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Roofing
What is the best way to seal flat roofs together?
I would like to do a warm conversion of the flat roof over my domestic two-storey rear extension.
There is another two-storey rear extension adjoining whose flat roof is about 100mm higher than mine. Where it meets my roof, its covering is laid down this step and sealed onto my membrane. Under this, there is a vertical gap between the buildings of about 50mm. Both properties have cavity walls along this length. Both roofs are cold type and covered with weather beaten old felt.
If I keep my existing cold roof, this 100mm step will be a convenient upstand for sealing a replacement membrane against the said adjoining extension.
However, I would prefer to convert my roof to a warm roof type because there is no practical way of ventilation. My difficulty arises from the new roof height after conversion, which will eliminate the said upstand. I don’t know how to make a weather tight seal between, probably, two flat roofs at the same level.
The weather proof membrane will be laid professionally, but I want to install the new roof components myself.
Is there a way to seal two flat roofs at the same level, one of which is felt? If so, what membrane material should I ask for?
If not, would I need to raise my cavity wall to provide a parapet along the join, to give each of the two roofs its own upstand to seal against?
2 Answers from MyBuilder Roofers
Best Answer
Hi,
If your looking to install warm deck I’d recommend using a rubber membrane. Firstly the joists will need to be boarded using 18mm OSB3 which will need to be cut and joined at each joist with a 3mm gap. Build the upstand all the way around the edge using 4x2 timber with 120mm screws to anchor in position. You can then cut and fit 70mm foil back insulation board over the top of the OSB and finally a 50mm tissue/paperback insulation board on top of that overlapping in the opposite direction. When the warm deck is installed it will come level to the 4x2 timbers installed earlier so where an upstand is needed simply nail a 2 inch lath on top of that where an upstand is needed. The warm deck can now be anchored down with some thermal plugs (similar to plaster board fixings) and 90mm screws. Your now ready for the (EPDM) rubber membrane to be installed preferably a fleece back membrane like Duo Ply for durability. Any seam can be joined and flashed with 6 inch reinforced flashing tape, as an extra precaution we always underlay with a 3 inch EPDM tape first. Best of luck, if you need any more advice we’re here to help.
Simon @HS Roofing
Answered 1st Sep 2018
if your creating a warm roof it will need 100mm cross bonded with 25mm kingspan this will raise your side by 125mm so it will be 25 mm higher than your neighbors then depending on what materials your using will depend on how you seal both roofs, you can seal E.P.D.M. to felt , felt to felt, or grp to felt,
盖屋顶的人如果主管应该找到一个漂亮的圣raightforward job to seal both roofs.
good luck Alex.
Answered 30th Aug 2018
