Garage conversion - wall/floor detail, soleplate and DPM...

[walty666]

Hi all,
I've reposted this to the 'Building' forum as it wasn't get much of a response in the 'Your Projects' forum...

I'm DIYing a conversion of a single block work shed in our garden to make a home office. We've had some roof leaks from the one verge due to poor mortar bedding which has saturated one of the walls but this has now been fixed. Now it's dried out I'm looking to insulate, stud and dry-line it but have been going round in circles with regards to the floor/wall detail, especially with regards to where to fix the soleplate for the studwork, and how to ensure we don't affect the integrity of the DPM.
I've attached a drawing showing the existing situation and one option I'm considering.

There's one slight anomaly/issue - when it was built the builder didn't lap the DPM below the floor slab with the wall DPC, but instead cut it off at floor level leaving a 4" strip of footing between the floor slab and wall which is damp to the touch. This moisture may have been a result of the leaky roof but it may also be due to ground moisture.
What I was thinking of doing is as follows:

1. Applying liquid damp proofing over the 4" gap to seal it.
2. Lining the outer walls with horizontally lapped breather membrane
3. Laying a DPM over the floor slab (not sure on the integrity of the one below the slab so just in case...)
4. Installing tanalised 4x2 soleplate 2" from the wall over the DPM but with a DPC below, screwing down through both into the concrete, then bringing the DPM up the back of the soleplate for a nominal distance
5. Firing studs up from the soleplate to a top plate in the roof trusses, insulate between, VCL and 12.5mm plasterboard
6. Laying 50mm Celotex up to the inner edges of the soleplate with 22mm T&G chipboard over, allowing a gap for the T&G expansion

However, there also seem to be the following options:

1. To take the DPM and flooring out further and fix the soleplate to the chipboard floor - seems like an easier option but possible concerns over floor movement resulting in the stud moving/plaster cracking
2. To build a single course inner brick wall on the slab with DPC below, run the DPM over it and screw the soleplate to the brick through the DPM - seems like a bit more work.

There's no low level vents in the outer block wall, just the soffit ventilation at the top of the wall.

I'd appreciate all/any comments you have relating to the above/attached drawing.

Thanks
Ross

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[pinenot]

There's nothing wrong with your proposals, all would work somewhat, but I think there's an improvement to your first that might be worth considering.
Lay the liquid dpm across the entire floor, and up the internal face of the ex wall as you show, perhaps 100 mm past the the ex dpc course. Double the dpm at the missing section around the wall, stud wall sole plate as per dwg.
The current ventilation should assist any drying out once you've closed the wall in...pinenot

 

[walty666]

Thanks pinenot!

So do the whole floor with liquid DPM and 1200ga DPM?

What about the breather membrane on the walls? I assume the bottom of this needs to finish on the outside of the DPM. That's why I'd left the gap so that any moisture on the outside of the breather membrane had somewhere to drain to. I'm not expecting torrents but possibly the odd drip now and again. If I paint liquid DPM up the wall, it'll be on the outside of breather membrane, so any moisture that makes it's way through the wall is likely to run down on top of the liquid DPM?

In terms of the sole plate fixing location, is one option perceivably better than another? Single brick course vs slab vs chipboard flooring? The floor will be floating so no joists to support the stud.

 

[walty666]

These are the various options I've found on the net. Any thoughts on which is best?

1. Soleplate on floating floor:

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2. Soleplate on slab:

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3. Single brick course beneath soleplate:

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4. Double brick course beneath soleplate:

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Cheers
Ross

 

[tony1851]

Your drawings don't show the adjoining finished ground level. Depending on that, your details may or may not work.

You would not fix a membrane direct to the inner face of the blockwork. It would be fixed to the cavity side of the studs, and slope outwards at the bottom to form a cavity tray

 

[walty666]

Ground level is at least a foot below DPC and floor slab level.

 

[pinenot]

No need for a dpm up the wall, just 100mm past the dpc course.

What your wanting to do here is a reverse of a timber frame build i.e. the block wall is already up now your building the timber frame inside of it.

