to be 739cm long...

Oops, copy and paste error. That should have read 1 : 1.58 : 1.89, however....

Are the Option 2 dimensions OK? For the room itself, yes, but your interior geometry needs fixing (speaker/listening position).

Option 2 it is then! Sorry, but what's incorrect about the speaker/listening position? I have listening position at 38%...?

2.1m is very low for the ceiling. To put that in perspective, that's roughly the height of a typical doorway...

Unfortunately I cannot go past 2.5m for the external ceiling height (UK permitted development regulations) so the only way of bringing up the inside ceiling height is to reduce the gap between inner and outer roof (eg by 5cm by going to 2x6 joists). The planned roof is flat with a gentle fall of 10cm across the 3.8m external width.
However, increasing the internal ceiling height to 2.15m (as a result of reducing the joist size) means the ceiling is now exactly half the length of the room. Probably best leaving it as 2.1m, or changing the length?

Would the one layer of OSB combined with the Siberian birch cladding be similar mass to my internal leaf? Is the classing absolutely solid, and 21mm thick throughout? No gaps, holes, slots, grooves, etc? If so, then you should be OK, but if this is done as a series of planks that overlap, then that would not be so good.

I think so - I'm planning on getting http://www.silvatimber.co.uk/cladding/s ... 121mm.html.

Looks like I need to read up on HVAC, air flow and soffit mounting!

Could I bother you with this construction question: I have specified cement bonded particle board for the outside roof leaf. Is this required or would OSB be effective enough?

Thanks again
Gareth

[Soundman2020]

Regarding the size of the joists, I have read conflicting information. If I could use 2x6 that would be great as it would give either a slightly larger air gap, or perhaps more usefully a slightly higher ceiling.

Ask your structural engineer! Span tables show that you should be able to do that with 2x6s, provided that you use proper structural grade joists made from high strength woods such as red maple, spruce, yellow cedar, beech, Douglas fir, etc.

No, I haven't done any maths on this. Have you a link to a reference on this so I can calculate?

HVAC is a big subject! You'll need to do a lot of research on this. Basically you need to figure the volume of your room, how many times that volume of air needs to be replaced each hour, to get the flow rate (volume movement). In other words, how many cubic feet (or cubic meters) do you have to move every minute. Based on that, and using the rule of thumb that the velocity of the air moving through the registers cannot exceed 300 fpm in a studio, and would be better at 200 fpm, you can figure out the cross sectional area that is needed for the registers. Then you can work backwards from there to figure out the cross-sectional area that you will need inside your silencer boxes, then backwards again to get the acceptable cross sectional area for your ducts, which will lead you to the size of ducts that you need. Based on all of that you can figure ou the static pressure that all of the above will produce, and based on THAT you can finally figure out what fan you will need. The fan needs to be able to move the correct flow rate at the correct flow velocity when faced with the static pressure of your system.

That's just for the air movement. Then you need to dimension the actual HVAC system itself, to see how many BTU per hour it needs to deal with. You figure that by calculating the latent heat load and the sensible heat load for each room, under minimum and maximum occupancy, taking into account the climate where you live. The goal is to dimension a system that is able to keep the humidity at about 40% and the temperature at about 22°C under all conditions, from coldest day of winter to hottest day of summer, wettest to driest, and with nobody at all in the studio and all the gear turned off, to having it packed full of musicians and WAGs with all the gear turned up to 11.

Like I said, it's a big subject!

I hadn't thought about putting the silencers on the extremities of the leaves - I could do this (perhaps hidden inside a bass trap?)

That's possible, yes, provided that it doesn't take up all the volume of the bass trap! The bass trap needs to do some trapping of bass as well, and it won't be able to do that if it is stacked full of silencer instead. I normally put my silencers in the cavity between the inner-leaf ceiling and the outer leaf, or in the wall cavities. But sometimes there's no choice, and you have to put them inside the room.

Option 2 it is then! Sorry, but what's incorrect about the speaker/listening position? I have listening position at 38%...?

