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DPM under Wallplate? OSB vs Plywood?


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Can anyone tell me the purpose of the DPM under the wallplate?

 

EavesDetail.jpg.075ca8506e93b6686a6565f94cec364d.jpg

 

Does this present an opportunity to somehow add a membrane layer over the Posis, under the deck for air-tightness? Or that not the best way? I had otherwise planned to implement an airtight layer under the Posis with a deeper service void for downlights etc.

 

Finally, 22mm OSB or Plywood for roof deck? I'm erring towards OSB.

 

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Posted (edited)

This detail is ok. I don't love it however. 

 

120mm seems a bit slack when it comes to insulate for the roof. I would improve on this. 

 

How are you planning on keeping the zinc roof on? It appears to be just sitting there. 

 

22mm ply would be better for screw pull out strength if you're planning on fixing into it.

 

Why the service void under the pozi joists? Why not run everything through the webs? 

 

 That eave detail is very very tight to the wall. I know it's an aesthetic decision but if the chutes get blocked you are asking for water infiltration to the ICF or behind the ICF render and the timbers that hold the chutes on. It isn't robust enough for a moist climate in my opinion. In any case you'll get lots of staining on the render as is. 

 

Your logical place for an airtight layer is the ply, joined to the concrete core of the ICF via the wall plate. 

 

1. Set the wall plate on a thick bed of permanently flexible mastic type sealant. 

 

2. Run the ply to stop atop the wall plate. 

 

3. Tape the wall plate to the ply. 

 

 

 

For the eave detail add rafters tails to move the chutes well out from the wall. 

 

 

 

 

 

Edited by Iceverge
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I see this sometimes when going from masonry to timber frame.  A strip of DPC under the soleplate.  I am not sure what your top plate is doing.  It does not seem to support anything.

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19 hours ago, Mr Punter said:

I see this sometimes when going from masonry to timber frame.  A strip of DPC under the soleplate.  I am not sure what your top plate is doing.  It does not seem to support anything.

 

I agree about the detail being poor, it's like many aspects of our design. We only paid 16 grand for our Architectural design, we can't expect proper details can we? *jokes*

 

I'm installing the wall-plates with the 5° tilt on them to match the roof pitch. I cannot fathom why they were drawn like this, but my Architect has left the project after being paid and I have to try to figure out the details.

 

I'm also not a fan of the eaves detail arrangement and am trying to figure that out a bit better since we are upgrading the roof from 120mm insulation to 200mm.

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Posted (edited)
20 hours ago, Iceverge said:

This detail is ok. I don't love it however. 

 

120mm seems a bit slack when it comes to insulate for the roof. I would improve on this. 

 

How are you planning on keeping the zinc roof on? It appears to be just sitting there. 

 

22mm ply would be better for screw pull out strength if you're planning on fixing into it.

 

Why the service void under the pozi joists? Why not run everything through the webs? 

 

 That eave detail is very very tight to the wall. I know it's an aesthetic decision but if the chutes get blocked you are asking for water infiltration to the ICF or behind the ICF render and the timbers that hold the chutes on. It isn't robust enough for a moist climate in my opinion. In any case you'll get lots of staining on the render as is. 

 

Your logical place for an airtight layer is the ply, joined to the concrete core of the ICF via the wall plate. 

 

1. Set the wall plate on a thick bed of permanently flexible mastic type sealant. 

 

2. Run the ply to stop atop the wall plate. 

 

3. Tape the wall plate to the ply. 

 

 

 

For the eave detail add rafters tails to move the chutes well out from the wall. 

 

 

 

 

 

 

As above, our Architect is no longer involved with the project, so I cannot get it redrawn. We have already made the decision to beef the insulation up to 200mm.

 

I am trying to ascertain a better detail for the Zinc roof, in conjunction with the actual contractor, but am open to ideas on that. This, of course needs to take into account the increased insulation thickness.

 

22mm Plywood or 22mm OSB for the roof deck? I'm erring towards OSB.

 

The service void was drawn because the Architect had no inclination to learn about the ICF system. I am having to be fluid on these details and re-think everything.

 

What type of sealant to bed the wall-plate onto please? Any suggestions?

 

I'm concerned about the mix of materials at the face of the wall and the risk of cracking. I have built the building as drawn so far, so the only workaround I can come to is to add a further layer of EPS to the whole portion of the building that will be rendered. This improves another poorly designed detail as was discussed here

 

The above drawing is only of only 2 short walls that will have gutters (probably only about 10% of the total wall length, the rest has no gutter at all. The Zinc Roofer has proposed something a little different to that drawn for those walls...

 

Quote

If you are describing the eaves flashing , then this is fine as drawn , but I would extend this slightly so the render does not got behind the zinc gutter , then fit a zinc fascia and drip edge detail .

 

All help here is greatly appreciated.

