Garage Conversion Wall & Floor Construction Details

1 Nov 2021
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United Kingdom
Hello All,

I am planning to convert a detached, 1960's, brick-built (singe-skin with piers) garage with a pitched, tiled roof into a new kitchen-diner and connect it to the existing house with a nice walkway with plenty of glass. The garage is approximately 3.5x6.0m.

I am planning to construct the internal walls using timber studwork and the build the floor in a similar manner. I have drawn up a sketch showing the intended construction, which I think I am happy with - and is attached to the post for people to view - but do have a a couple of questions remaining for anyone who's done this before.


1. Is an external layer of sheathing (OSB) required on the outer face of the studwork walls for stability? (of course this would go on before the Tyvek wrap like any other timber frame building.

2. Should I provide brickwork ties between the new studwork framing and existing outer skin of brickwork, again, to strengthen and stabilise the whole structure. I am planning a large opening of around 2.5m for some bi-fold / sliding doors in a longer side. I have a gut feeling this would be a good idea, but am unsure whether the benefit would actually be realised without a positive connection between the perimeter block and existing slab.

3. How should the potential for water to get into the cavity and pool at the base of the wall be combatted, or is this not an issue I should worry about?

Any input or feedback is much appreciated, be it specific to the items above or the project in general.



Garage Conversion - Floor & Wall Detail.PNG
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I'll put you up for the drawing of the year award. Our Isambard will be jealous.

Anyway, no need for a cavity - otherwise you'll need to drain it at the bottom and vent it at the top, and tie the two walls.

You could line the brick wall with a vDPM, form the timber frame without sheathing or a membrane, and just fix the frame at the sole and header

And it's always better to have an addition layer of insulation across the whole inner face of the stud wall to avoid pattern staining. You don't need fire rated board.
Thanks for the fast reply Woody.

So I'll lose the cavity. I was concerned that fixing studs into the single external skin would attract damp or blow out mortar during installation. But fixing top and sole only would solve that issue.

When you say vDPM, do you mean damp proof course placed vertically between the studs and brickwork? What do you think about tanking slurry waterproofing the internal face of the outer leaf and then using DPC or keeping the Tyvek in the fashion mentioned above as a belt and braces approach?

I will keep the perimeter blocks and just shift them up to meet the inner face of the outer leaf with a mortar joint between and keep the lapping of the DPM over the blocks and bring it up the wall a couple hundred mill.
Tanking has no value, and introduces the risk of it cracking. A vertical polyphene DPM performs the same function simply and better.

Any breathable membrane needs an air gap and air movement to work as intended. And having two barriers is not belt and braces, but a waste of time - and risks permanent interstitial moisture. Just have one properly detailed barrier of whatever you decide.

The vDPM and hDPM junction is important, so that any moisture running down the membrane does not get over the hDPM. So the hDPM should be lapped on the room side and preferably taped too.

And with this method of no cavity, the vapour barrier on the internal face, with properly sealed joints and penetrations is crucial.
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Great detail thanks guys, can I ask what it says, regarding how you will improve bond on this block you have for the stud work.

Great detail thanks guys, can I ask what it says, regarding how you will improve bond on this block you have for the stud work.

Scabble - roughen up the surface of the concrete so that the mortar will adhere better
I'd do away with the block course (it does not seem to be necessary - and it creates a section of wall that is less insulated and a technical thermal bridge), and/or extend the vDPM down to the slab

But that's just tinkering with the details as your drawing covers the principles and is OK.

You can foil tape the PIR (joints and timber frame) to create a vapour check layer, rather than fitting additional polypthene
Jesus jks you got a ‘and is ok’ from woody :!:

Like a Hollywood handshake :D

cheers guys.

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