38% is just a guideline, not a rule. But the problem is more with the speaker setup. The room is not big enough to allow the speakers to be away from the front wall, (it needs to be at least 2.5m away, and preferably more, to prevent SBIR artifacts from the front: 3m would be better), so your only other option is to push the speakers right up against the front wall, leaving only a 10cm gap for a panel of porous insulation. For those dimensions, your speakers need to be placed 89cm from the side walls (meaning they will be 142cm apart), and 125cm above the floor. Set them on very solid, very heavy (massively heavy!) stands. Some people use concrete blocks staked up to make their stands, some use bricks, other used large dimension hollow steel profile filled with sand. "Heavy" is the key.

Now set up your chair on the room center-line, 160cm from the front wall. That's the position where your ears should be. Set up a vertical pole 200cm from the front wall, also on the room enter-line (in other words, 40cm behind your head). Tape a laser pointer to the top of each speaker, directly above the acoustic axis, and exactly perpendicular to the front face of the speaker. Rotate the speakers in place in their stands until both laser dots are hitting the exact same spot on the vertical pole. Now go back and double-check all of those measurements. NOTE! All of those numbers refer to the ACOUSTIC AXIS of the speaker, and not the top, bottom, side or center of the cabinet, nor the location of the tweeter. Find out from the manufacturer where the acoustic axis is for your specific model.

So the acoustic axis of the speaker will be 89cm from the side wall, 125cm above the floor, and aimed perfectly at the pole that is set up 40cm behind your head, which is 160cm from the front wall.

Bingo! Your geometry is correct. That's the theoretical optimum setup. It might be possible to improve on that, by taking careful measurements with REW and adjusting distances and angles slightly, but that should be a pretty darn god spot.

so the only way of bringing up the inside ceiling height is to ...

OR you could build your ceiling "inside-out"...

Could I bother you with this construction question: I have specified cement bonded particle board for the outside roof leaf. Is this required or would OSB be effective enough?

Go for whatever provides the greatest mass, and highest level of waterproofing!


- Stuart -

[garethmetcalf]

Thanks once again Stuart.

I will research the HVAC, but in the meantime I noted with interest the inside out comment and also read that you've said the same on Jonosteer's backyard studio post. I understand the concept, so are you suggesting to him that because his room is so small it would be better to make it inside out. What I'm unsure of is whether you're suggesting to him (which applies to me as we have similar sized spaces) to make every wall and ceiling inside out?? I'm worried how dead the room would end up!!??

Cheers
Gareth

[Soundman2020]

I noted with interest the inside out comment and also read that you've said the same on Jonosteer's backyard studio post. I understand the concept, so are you suggesting to him that because his room is so small it would be better to make it inside out. What I'm unsure of is whether you're suggesting to him (which applies to me as we have similar sized spaces) to make every wall and ceiling inside out?? I'm worried how dead the room would end up!!??

There's basically two approaches to treating a studio acoustically: 1) You can start off by making it very live (reverberant) then toning it down by adding absorbers and diffusers until you get the result you want. 2) You can start off by making it very dead, then adding reflection and diffusion until you get it where you want. Since small rooms need a lot of bass trapping anyway, and probably need more than 50% of the entire surface area absorbent anyway, personally I prefer to go with #2. It makes more sense. Especially when you consider that devices designed to absorb sound tend to be very thick (many inches), whereas devices that are designed to reflect sound can be very thin (fractions of an inch, or maybe one inch at most).

So for small rooms, inside-out makes sense: start with a dead room, and liven it up as needed.

If you are planning to soffit-mount your speakers (highly recommended!) then it makes even more sense to have the front walls inside-out, as the area in between stud bays will be inside the soffits, and available for both absorption and also to accommodate the rear corner of the speaker (thus the front face doesn't need to extend so far into the room).

- Stuart -

[garethmetcalf]

Thanks once again, and that makes sense.

So, essentially for my room building walls inside out will give me 4" thick absorption all the way around (as the studs are 2x4) on the walls, plus I'd planned superchunks in each of the four wall corners (as a minimum). I'm guessing I'd build initially without the superchunks, just with the wall absorption and take measurements before planning/building any further treatment?

