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Old 03-30-2011, 08:58 AM   #1
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Cold Deck flat roof and insulation question...


I'm finally back on the project of improving the last section of our house. What we have is a 1960s contemporary that was built with a vented cold deck roof with BUR tar and gravel--rough size is about 800sf. Original roof was replaced sometime in the 1980s so roof is 30+ years old. Currently, it is not leaking, but there are a couple of spots where the copper flashing sealing has started to fail.

Given the budget, I'm tempted to clean and re-coat the existing roof, except the only insulation is about a 2" foil backed layer of ?rock-wool. So I think I need to strip and replace the deck or remove the drywall underneath to get into the rafter cavities to insulate.

-Will I likely find the base coat mopped to the plywood decking and therefore should I expect to have to replace the plywood? (If so, I can do the insulation from the top at the same time & save the drywall...)

-Since we are on a strict budget, can the copper flashing be re-used?

-Hardest question--what type of insulation/application? This thread discussed some of the possibilities, but was a bit inconclusive.

If I use standard fiberglass batts, I am limited to R25 (8"-8 1/2") or R30 (high density) to maintain a 1" airspace above the batts for the deck ventilation. (2x10 16" OC, continuous vented soffit)

I thought about foam, but can the installers fill the joist cavities and leave a gap, or would you need to spray and then install furr strips?


Last edited by paredown; 03-30-2011 at 09:01 AM.
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Old 03-30-2011, 11:57 AM   #2
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Cold Deck flat roof and insulation question...


Take off the old roof. recoating a 30 year old roof isnt going to last long. Will you have to replace plywood, maybe some times a base sheet was nailed down, sometimes moped, even if its moped you could just scrape it down smooth. You could also remove the plywood and insulate from the top. The 2" insulation thats on there now has a R value of about 12 if its wet could be alot less to none. If you insulated the joist with R30 then you could go with out insulation on the roof, or you could add extra. Figure about R6 per inch of ISO. Then you could install a single ply roof like EPDM, TPO or PVC. All are available in white but TPO and PVC are hte most common white single plys. They do require extra equipment to install and is best left for the pro's. Post some pics it would help a ton. Oh the flashings, probably best to turn them in for scrap, 9/10 if htey are remove the copper gets damaged anyway.

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Old 03-30-2011, 12:02 PM   #3
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Cold Deck flat roof and insulation question...


I would recommend insulating from the top especially since your roof is 30 years old and may need to be stripped. Will the bur be mopped to the wood? Your guess is as good as anyones. Often times it is, sometimes the vapor barrier is mechanically fastened and the roof mopped to that.

Even if it is mopped, you may not need repalce all the wood. What I mean is often times you can scrape the wood good enough that when you install your cover board there won't be any humps or bumps. The other thing is depending on the configuration of the roof, I don't know how much space is between the roof and ceiling, you may get good enough access to insulate without replcing all the wood. Then again if you do have to repalce all the wood, 3/4" CDX plywood doesn't cost that much and you'll need 3.125 4x8 sheets per 100 square feet.


I wouldn't reuse any flashings, but in theory yes copper can be re-used if it's not all mangled and beat up. In the grand scheme of things the flashing is so minimal of a cost in a roof replcement project, we pretty much always replace the flashings withotu question.

leaving 1" air space may not be good enough unless there will be a vent of some kind of every rafter bay. If there is no vent in every rafter bay you will some how need to create horizontal air flow between the rafters. This can be done by drilling a series of 1" holes in the center of the rafters so air can move from one rafter pocket to the next and make its' way to the vents. If you cover these holes with insulation there will be little to no air flow.

I won't comment on spray foam since that is not my specialty, but I see no reason why a professional insulation contractor can't spray the ceiling from the roof side while you are replacing the roof. You say you are on a strict budget though and spray foam is surely to cost much more than fiberglass. Spraying the roof substrate would be silly. If that's what you want to do just get PolyIsocyanurate insulation boards and convert your cold deck to a warm deck... assuming it is at all possible not knowing the other conditions of the construction. If you do insulate the roof, then pelase don't ventilate the roof or you'll be throwing out a ton of money.

Sounds like you are taking on a bid challenge and alot of what-ifs.
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Old 03-30-2011, 12:51 PM   #4
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Cold Deck flat roof and insulation question...


Well, thanks--glad to see the "A" team is on the board today.

Here's some shots of the roof & house section. In one of them the shot is looking up at undereaves & the continuous vent and the downpipe scupper. (Chimney that shows on one shot is actually on the next section of roof.) The best I have as a full-on front shot shows about 2/3 of the front--to the right is the carport also under the same section of roof. I'll try for a better shot of the whole roof tomorrow.

I'm reluctant to change the look of the roof, so I would prefer to maintain the profile as it is, which would seem to suggest that insulating in the cavities would be the best.

Grumpy, when you talk about cross-flow between the rafter bays--does the continuous venting solve this problem?

Also, on a roof like this would you use T&G or just butt regular 3/4" CDX?

I'm thinking of contracting the new roof application, but thought that I could manage the strip-off, insulation and re-sheathing (with the advice and encouragement of the experts on the board).
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Cold Deck flat roof and insulation question...-o_frhouse2.jpg   Cold Deck flat roof and insulation question...-old_roof2.sm.jpg   Cold Deck flat roof and insulation question...-old_roof_sm.jpg   Cold Deck flat roof and insulation question...-orig_downspout.jpg   Cold Deck flat roof and insulation question...-under_eave.jpg  

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Old 03-30-2011, 01:09 PM   #5
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Cold Deck flat roof and insulation question...


