Roofing

What is the best way to seal flat roofs together?

Anonymous user 23/02/2024 - 3.18 PM

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?