Would you propose an inside out ceiling as well, or should I keep this normal with the plastboard on the room side? I only gain 5cm by changing to inside out which then makes the ceiling exactly half of the internal length (4.3m) so I'd have to reconside that dimension..

I'm also going to look at increasing the air gap between leaves on one side to allow for two silencers to be installed there.

This is all extremely helpful, thanks. I'm hoping to have a concrete base laid in the next couple of months so I can start the build once the spring appears. Once I've finalised dimensions I can press on with that!!

Cheers
Gareth

[Soundman2020]

essentially for my room building walls inside out will give me 4" thick absorption all the way around (as the studs are 2x4)

Almost! But it only gives you 3 1/2" of insulation, since 2x4 studs are only actually 3.5" deep... (In the UK; they are probably 89mm).

I'm guessing I'd build initially without the superchunks, just with the wall absorption and take measurements before planning/building any further treatment?

Right, but you'd certainly need either superchunks or hangers in the corners. If you really want to keep meticulous records of your acoustic progress, do one REW test with the bare empty room (no insulation on the walls at all), then put the insulation in, do another REW test, but a superchunk (or hangers) in, another REW test, etc. Of you do REW tests at each stage of the treatment, you'll easily be able to see what is working, what isn't, and what you still have left to do.

I only gain 5cm by changing to inside out

Are you sure about that?

which then makes the ceiling exactly half of the internal length (4.3m) so I'd have to reconside that dimension..

If you have only 5cm difference on a dimension that is 430 cm long, then you already have a problem! That's only a 1% difference, but you should try to avoid dimensions within 5% of each other. So if you have one dimension of 430cm, you should avoid other dimensions in the range 408 cm to 452.

- Stuart -

[garethmetcalf]

Hi
Further to the very useful comments so far, I have spent the last couple of weeks reading more, and playing with sketchup to refine my thoughts.

My final internal room dimensions are 2.1m (H), 2.94m (W), 3.99m (L) which follow the Louden 1:1.4:1:9 ratios. This will be constructed with inside out walls, but a 'normal' ceiling to maximise headroom (due to my overall external height restriction of 2.5m).

I have looked a lot more at soffit mounting and am going to go for it, it seems others on this forum have had success with Mackie HR824's so should be good. I like quite a lot of the design ideas in Bigsby's studio: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=15430 - I note that his soffits are quite into the corner of the room.

Question: Below are three different suggestions for my speaker/soffit location/angles. My biggest concern is the issues I'll get from the computer monitor being in the path of the sound, so would appreciate advice about how best to locate the speakers and their soffits to minimise these problems. I have put the monitor at 80cm from the mix position (which is at 38% of the length) because that's how I'm used to having it currently on my desktop. I guess one other option could be a larger screen positioned closer to the front wall, but I don't know how realistically well this would work for editing etc.

soffit mount angles.jpg

As always, advice gratefully received. The Sketchup file for the above picture is attached in case it's helpful.
Thanks
Gareth

[Soundman2020]

but a 'normal' ceiling to maximise headroom (due to my overall external height restriction of 2.5m).

Ummmmm... don't look now, but "normal" ceiling construction does not maximize headroom! If you want maximum headroom, then inside-out is the way to go...

(think it through... )

I have looked a lot more at soffit mounting and am going to go for it,

Smart move!!!

it seems others on this forum have had success with Mackie HR824's so should be good.

Yep.

Below are three different suggestions for my speaker/soffit location/angles.

Definitely not "A". I would go with "B" or "C".

My biggest concern is the issues I'll get from the computer monitor being in the path of the sound,

Then keep it out of the path of the sound!

There's lot's of factors involved here, but the main idea is to keep your video screens down low, where they won't be in the path from the tweeters to your ears. It's not so critical if they are partially in the path from the woofers to your ears, since they won't have such a large effect on lower frequencies. So check the dispersion angles vs. frequency for your speakers, and keep the video screens away from any wavelength that is similar in size to the width of the screen, or smaller. Also, angle your screen(s). Don't have them straight up, and don't have them parallel to the front wall.