Edited by Mulberry View
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It is timber sitting on concrete, so it makes sense to avoid any chemical reaction for very small cost.

 

As above, it would be a shame for that roof to be sucked away by a gale. Uplift loads can be huge. It needs expert specification for the fixings. An SE for the loads and experienced roofer for the practicality.

I don't know for sure but most domestic  roofing has battens and an air gap. This is sitting on the pir. Apart from any condensation issues, the pir might crush or the fixings rip the metal under foot traffic, unless thought out.

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On 25/05/2024 at 17:06, Mulberry View said:

Finally, 22mm OSB or Plywood for roof deck? I'm erring towards OSB.

 

Don't go OSB with your standing seam roof deck. Use ply as a mimimum. 18mm thick is fine.

 

VM Zinc have an approved warm roof buildup. https://www.vmzinc.com/en-gb/structural-roof Detailing for the vapour barrier is critical which is why the preferred method for standing seam roofs is ventilated.

 

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6 hours ago, Mulberry View said:

we can't expect proper details can we? *jokes*

You are right though. This isn't good enough, leaving you to sort the clever stuff out.

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3 hours ago, SimonD said:

 

Don't go OSB with your standing seam roof deck. Use ply as a mimimum. 18mm thick is fine.

 

VM Zinc have an approved warm roof buildup. https://www.vmzinc.com/en-gb/structural-roof Detailing for the vapour barrier is critical which is why the preferred method for standing seam roofs is ventilated.

 

 

Thanks @SimonD. Our VM Zinc installer will, I'm sure, follow the prescribed build-up to protect our warranty, I just want to make sure I'm right up to there. It is my responsibility to install the roof structure (Posi Joists) and the deck (Plywood). So, you're saying 18mm if fine? My Architect 'specified' Class 3 Plywood, is this what I need to order? Does it need any other preparation or treatment by me before I hand over to the Zinc installer?


Yes, I'm a chronic overthinker, but hopefully for the right reasons.

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4 hours ago, saveasteading said:

It is timber sitting on concrete, so it makes sense to avoid any chemical reaction for very small cost.

 

As above, it would be a shame for that roof to be sucked away by a gale. Uplift loads can be huge. It needs expert specification for the fixings. An SE for the loads and experienced roofer for the practicality.

I don't know for sure but most domestic  roofing has battens and an air gap. This is sitting on the pir. Apart from any condensation issues, the pir might crush or the fixings rip the metal under foot traffic, unless thought out.

 

I have had a spec for wall-plate fixing from me SE, that's all cool. I was immediately concerned about the amount of steps to install the wall-plates as I was planning to do these during the concrete pour. I have abandoned this thought process and will now wet-set the fixing studs (using a jig that I'll make to ensure accurate placement/straightness).

 

This way, I can do the wall-plates with a bit less urgency and hopefully once the concrete has hardened. By which time hopefully I'll have taken a balanced view of all the great input I've had here as to how this will be detailed.

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3 hours ago, saveasteading said:

You are right though. This isn't good enough, leaving you to sort the clever stuff out.

 

Worse yet, having to second guess weirdly drawn details, like this....BeamWeirdDetail.jpg.096991aa9b9fc9f8e6aca642e8e8798c.jpg

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On 27/05/2024 at 19:57, Mulberry View said:

 

Thanks @SimonD. Our VM Zinc installer will, I'm sure, follow the prescribed build-up to protect our warranty, I just want to make sure I'm right up to there. It is my responsibility to install the roof structure (Posi Joists) and the deck (Plywood). So, you're saying 18mm if fine? My Architect 'specified' Class 3 Plywood, is this what I need to order? Does it need any other preparation or treatment by me before I hand over to the Zinc installer?


Yes, I'm a chronic overthinker, but hopefully for the right reasons.

 

Yes 18mm plywood is fine. And Class 3 is what you need - that just stands for exterior grade nowadays. No other preparation needed but I would try to avoid the really cheap Far Eastern hardwood plywood as it can have losts of voids.

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  • 2 weeks later...
On 26/05/2024 at 14:41, Iceverge said:

 

1. Set the wall plate on a thick bed of permanently flexible mastic type sealant. 

 

 

@Iceverge Would you be kind enough to show me the sort of product you think would be suited to this purpose? You thinking an expanding foam of some sort? Or Roof Sealant?

 

Would I be able to tighten the wall-plate bolts down sufficiently without compressing the sealant beneath them?

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1 hour ago, Mulberry View said:

You thinking an expanding foam of some sort?

argh. no. I would suggest this one. because it stays permanently plastic under the skin. ie when the building moves or distorts, this will move with it.

2 longitudinal beads then place to material on it. then fix.

I think there are others similar, Most mastics set hard and lose this ability to stick while deflecting.

 

 

115-mastic-28122_yi9eqp?_a=ATAABAA0

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