Now a couple of important things - is there any block piers built, reinforcing the single skin wall? You say there are air bricks/vents in the single skin at present, what size, how many and how are they placed along the wall(s)

A couple of important things though to bear in mind - as the block wall is free standing, you don't need brick ties (minimizing cold bridge) and if you build the stud from 4"x2" there will be no need for any, as they will be free standing.
As what I have already said - your building a timber frame building in reverse, you might want to look @ trada for some insparation, here's just one link - http://www.trada.co.uk/news/view/68ECDC4E-1AD4-4244-8C29-C46CEAA5B6A4/
You sound pretty clued up and I'll be here for anything else you need to know, so go ahead, you should get some workable clues on trada pages, and if you need anything else just ask...pinenot

 

[tony1851]

I'll be here for anything else you need to know

 

[walty666]

Thanks pinenot. Had a look at the TRADA site. Some interesting stuff on there!

Just to confirm, there's no ventilation in the outer walls at all. Just soffit insulation at the wall/roof interface. I could put some in but it'll mean disrupting the render on the outside.

Yes there is a 125mm x 440mm pier half way along the longest wall. The blockwork also thickens in the corners of the room. By setting the 4x2 sole plates 2" or 3" from the wall they'll extend an inch or two past the piers. This should allow me to run 25mm or 50mm insulation against the piers.

What I'm most confused about is the breather membrane and DPM location. If I put the breather on the back of the studwork but run the DPM to the wall as you suggest any moisture will run down the back of the breather on to the top of the DPM potentially allowing the sole plate to get wet!? I'm not expecting torrents but thought the breather membrane should terminate outside the DPM? Any thoughts on this?

 

[^woody^]

Any thoughts on this?

I have ........ ignore pinenot and see Tony's post four above yours

 

[walty666]

I get the idea that the breather needs to be on the back of the studwork rather than the wall but should it drain on top of the DPM, or should it be tucked in between the DPM and wall?

 

[pinenot]

Any thoughts on this?

I have ........ ignore pinenot and see Tony's post four above yours

Yer heids wasted man

 

[pinenot]

Thanks pinenot. Had a look at the TRADA site. Some interesting stuff on there!

Just to confirm, there's no ventilation in the outer walls at all. Just soffit insulation at the wall/roof interface. I could put some in but it'll mean disrupting the render on the outside.

Yes there is a 125mm x 440mm pier half way along the longest wall. The blockwork also thickens in the corners of the room. By setting the 4x2 sole plates 2" or 3" from the wall they'll extend an inch or two past the piers. This should allow me to run 25mm or 50mm insulation against the piers.

What I'm most confused about is the breather membrane and DPM location. If I put the breather on the back of the studwork but run the DPM to the wall as you suggest any moisture will run down the back of the breather on to the top of the DPM potentially allowing the sole plate to get wet!? I'm not expecting torrents but thought the breather membrane should terminate outside the DPM? Any thoughts on this?

What I should have explained is that there's no need for any breather membrane. Your timber build will mirror some of the procedures, but not all. Remember I said that your block wall are load bearing, not the timber frames, that means theirs no requirement for sheathing, which in turn means there's no requirement for a breather membrane.

As I said, whole floor liquid dpm - returned up the wall 100mm beyond the dpc (double thickness at the perimeter, where you say the original dpm is missing) - sole plate on dpc strip, as per your original sketch - no ties to ex blocks (minimized cold bridging) - insulate between studs and plasterboard.

Sorry, my mistake re the low level vents. Assuming you can drill say three half inch holes at approx 1000mm back from each corner, on the long walls, and fix a meshed hood over these (stop any bugs, flies etc.) and that the air flow thus gained, can vent uninterrupted up to the soffit vents, like this -
View media item 61250What you haven't stated yet is your preference of flooring detail...pinenot

 

[walty666]

Thanks again. Floor's floating 22mm chipboard on 50mm Celotex on DPM.

Not sure about leaving out the breather membrane on the back off the studwork as nearly all the local council BC advice seems to suggest it's a requirement.

 

[pinenot]

What your doing is likened to a perpendicular floor structure but instead of timber board cover, yours is a plasterboard. If the studs are free standing from the block work wall, there should be little chance of condensation and if you create the ventilation between walls, what little could be caused, will be allowed to vented away. However these are my thoughts on the matter, it's up to you to put your thoughts into practice...pinenot