October 7Oct 7 This is an external door installed onto XPS with EPDM as the DPM. The EPDM was left long on the inside. I turned it up at the sides in case it was needed. It's an ICF build with Waterproof concrete and no conventional DPC. It's an RK door with a skimpy low threshold, almost non existent upstand. The floor is Block & Beam, there is a 1200ga DPM under the PIR, this is what you can see in the image laying back onto the PIR. This is with the EPDM lifted. The inner threshold is 50mm below the door threshold, but still XPS. The screed needs to come up almost to the underside of the threshold, with 3-4mm Microcement as the final finish. How should this detail be finalised? Should these membranes just be taped together? Any expansion joint along the threshold? There's another 80mm of PIR to go on yet, this will bring it level with the blue XPS, with everything 50mm below final FFL. I have 2 more Norrsken doors following roughly the same principle. As always, your help is invaluable to me, and I'm procrastinating on it at the moment!
October 8Oct 8 I'd treat the EPDM and the 1200g DPM as one continuous layer and lap them rather than just butting them up and taping the edge. Give it a good lap, 150mm or more, bonded with double sided butyl tape or the EPDM primer and seam tape, with both surfaces clean and dry and the lap rolled down firmly. The aim is a continuous tray from under the threshold right across the floor. At the sides, keep your EPDM upstands and bring the DPM up behind them, then carry both up the reveals to at least finished screed level so the screed is fully contained. Under the threshold I'd bond the EPDM to the sill support or the underside of the frame rather than leaving it loose, and seal the frame to the membrane with a compatible sealant. With an almost non existent upstand that joint is your main defence against wind driven rain, so a fall away from the door or a drainage channel outside is well worth having too. On the expansion joint, yes. I'd run a strip of perimeter edge foam (5 to 10mm) along the threshold and round the reveals before screeding. It stops the screed bonding to the frame, takes up movement and gives the microcement a clean line to finish against, so it's less likely to crack at that junction. Trim it flush afterwards and finish with a flexible sealant once the microcement is down. Also worth checking the screed and microcement makers' moisture limits before finishing, as 50mm of screed over PIR can take a fair while to dry out.
October 8Oct 8 16 hours ago, Mulberry View said: This is with the EPDM lifted. The inner threshold is 50mm below the door threshold, but still XPS. The screed needs to come up almost to the underside of the threshold, with 3-4mm Microcement as the final finish Inward or outward opening?
October 8Oct 8 Author 4 hours ago, JpManning said: I'd treat the EPDM and the 1200g DPM as one continuous layer and lap them rather than just butting them up and taping the edge. Give it a good lap, 150mm or more, bonded with double sided butyl tape or the EPDM primer and seam tape, with both surfaces clean and dry and the lap rolled down firmly. The aim is a continuous tray from under the threshold right across the floor. At the sides, keep your EPDM upstands and bring the DPM up behind them, then carry both up the reveals to at least finished screed level so the screed is fully contained. Under the threshold I'd bond the EPDM to the sill support or the underside of the frame rather than leaving it loose, and seal the frame to the membrane with a compatible sealant. With an almost non existent upstand that joint is your main defence against wind driven rain, so a fall away from the door or a drainage channel outside is well worth having too. On the expansion joint, yes. I'd run a strip of perimeter edge foam (5 to 10mm) along the threshold and round the reveals before screeding. It stops the screed bonding to the frame, takes up movement and gives the microcement a clean line to finish against, so it's less likely to crack at that junction. Trim it flush afterwards and finish with a flexible sealant once the microcement is down. Also worth checking the screed and microcement makers' moisture limits before finishing, as 50mm of screed over PIR can take a fair while to dry out. So, I'm lapping the EPDM with the lower DPM that runs under the PIR as opposed to the slip membrane that'll go over the PIR?
October 9Oct 9 Yes, lap it with the lower DPM under the PIR. That's your actual damp proof layer and the one that should link up with the DPC, so the EPDM under the threshold needs to be continuous with it. The slip membrane over the PIR is just there to separate the screed from the insulation and stop it running into the joints, it isn't doing any waterproofing. Where you can, lay the lap so the upper sheet sits over the lower one, so any water that does get past the threshold is shed down onto the DPM rather than behind it. Then turn the slip membrane up at the edges behind your perimeter foam as normal. If the DPC detail at the door is a bit unusual, worth a quick word with building control so they're happy with it before it's covered up.
