DPM?

[junglejim]

Slowly progressing with my timber frame build and getting ready to lay dpm, add pir, ufh and screed.

 

 I’ve got a couple of questions:

1. My architect drawing shows flow dpm sealed to dpm under sole plate and lapped up external walls. (Red and blue lines in drawing) The internal walls were all installed at the same time as the external walls.  So where these intersect I can’t fold up the dpm. The easiest solution would seem to be not to fold up the dpm strips that are under the sole plate and instead fold them down but tape / seal the new floor dpm to all junctions with overlap so that can fold up. It would mean that the there is only one layer of dom above the upstand rather than the 2 in the architect spec. In the corners presumably I’d use ‘hospital bed style’ to neatly tuck in without any break in the dpm.

 

2. there are sections of the slab that to my untrained eye seem quite uneven. I can chisel off small lumps but wondering if I should be taking  additional measures to perfect the dpm? I haven’t got any photos yet and i could well be overthinking it! I don’t think self levelling screed will work because it would interfere with the dom under internal walls. I guess I’m wondering when I should worry about this and what solutions there are? I’m thinking the addition of a rubber layer (say 1-2mm) might be useful?

 

3. also with insulating the slab. The architect drawing shows the perimeter strip sitting on top of the sheet insulation. I’ve also seen lots of people do the strip first at full height then add the sheets. Wondering what the logic is for each method?

 

 Thanks as always does the help given.

[JohnMo]

5 minutes ago, junglejim said:

3. also with insulating the slab. The architect drawing shows the perimeter strip sitting on top of the sheet insulation. I’ve also seen lots of people do the strip first at full height then add the sheets. Wondering what the logic is for each method?

Each method works, but I did perimeter insulation at full depth of insulation and screed, my thoughts were less chance of a thermal bridge due to gaps. I used 70mm PIR which was the maximum use for my wall build-up instead of the normal blue stripes.

 

8 minutes ago, junglejim said:

there are sections of the slab that to my untrained eye seem quite uneven. I can chisel off small lumps but wondering if I should be taking  additional measures to perfect the dpm?

You could add a thin layer of sand or use thin eps insulation as a sacrificial layer below the DPM.

 

 

[junglejim]

Here’s some photos of the slab…any thoughts?

 

 I’m thinking of laying a geotextile to protect dpm. What do you think? 

[Big Jimbo]

That looks rough as IMO. A thin layer of EPS before you put the membrane down would do the job.

[junglejim]

On 01/11/2025 at 17:43, junglejim said:

Slowly progressing with my timber frame build and getting ready to lay dpm, add pir, ufh and screed.

 

 I’ve got a couple of questions:

1. My architect drawing shows flow dpm sealed to dpm under sole plate and lapped up external walls. (Red and blue lines in drawing) The internal walls were all installed at the same time as the external walls.  So where these intersect I can’t fold up the dpm. The easiest solution would seem to be not to fold up the dpm strips that are under the sole plate and instead fold them down but tape / seal the new floor dpm to all junctions with overlap so that can fold up. It would mean that the there is only one layer of dom above the upstand rather than the 2 in the architect spec. In the corners presumably I’d use ‘hospital bed style’ to neatly tuck in without any break in the dpm.

 

2. there are sections of the slab that to my untrained eye seem quite uneven. I can chisel off small lumps but wondering if I should be taking  additional measures to perfect the dpm? I haven’t got any photos yet and i could well be overthinking it! I don’t think self levelling screed will work because it would interfere with the dom under internal walls. I guess I’m wondering when I should worry about this and what solutions there are? I’m thinking the addition of a rubber layer (say 1-2mm) might be useful?

 

3. also with insulating the slab. The architect drawing shows the perimeter strip sitting on top of the sheet insulation. I’ve also seen lots of people do the strip first at full height then add the sheets. Wondering what the logic is for each method?

 

 Thanks as always does the help given.

Just resurrecting this in the hope someone can advise on my first point? Ta

[Mike]

I'm not clear on exactly what you mean - or rather that at a junction of an internal wall and external wall I can't envisage a problem, which probably means I've not understood you question. However any DPM / DPC is likely to be too thick to fold into hospital corners. There are pre-formed DPC corners on the market, for a price, that may help. But the fundamental rules are that all DPM and DPC junctions must overlap by at least 100mm and be to the satisfaction of your BCO.

[junglejim]

On 03/12/2025 at 23:41, Mike said:

I'm not clear on exactly what you mean - or rather that at a junction of an internal wall and external wall I can't envisage a problem, which probably means I've not understood you question. However any DPM / DPC is likely to be too thick to fold into hospital corners. There are pre-formed DPC corners on the market, for a price, that may help. But the fundamental rules are that all DPM and DPC junctions must overlap by at least 100mm and be to the satisfaction of your BCO.

Thanks for your reply Mike. Sorry I’m not explaining clearly. My issue (I think) is that I can’t access the small sections where the partition wall butts against the perimeter upstand/soleplate. The issue is 2 fold; 1. I can’t bend the dpm that is under the soleplate upwards as it’s folded down between the partition. This may not be an issue but 2. I can’t get in between the partition junction to seal the laps. 
there is no exposed concrete as the dpm strips overlap but they aren’t taped or sealed between partition and upstand. 

in my mind it would have been better to lay the dpm across the whole slab before the partition walls but this was done by the TF installer as standard.

[Mike]

Thanks for the added info; I've lightened your photo which helps too see what's going on too.

 

 

So part of the problem is pushing the clear polythene, or maybe the black plastic marked 'A' (or maybe you've tried both) both through the gap where I've added the arrow?

 

Bearing in mind that the minimum overlap is 100mm, you don't need to get a large piece of DPM through that gap - an offcut of 300 x 200 - ideally a little more - would be enough, if it overlaps the DPCs by 100mm. If the partition is too tight to the external wall, then I'd carefully (without damaging the existing DPCs) take a few mm off the timber using a multitool with a long blade. You could double up the vertical timber if there is any question over the somewhat reduced strength.

 

Having said that, if the timbers on that that internal partition are 100mm wide, it looks like the DPC beneath doesn't project enough to provide the minimum 100mm overlap? If that's the case then you will need to get a new wider DPC underneath its full length.

 

On the external wall, I'd have preferred the DPC to have been turned up but, if it's turned down and still overlaps the DPC & other DPMs by 100mm then I'd find that acceptable, but you'd need to verify that your BCO & any 3rd party certifying insurer that may be involved takes the same view.

[Nickfromwales]

Where’s top of screed?