And one other thing: think in 3D, not 2D!

PM me, and I'll be happy to share a design that I just completed for a paying customer (who authorized me to share privately), for a very small room with extremely tight sizes, with soffit-mounted speakers and video screens that are not quite in the way...

- Stuart -

[garethmetcalf]

Hi

So, further work and thinking has got me to the following design. If I make the ceiling inside out the internal height raises to 2.15m, so I've increased my width and length accordingly to 3.01m x 4.09m.

I find sketchup difficult but I think I'm getting there: https://www.dropbox.com/s/ndqzbgyh07vw6 ... 2.skp?dl=0

I think I've got the soffit location and angles correct, and have managed to squeeze some silencer boxes into both the soffit and rear wall for the air in and air out. The door and ideally a window will go along the bottom wall in the picture below, but I'll finalise their positions once I've got the design basics in place.

Questions:
1. Am I now looking at something workable as a design?
2. Do I need to add any splayed treatment to the left and right of mix position? (many other designs on here have 6 degree or so of helmholtz type treatment)
3. Would a mini-split such as this be OK? http://www.appliancesdirect.co.uk/p/eiq ... Gwod82YLRA

Thanks once again!

Screen Shot 2017-02-07 at 16.26.13.png

[garethmetcalf]

Hi guys

Hoping for some feedback on my design above, now I've ventured into the world of sketchup!

Sketchup file available: https://www.dropbox.com/s/ndqzbgyh07vw6 ... 2.skp?dl=0

Questions:
1. Am I now looking at something workable as a design?
2. Do I need to add any splayed treatment to the left and right of mix position? (many other designs on here have 6 degree or so of helmholtz type treatment)
3. Would a mini-split such as this be OK? http://www.appliancesdirect.co.uk/p/eiq ... Gwod82YLRA

Thanks very much!!

Gareth

[nicklear]

Hi Gareth - I can't give you any advice, I just wanted to say your build is similar to mine and I've also been learning Sketchup to give Stuart a proper model to look at - I've nearly finished my first draft. I've found it like pulling teeth, but I'm getting there now. Will be following your build, keep up the good work.

[garethmetcalf]

Thanks Nicklear, I have read your thread with interest too!

I think my build will be a while off because I've got to save the money, but I'm hoping to get my design good enough that I can get the concrete base in a couple of months time. I'll see where I'm at with the budget and savings at that point!

[garethmetcalf]

Hi

I've been trying my best to figure out the air flow calculations. I would appreciate any advice on the following to see if it's correct and useable for my room.

room volume 26.47
in cubic feet 934.72 1 Cubic Meter = 35.3146667 Cubic Feet
6 change per hour = 5608.34 cubic feet to move in an hour
qi (CFM) = 93.47 in minutes 158.9030746 m3/h

vi (chosen air speed) = 350.00
Ai 38.46 square inches
7.00 duct cross section (inches)
0.18 duct cross section (m)

Size of silencer = twice duct cross section plus materials
materials are 18mm thick: 2 layers for the box and 1 layer for the baffles

Air silencers: length, m width, m thickness, m
dimensions: 0.66 0.33 0.46 actual height

Does the above seem reasonable?

It seems quite hard to find fans in the UK - I would like one that mounts to the exterior of the building and I'm thinking it will extract the air. Is that OK, or should it push fresh air in?

I've tried a few static pressure calculators and it seems quite hard to figure it out, I think it'll be 0.1 psf in.wg - based on 8 feet of duct (including the silencer), and 8 90 degree turns.

Again does this seem right?

Assuming all the above is correct, I think I could buy one of these fans and it will work...
https://www.tlc-direct.co.uk/Products/ADMV150ECOP.html
Data sheet: https://www.tlc-direct.co.uk/Technical/ ... nt-eco.pdf

Any help with this gratefully received.

My sketchup diagram is coming along well and I'm meeting a builder tomorrow to discuss the concrete pad...