October 10Oct 10 On 09/10/2026 at 10:03, Mulberry View said: Inward. Watch the gap for stones / debris brought in on your shoes. 6-7mm gap is minimum but 10mm better. Just one gotcha to avoid before committing to final floor levels.
October 10Oct 10 Author 9 minutes ago, Nickfromwales said: Watch the gap for stones / debris brought in on your shoes. 6-7mm gap is minimum but 10mm better. Just one gotcha to avoid before committing to final floor levels. The RK door is set up for 9mm clearance. You can see the skimpiness of the threshold arrangement in that drawing. Essentially, my EPDM sits on the datum line where the arrow points and just has slim packers (around 5mm) between it and the underside of the threshold.
October 10Oct 10 37 minutes ago, Mulberry View said: The RK door is set up for 9mm clearance. You can see the skimpiness of the threshold arrangement in that drawing. Essentially, my EPDM sits on the datum line where the arrow points and just has slim packers (around 5mm) between it and the underside of the threshold. So you have to turn that membrane down to get the suggested (obligatory) 9mm?
October 10Oct 10 Author 2 hours ago, Nickfromwales said: So you have to turn that membrane down to get the suggested (obligatory) 9mm? I'm thinking of laying the upstand flap of the internal 1200ga membrane over onto the flat light blue XPS face, then laying the EPDM from the door threshold over the top of it and bonding the 2 together on the horizontal plane, or vice versa. This is a rough diagram of what I propose. The magenta is the EPDM, the green is the 1200ga DPM. I hope it's not too hard to see... I wouldn't be able to retain the turned up portions of the EPDM on the jambs if I want it to head downwards, but are they really doing anything? I could triangle cut them to allow the EPDM to fold down. Does a threshold DPM usually turn up the jambs anyway? Apologies that this sketch is not at all in proportion! Edited October 10Oct 10 by Mulberry View
October 12Oct 12 On 10/10/2026 at 16:59, Mulberry View said: I'm thinking of laying the upstand flap of the internal 1200ga membrane over onto the flat light blue XPS face, then laying the EPDM from the door threshold over the top of it and bonding the 2 together on the horizontal plane, or vice versa. This is a rough diagram of what I propose. The magenta is the EPDM, the green is the 1200ga DPM. I hope it's not too hard to see... I wouldn't be able to retain the turned up portions of the EPDM on the jambs if I want it to head downwards, but are they really doing anything? I could triangle cut them to allow the EPDM to fold down. Does a threshold DPM usually turn up the jambs anyway? Apologies that this sketch is not at all in proportion! Looks good to me, just be mindful to apply weight, over straight timbers / other, to keep things compressed whilst your bonding agent cures fully. Then, in the absence of an upturn, be sure to have a continuous fat bead of CT1 from the far left, to the far right, and to continue that ‘water stop’ at least an inch each side. Your enemy here will be melting snow, as rain is much simpler to defend against.
October 13Oct 13 Author 17 hours ago, Nickfromwales said: Looks good to me, just be mindful to apply weight, over straight timbers / other, to keep things compressed whilst your bonding agent cures fully. Then, in the absence of an upturn, be sure to have a continuous fat bead of CT1 from the far left, to the far right, and to continue that ‘water stop’ at least an inch each side. Your enemy here will be melting snow, as rain is much simpler to defend against. Of my 3 external doors, 2 of them are under significant "canopies" and one is on the North side in a very "unexposed" position. That doesn't lessen the quality I wish to achieve in the detailing, but the 2 sheltered doors will definitely never see much in the way of direct rain or snow. Thanks for your help everyone and onto the next. 🤣
Just now1 min I would agree with JpManning and Nick here. From the leak finding side, this threshold detail is exactly the kind of spot where I have seen water get in on new builds, and it never shows itself right at the door. Water tracks sideways along a broken lap and pops up metres away, which makes it a real pain to trace back once the screed is down. Getting that lap continuous and sealed now is cheap insurance.
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