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Outer timber wall construction question
Old 23rd January 2013
  #1
Gear Maniac
 
🎧 10 years
Outer timber wall construction question

Hiya - I'm building a new mix room - self-contained external build as discussed in previous post - Timber construction on a reinforced slab. Approx 6 x 4 m (TBC)

My question is regarding the eternal wall. Everyone seems to suggest (from out side going in) Cladding/Building Paper/OSB/Stud work/plaster board infill between studs, Rock wool infill.

What about putting the plasterboard (say 2-3 layers) between the OSB and against the studs. Then I can use full sheets and not worry about all that cutting and backer rod. If I used an over sized footer and header the OSB layer could still line up with these and the building paper should keep moisture from damaging the pasteboard. More space then for the rockwood and a much quicker build.

What do you think?

Also can someone explain how the DPM ties in with the external wall construction? (UK)

Cheers,

Jon
Old 23rd January 2013
  #2
Lives for gear
 
Rod Gervais's Avatar
 
🎧 15 years
Jon,

This is certainly a good approach - with a few modifications.

The structural sheathing needs to be in direct contact with the studs in order to create the brace frame for the building. This places the drywall outside of the building shell - and as such typical drywall is not an adequate product for the job.

You now need to use an exterior grade drywall (we use this when building structures that have Dryvit finishes) such as:

Exterior Gypsum Sheathing by American Gypsum

I do not understand what you mean by DPM... please elaborate.

Rod
Old 23rd January 2013
  #3
Gear Maniac
 
🎧 10 years
Thanks Rod,

I'll look for the UK equivalent plaster board then for external work if the building paper not be sufficient to keep the moisture out.

DPM = Damp Proof Membrane - Either under the slab or ontop but under a screed. Basically to stop moisture rising up through the concrete slab. I'm a little uncertain how this ties into the timber frame external wall. On a Brick build it would overlap into the damp proof course of brickwork.

Many thanks,

Jon
Old 23rd January 2013
  #4
Gear Maniac
 
🎧 10 years
um - seems it's hard to get hold of external plasterboard in the UK. Loads of water resistant stuff but nothing designed for external use (as such).

Does anyone know of a UK supplier?

Cheers,

Jon
Old 24th January 2013 | Show parent
  #5
Lives for gear
 
Rod Gervais's Avatar
 
🎧 15 years
Quote:
Originally Posted by Jon Dixon ➑️
I'll look for the UK equivalent plaster board then for external work if the building paper not be sufficient to keep the moisture out.
Try this:

GTEC Aqua Board Tapered Edge 1200 x 850 x 12.5mm - Specialist Boards | Jewson

Quote:
DPM = Damp Proof Membrane - Either under the slab or ontop but under a screed. Basically to stop moisture rising up through the concrete slab. I'm a little uncertain how this ties into the timber frame external wall. On a Brick build it would overlap into the damp proof course of brickwork.
Jon, the DMP (in order to work) would have to run over the top of the foundation and beneath the wood framed exterior wall assembly - then tie into the DMP running up the wall.

If this is not the case than I do not see how you can install this in such a manner as to be effective.

However - if it simply meant to damp proof the slab - then it could simply run beneath and tie to the inside face of the wall at the bottom.

Rod
Old 25th January 2013
  #6
Gear Maniac
 
🎧 10 years
Many thanks Rod,

This is pretty pricey compared to normal plasterboard - Do you think I could use this just on the outer layer and regular plasterboard for layers 2 & 3?

I'm intending to build straight on to the slab, so do you mean layer the DPM on top of the slab and then place my timber sole plates on top of that and fold it up the outer wall before cladding or would you suggest I build a few courses of block work as a foundation just to lift me 'out of the ground' so to speak? (I'm not sure yet if I need to have the slab slightly above ground level or just below....head height is the issue

Cheers,

Jon
Old 25th January 2013 | Show parent
  #7
Lives for gear
 
Rod Gervais's Avatar
 
🎧 15 years
Quote:
Originally Posted by Jon Dixon ➑️
Many thanks Rob,

This is pretty pricey compared to normal plasterboard - Do you think I could use this just on the outer layer and regular plasterboard for layers 2 & 3?
No - one of the things we need to deal with when it comes to this type of construction is that the manner in which we deal with things determines what products we use.

Regardless of where in the layers this goes - once you take it outside of the buildings structural sheathing, it now becomes part of the outside world, and the integrity of the weather seal (the siding) for the building is dependent on it. Whether the sheathing were to deteriorate in the outermost location or the sheet behind the one the siding is installed over the result would be exactly the same. This is not a matter of direct weather exposure - if it was that simple the siding would protect it.

