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Boy Genius 03-01-2013 02:05 PM

How should I upgrade and insulate my flat gravel and tar roof
I have a house built in 1951 with open beam (2x8 and 2x14) ceilings. They are sheathed in 1x4 and 1x10 tongue and groove sheathing respectively. The sheathing is covered in felt, hot mop, and pea gravel. The roof is leaking and needs to be replaced. I want to tear off the old roof, build it up with blue wood 2x2 or 2x3 stringers on top of the sheathing in line with the joists, insulate the space between the new stringers, top it with blue plywood, and cover the plywood with a commercial membrane. Should I have some kind of vapor barrier and what kind of insulation would be best to use in this situation. I live in Southern California and want to keep the heat out of the house in summer and in the house when it gets cold at night.

OldNBroken 03-01-2013 02:42 PM

Only have time for a quick reply right now so not real detailed.

No need for all the framing and sheeting. Just look into installing polyiso rigid insulation under the new roofing (regardless of type) after the old roof is demo'd.

It is the ideal time to add insulation in a flat roof and the ROI is there in time so don't necessarily look at the added expense as lost money. Polyiso r-value is around 6 per inch so go from there. Usually commercial specs are 4-6" and done in staggered layers to improve the thermal value. Vapor barrier, if required, is simple 6 mil visquene under insulation.

The only place you need added framing is larger penetrations and around the perimeter to match the profile to the increased height of the insulation.

Lastly a layer of rec board or densdek for fire rating and the new roofing is installed over that.

If you already plan on a new roof, now is the best time to get all of this done correctly.

Sorry, gotta go

jagans 03-01-2013 04:04 PM

If you have gotten 61 years out of your existing Built up Roof, I would say that is one hell of an advertisement for another BUR. The rule of thumb over wood is to mechanically fasten a sheet of red rosin paper, and a fiberglass base sheet to the deck then mop from there up.

Are you sure that your deck is only 3/4 of an inch thick? what is the spacing on your exposed joists?

Boy Genius 03-01-2013 08:30 PM

Thank but...
I need to insulate the roof... I'll look into the Polyiso. I'd like to use more
modern materials that will resist moisture since I'm in a marine environment. Thanks for the quick reply.

Boy Genius 03-01-2013 08:41 PM

Joist spacing
The 2x8 joists are on 24" spacing with 1x4" t+g sheathing. The 2x14 joists are on 48" spacing with 1 1/2x 10" t+g sheathing. Custom home, custom headaches...

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