EPDM Flat Roof (plus insulate & vent)
The built up tar and gravel roof on my 100 year old home (Southern Ontario, Canada) needs to be replaced. The roof measures approximately 14′ x 25′. Ignoring the framing which creates the slope, the ceiling joists are true 2″ x 6″. The previous owners installed R-22 Batt insulation in the joist cavities, then vapour barrier, and drywall to form the second floor ceiling.
Due to the relatively shallow depth of the framing members, I cannot add anymore insulation on the inside. Is it practical to add insulation on the outside? Once the existing roofing material is removed, down to the “sheathing” (in this case planks), can sheets of rigid insulation be added, with a new plywood deck over top, to create a new surface for the roof membrane?
I also do not think there is any venting in the current roof, even though there is perforated aluminum soffit in various areas around the perimeter. I have exposed an interior 8′ x 8′ area, (i.e., removed the ceiling drywall, vapor barrier, and batts) due to a leak. It seems the batts are haphazardly installed and resting right up tight against the underside of the roof decking planks. Would adding the exterior insulation also be a good chance to correctly vent this roof, and if so how should this be done.
I am looking into an EPDM membrane for this job, as it seems to be highly rated for durability, plus it seems it can be a good DIY install. I do have one issue with EPDM. How do I join it to another flat roof section covered in asphalt based rolled roofing? I know that EPDM and asphalt cannot touch or else the EPDM will fail. Is there a product I can apply to the asphalt roofing which will allow me to lap the EPDM over top?
Thanks very much.
Replies
It is common to use a rigid insulation bd on roofs under the membrane. recommended in fact. The supplier of the EPDM can hook you up with something compatible.
I don't think there is a good way to join to a bituminous roof adjacent. I suppose if I had to I would have a sheet metal shop make up a flanged expansion joint and seal the EPDM to one flange and the Bit roof to the other side, but in most cases I can imagine, it would be better to just do it all in EPDM
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You actually don't need plywood over the foam. Typically around here they use what they call "coverboard" which is basically fiberboard sheathing ("Biltrite" for example) over sloped expanded polystyrene foam. Ventilation would be nice, but tough to do in your case. you could maybe put 2x2's down on the foam and then plywood, but then how would you get air into that assembly? It could be done, but it would take some thought. Besides, it doesn't really make sense to vent ABOVE the foam, being that the foam is going to block moisture from inside the house anyway. I'd think in your case I'd go with a hot roof.
Edited 4/3/2009 8:06 pm ET by Marson