What you need to is a cost analysis of the 2 approaches and then which ever is the least expensive is the way to go. I would point out that the full exterior sheathing approach will provide a slightly greater level of isolation, but that would not be the deciding factor if I was faced with this.

Quote:
I'm intending to build straight on to the slab, so do you mean layer the DPM on top of the slab and then place my timber sole plates on top of that and fold it up the outer wall before cladding or would you suggest I build a few courses of block work as a foundation just to lift me 'out of the ground' so to speak?
Please develop a building section so I can better picture what you're faced with.

Rod
Old 29th January 2013
  #8
Gear Maniac
 
🎧 10 years
Thanks again Rod - OK - thought things through a bit more and here's my proposed building details for the slab/wall contraction. I want to keep within a maximum external roof height (flat roof) of 2.5m so this means I need to have the internal floor space lower than 'grade' (i.e. the external ground). The site is away from other buildings and no known drainage issues.

No exact measurements as yet but keen to get feedback on this as a proposal:



I realise that the 'next step' in terms of isolation would be to form separate foundations for the footings and then pour the 'innner' slab as an isolated entity but I don't think I need that level of isolation to be honest

Many thanks,

Jon
Attached Files
File Type: pdf DPM.pdf (36.5 KB, 132 views)
Old 29th January 2013 | Show parent
  #9
Lives for gear
 
Rod Gervais's Avatar
 
🎧 15 years
Just a few thoughts here Jon,

Quote:
Originally Posted by Jon Dixon ➑️
here's my proposed building details for the slab/wall contraction. I want to keep within a maximum external roof height (flat roof) of 2.5m so this means I need to have the internal floor space lower than 'grade' (i.e. the external ground). The site is away from other buildings and no known drainage issues.
I would forget the gravel for drainage against the building - I assume (seeing as you indicate drainage) that there is somewhere to drain any water to.

That being the case you would be better suited to install an actual drainage system to make sure you never have an issue. I wish I had a dime for every time properties with no water problems suddenly developed them when someone blasted to deal with some ledge or large rocks - and sometimes this has been on properties quite a distance from the property I as working on.

Fixing things after the fact when this happens is quite expensive.

I also take it this is all new construction.

So pour a footing rather than resting everything on a monolithic slab - with the masonry walls resting on the footing - slab pours inside of innermost masonry.

That picks the slab up above the exterior drain point (meaning top of footing) . At the point where the wall meets the footing you should place a canted mortar shelf about 4" up the wall and 4" off the wall to drain water away from the wall itself.

Then I would install a footing drain, right on top of footing and tight to the cant. - fill to slightly below the top of grade (say 12") with stone - the stone and pipe should be wrapped in drain fabric so it doesn't all just silt up over time. The pipe should run to to sunlight (even if it's level water will still run.)

If you cannot run a drain to daylight - then the gravel is going to serve little to no purpose anyway.

You could always do a complete wrap of the foundation (picture laying down a waterproof membrane before pouring any concrete and then wrapping that up the wall to a point just below finished grade.

we do this quite often when we are concerned with potential water issues - and it will also deal with keeping any moisture in the ground from migrating into the building.

I know you said not concern, but (again) with new construction especially with a finished space below grade - I always tend to lean towards cautious. -

Quote:
I realize that the 'next step' in terms of isolation would be to form separate foundations for the footings and then pour the 'inner' slab as an isolated entity but I don't think I need that level of isolation to be honest
For a stand alone building I would never concern myself with this.

Rod
Old 30th January 2013
  #10
Gear Maniac
 
🎧 10 years
Thanks so much Rod, Great feedback,

I'll post up a revised drawing soon. I'm not actually on site yet so everything is only 'in theory at the moment'

Regarding drainage I should have made it clear than it's a level plot but as far as I know the only way for water to drain below would be to install a soak away. So, after a bit more re-shearch it seems the best way would be to instal 'french drains' on all four sides of the building running to a soak-away several meters away. Although this involves a lot of digging out site access is good so with a mini digger I don't think it should be too costly. Also the building with only a maximum of .5m below grade so it's massive amount of earth moving.

I take on board your suggest for separate footings and slab.

Thanks again,

Jon
Old 31st January 2013
  #11
Gear Maniac
 
🎧 10 years
Hi - Here's a new diagram=:

http://www.7hproductions.com/wp-cont...M-2-test-2.jpg

This pretty much to scale not sure if I should simply wrap the footing with the primary DPM or run right under the whole build - in effect having two DPM under the slab - might be over kill as may the need for the 'French' drain. Especially given the relatively 'high' slab (compared say to a full basement)

Thanks

jon
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