Advice on garage conversion

7 Oct 2016
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United Kingdom
Hi - we are converting our garage and I was after some advice along the way.

We have already poured foundations to support a wall and two windows to infill where the 2 garage doors were to the satisfaction of the building control officer. We have agreed with building control to make the external skin 2 bricks deep so that it ends up the same thickness as the existing central pier of the garage which we plan to leave alone. We will then have a 50mm air gap and stud etc.

We are unsure how we should install a cavity tray and if we in fact need one in conjunction with weep holes?

I would be very grateful for a steer on this as we have employed a great bricklayer(old skool) but he is not up to speed with current building regs…..

Thanks in advance for any assistance.

Cheers Alan
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Your description is of a solid wall one-brick thick, cavity and timber frame totaling nearly 400mm thick - which is bizarre for a garage conversion.

You would not normally use cavity trays for solid walls, but then you would not normally have solid walls with a cavity!

Anyway, cavity trays go above openings - unless the top of the opening is below a roof, and at floor level of timber frames.


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Hi Woody - thanks for your reply. Due to the width of the existing garage pier we wanted to make the new wall we are building the same width, so the building control guy agreed we could build a double thickness wall back to back as shown in photos. Essentially so we then have a flush wall with the piers on the inside. Then he said we neeeded a 50mm air gap then the insulated stud etc. So there will be a cavity, but behind the thicker outer wall?

Hope this makes sense - thanks Alan
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First, eeeew, those plain bricks should have been thrown off site, and that new wall should have been bonded in, not those crappy wall starters and straight joint. :sick:

Why don't you just have either
- traditional cavity wall, and just line the piers on the inner face with insulation
- a wider cavity or construct a wider frame
- solid wall and line the inner face with insulation

That proposed wall construction is a bit bizarre and make the detailing very difficult and is a lot or extra work for no benefit.

Your building inspector is not a designer, whilst timber frames do need an air cavity when screened with a single skin wall , the cavity should be vented and drained and that is not going to happen with a solid 215 wall in front of it. There is a much higher condensation risk within the cavity because of that thicker wall.
Appreciate your thoughts Woody and advice.

Based on your recommendations, we will then most likely leave a 50mm air gap (minimum) behind a single external skin and then have the stud timber framework with breather membrane on the back and insulation inside etc.

For the cavity tray, can we simply attach DPC or equivalent to the back of the timber framework have it drape down the 150mm fall and then tie in between the single skin brick with the weep hole insert?

I am assuming the cavity would not need to continue around the pier and we can simply fix insulation directly to the 3 sides as you suggested due to it being a solid wall?

Thanks Alan
Try this

Dpm wrapped around the pier behind the timber


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Ok. I think I get it. So we make the cavity large enough to mean the timber frame sits essentially on the back (inside) of the pier but we place DPM between the pier and back of the timber frame? This way we are not trying to come around the pier?

For the cavity tray install, as I described before?

Cheers Woody.
Yes the frame projects past the pier and the pier is infilled flush with the frame. The cavity thickness will depend on how thick your timber frame sections are CLS will give a wider cavity than standard 4x2

The criteria would be to set the frame out to allow for the insulation thickness on the face of the pier. I've put 50mm celotex across the pier on that sketch which should do as it's such a small area. Strictly it should be 60mm.. Confirm with the BCO if you are changing what you have already agreed.

No tray if you do as per the second image. Otherwise a tray would be built in to the outer leaf and project up about 200mm and stapled to the sheathing. The breather membrane would overlap the top edge of the tray by 100mm.

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