Does water pond on this roof? Looks like it would benifit from tapered insulation. I would just butt up the plywood, no need to for t&g. You could save money tearing it off your self and such Im going to say though about 1/2 way through your going to wish you has someone do it. If the base was moped down to the deck most of the time it will be a PITA. To me I think you would want to go back with a TPO or PVC or you could do a mod bit with a white mineral surface modified, Or you could BUR it again with white marble chips but the last two options you wont be saving any $. A TPO will be your best bet for a "cool" roof EPDM will be the cheapest. If you have someone do it, and thats what I would recomend, make sure they give you a detailed proposal and get 3 or 4 people out there to bid on it. If you like you can let us know and we could give you our thoughts. I wouldnt strongly recomend not doing a recover on this type of roof. If there is all ready two layers it may or may not be against coade.
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Old 03-30-2011, 01:21 PM   #6
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Cold Deck flat roof and insulation question...


The Realtor told us that the guy who put this roof on was the last good tar and gravel guy in the area- (and I'm pretty sure he was called after the first roof had failed, judging by some repairs inside.) From the owner I was told that it was originally a "ponding" style roof--which rings true, since the other flat section past the peak section in the middle of the house still "ponds" and the torchdown that is on there is not the right material to have water sitting on top of it.

As far as I can tell, he did put some sloped board on the roof, running the long length, with a slight slope to the front, and a slight slope to the rear. The front side drains through the two tight to wall downspouts and proper scuppers.

Because of the next section on the rear, he added a couple of cut- throughs on the back edge to drop the water onto the next lower section (not the best solution, though, since the water lands on the cants that break the slope around the chimney base on that section, and a goodly amount of water gets dropped in the corner of the two sections of the house--another problem to be addressed.

As far as I can tell it seems to drain, albeit rather slowly because of all the moss on there.

Last edited by paredown; 03-30-2011 at 01:26 PM.
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Old 03-30-2011, 01:44 PM   #7
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Cold Deck flat roof and insulation question...


The ponding style roof will more then likely be a coal tar pitch roof. This would be something your probably dont want to mess with your self. I would install tapered insulation to drain the roof to the areas you want. Some times droping it down to a lower section is the only way to go but doesnt look like it in this case. One could slope it all to the front and sump around the drains to have it drain in the drop drains, or a common slope and install a gutter.
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Old 03-30-2011, 08:16 PM   #8
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Cold Deck flat roof and insulation question...


If this roof is 30 yrs old and ponds like it does i will bet most definitely coal tar pitch a mop down asphalt roof chances are won't last that long.It's most likely nailed base sheet and mopped down plys. should tear off easy. If it is pitch have fun don't rub your eyes or your face oh yeah have plenty of beer ready after the tear off your going to need it for the pain and to be able to sleep
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Old 03-30-2011, 09:28 PM   #9
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Cold Deck flat roof and insulation question...


6-12 pack of beer and a cold dark basement.
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Old 03-31-2011, 09:59 AM   #10
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Cold Deck flat roof and insulation question...


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6-12 pack of beer and a cold dark basement.
Yeah we used to wear pillow cases over are heads with holes cut out for the eyes. Pitch and fiberglass tear off. And pitch and fiberglass back down. I have see Ford motor company roofs that were on 50 yrs and never leak they just wanted to replace them.

Last edited by Don Smith; 03-31-2011 at 04:38 PM.
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Old 03-31-2011, 10:58 AM   #11
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Cold Deck flat roof and insulation question...


Pitch hoods and pitch cream, long sleeve shirts and of course pants. Duct tape the ends if wanted/needed. Sucks when its 100+ deg's out.
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Old 03-31-2011, 12:03 PM   #12
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Cold Deck flat roof and insulation question...


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Pitch hoods and pitch cream, long sleeve shirts and of course pants. Duct tape the ends if wanted/needed. Sucks when its 100+ deg's out.
Seems crazy someone would do that for a job. You will only know unless you been there.
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Old 03-31-2011, 12:44 PM   #13
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Cold Deck flat roof and insulation question...


I may have unintentionally misled y'all--the current roof c. 1984 is not a ponding roof--the original was.

But if I wanted to find out what material I have on there, is there an easy test I can do?

Also, how much waste (half a small dumpster, a whole small dumpster??) do you think will I generate with the strip-off?

Cheers,
Dean
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Old 03-31-2011, 04:31 PM   #14
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....,
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Old 03-31-2011, 09:16 PM   #15
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Cold Deck flat roof and insulation question...


Quote:
Originally Posted by paredown View Post
I may have unintentionally misled y'all--the current roof c. 1984 is not a ponding roof--the original was.

But if I wanted to find out what material I have on there, is there an easy test I can do?

Also, how much waste (half a small dumpster, a whole small dumpster??) do you think will I generate with the strip-off?

Cheers,
Dean

Im sorry thought there was two roof on there. If your wanting to find out what kind of "tar" was used smell it, Coal Tar Pitch will smell really really bad, asphalt and if it was done in the 80's its more then likely asphat will have the same kind of smell as a asphalt road. If its hard to tell burn a little chunk of it that will let you know for sure, or you could break it down in to dust form and rub it in your eyes (DO NOT DO THIS!) it will burn like crazy. I would be atleast 80% sure its asphalt, if there is aroof underneath done in the 60's probably pitch. Although I do not know your area and how popular one style to the other is.

8sq will fill a small 10-15 yard dumpster.

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