Hi all, I'm in the process of building a single story side extension with parapet walls on 3 sides. The parapet wall will be built at say 1 block above the (epdm) flat roof. My 2 questions are if I continue the epdm sheeting vertical up the 2 or 3 courses of the parapet wall and incorperate it directly under the coping stone (40mm in) would this be sufficient enough for me not to need a parapet cavity tray or is it regulations to insert one? I am using brick on the outer wall with a 100mm full fill insulation cavity then solid breeze blocks on the inner skin, I already have a cavity tray at ground dpc. As far as I know i have to close the cavity using slate then lay out a sheet of dpc before bedding the coping stones on top.
My second question is if I had to use a parapet cavity tray, when it comes to putting a hole in both skins for a wall drain pipe into a hopper would the cavity tray have to sit below the wall drain pipe or would it go through the cavity tray and be sealed?
I hope you can understand my questions. Thanks all. Stay safe
Are you a tradesperson and able to answer this question?
Hello. From you have said. You are doing everything correct.
I would not put in the parapet cavity tray.
You have one at the bottom and above the windows/doors.
You are taking the EPDM right up to the underside of the coping stones.
Then you are doing everything correct.
Wesley
The purpose of a cavity tray DPC in a parapet is to direct any water that gets past the coping out of the wall, to prevent moisture damage to the inner leaf, and prevent the full fill insulation becoming saturated. Parapet copings are often concrete and the joints between them are prone to allowing water ingress over time.
The EDPM could create a slip plane under the copers, maybe better to use a hi load dpc, sand impregnated bitumen in the top mortar course and fold the 30 - 40mm EDPM edge onto the top of the block, sealed and secured in place with a trim first
Personally I would show on my drawings that the parapet be built in brickwork. I would use a wide width of high-load d.p.c. to form a cavity tray at one course below the coping stone on inside leaf and two course below on external leaf with weep-holes externally. All as House Architecture noted. I agree with them that precast concrete coping stones tend to have joint problems after a number of years as do genuine stone copings. I tend to suggest the top course of brickwork has brick reinforcing mesh laid within its bed joint to the coping which ties those top courses together adding more weight to the copings for resisting strong winds - that of course depends where your building is and whether it is susceptible to high winds
Personally I wouldn't take the EPDM up to the coping. Its not easy to bed down onto this with a coping. Also copings are usually in sections, ergo you get gaps between them where moisture can penetrate over time. For the cost of a roll of wide DPM I would install the cavity tray 1 course below the coping and lap this with the EPDM.
Andrew
EDG:Architecture Ltd
