# Best way to replace/insulate a flat tar-gravel roof



## federer (Aug 20, 2010)

i have the same exact issue right now. i am getting my flat roof replaced. in terms of material there are all kinds out there now. do some reading and you can pick one that best fits your house. i am doing shingles for one half and a firestone app mod bit for the other half. 

as for your original question, i am doing the same as well. since i am tearing teh roof off, i am adding more insulation too. right now i like to use spray foam because it acts as a air seal as well.


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## Grumpy (Dec 12, 2003)

Is the roof designed cold deck or warm deck? Cold deck is insulated ceiling. Warm deck is insulated roof top. Either is ok, but to mix the two is not recommended. I've never seen a flat roof on a bungalow so I am having a hard time visioning the project. If there are any vents in the soffits or walls, the roof top should not be insulated (warm deck), and if there are vents in the soffit, walls or roof then the ceiling should be insulated. 

I think building up the roof to a shingle roof is a waste of time and money. A properly installed flat roof can last 20-30 years. However a 2 ply system )base+cap) will not last more than 10-12 years likely. A 3 ply (base plus mid plus cap) will last 15-20 years. It's all what the budget allows, but if you go cheap now you'll have to replace it sooner rather than later. 

With 3" of blown in fiberglass you'll get an R value of something like 9. With 1 1/2" of rigidi polyisocyanurate insulation, you'll achieve the same R value. The choice is yours but I highly recommend against doing both. Also remember that insulation is based upon trapping air, so you need not worry about 3" of empty space. It's not the insulation that insulates, but it's the air the insulation traps that creats the insulating value.

If you want to do the roof yourself, I usually recommend against it. But I definetly recommend against any novice using a torch. Try a self adhering modified bitumen membrane or maybe EPDM.


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## Hexar (Mar 5, 2010)

federer, are you doing it yourself or you're hiring a professional roofer?


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## Hexar (Mar 5, 2010)

Thanks a lot Grumpy!
Let me clear up some background. I live in Calgary, Alberta, Canada, so it is quite cold here in the winter. 

So I think the current roof is designed as "cold deck", I mean the current construction is, from top to bottom:
tar and gravel
plywood
air space
fiber glass insulation
a layer of tar paper (this may be part of the fiber glass insulation butt?)
a layer of vapor barrier
my ceiling drywall
my living space.

The house is indeed one story bungalow, and house plan is about rectangle, except the attached garage, so it is like a L shape. Along the east and west side of the house, I have soffits vents. I think, cold air can go through one side's soffits, go up, onto top of the fiber glass insulation, exit from the other side of the house, through soffits vents on the other side of the house, the exit to outside.

Here is a "drawing" I did in Microsoft paint... sorry for the childish painting...

That is why I kind of think, if they put rigid foam insulation on top of my roof plywood, then this air-space is still there, connecting to the outside, and the R-Value of the rigid foam insulation is wasted, since as you said, the air in that space is not trapped... unless we close those vents. But with vent closed, would I get dry rots?

Regarding 2-ply, he indeed only will warrentee 10 years.

I definitely dare not, yet, to bring myself up the roof with a torch, :no:, that is why I am thinking to "build up the roof to a shingle roof", I am good at nailing lamber stuff... 

I will try to take some pictures when I get home today.

Thanks again!


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## federer (Aug 20, 2010)

Hexar said:


> federer, are you doing it yourself or you're hiring a professional roofer?


i am hiring a roofer. but its still a pain in the butt. you dont know who to go with!


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## Hexar (Mar 5, 2010)

federer said:


> i am hiring a roofer. but its still a pain in the butt. you dont know who to go with!


I hear you! If I knew the low R-Value of a flat roof before I bought the house, I would not have bought it...


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## Hexar (Mar 5, 2010)

Grumpy, sorry it was super busy this afternoon, daught's violin class and their evening's activities took too much time, I will try to get the pictures tomorrow afternoon.


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## federer (Aug 20, 2010)

Hexar said:


> I hear you! If I knew the low R-Value of a flat roof before I bought the house, I would not have bought it...


well with spray foam you dont need to worry because you seal it off completely.


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## Grumpy (Dec 12, 2003)

If you have fiberglass in your ceiling and soffit vents you have a cold deck design. 

Here is a small problem, adding too much insulation may block air flow. For sure you want to leave no less than 3/4" beneath the plywood for movement of air. Also make sure vents are installed as a part of this cold deck design.

So how much insulation would block the air flow? I have no idea without seeing how the air flows. Normally holes would be cut in the rafters/joists to allow for horizontal air flow between rafters. If no holes are cut, then each rafter bay becomes its' own zone. Ideally these holes are placed at the center of the rafter so as to not weaken the strength of the rafter. Therefore let's say you have a 10" rafter with a 2" hole in the center, you can only insulate the bottom 4" or will block the holes. If you put the holes off center you will weaken the rafter. If you cut notches at the top of the rafter you will weaken the rafter. 

As far as spray foam, I am on the fence taking a taking wait and see approach. With flat roofing I'd be a little less scared vs a sloped roof. However with sloped roofign I did a few jobs that have spray foam beneath and am waiting a few more years to see what happens to the shingles. Modified bitumen is not as sensative to the ventilation issues that shingles are, which is why it's less scary for me. Infact it's not likely there would be a problem since warm decks are not ventilated but I have not seen any data to prove or disprove this theory. Scary might not be the right word, but you get my point.


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## federer (Aug 20, 2010)

Grumpy said:


> If you have fiberglass in your ceiling and soffit vents you have a cold deck design.
> 
> Here is a small problem, adding too much insulation may block air flow. For sure you want to leave no less than 3/4" beneath the plywood for movement of air. Also make sure vents are installed as a part of this cold deck design.
> 
> ...


well so far it looks like there is about maybe 3ft of height in the attic. so i have enough room to blow in fibgerlass up to R38 right? i prefer spray foam but its way more expensive and tehre is some existing insulation...


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## Hexar (Mar 5, 2010)

federer said:


> well with spray foam you dont need to worry because you seal it off completely.


Federer,
So with blow in/spray foam, 

1. do I/we need to take out the existing fiber glass insulation?

2. we don't need to leave a ventilation space above the sprayed foam insulation and under the roof ply wood?


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## Hexar (Mar 5, 2010)

Grumpy said:


> If you have fiberglass in your ceiling and soffit vents you have a cold deck design.
> 
> Here is a small problem, adding too much insulation may block air flow. For sure you want to leave no less than 3/4" beneath the plywood for movement of air. Also make sure vents are installed as a part of this cold deck design.
> 
> ...


Thanks Grumpy,
I have 8" tall rafters.  I don't know if there are holes drilled in the rafters. 

I know there are venting holes outside, near the softies, please take a look at the pictures attached, the side with four-red-arrows drawn is facing west, and there are holes under the softies face boards.

Other than the spray foam question that federer and I asked eariler, what other options do I have? 

In addition to insulation, what roof options are "relatively" better than others, in terms of DIY? 

Thanks again! You have been really helpful.


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## federer (Aug 20, 2010)

Hexar said:


> Federer,
> So with blow in/spray foam,
> 
> 1. do I/we need to take out the existing fiber glass insulation?
> ...


1.yes thats strongly recommended
2. if you spray to under the roof decking then no ventilation needed. you essentially seal the roof off from the attic, and the attic becomes part of the building envelope


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## Hexar (Mar 5, 2010)

federer said:


> 2. if you spray to under the roof decking then no ventilation needed. you essentially seal the roof off from the attic, and the attic becomes part of the building envelope


Thanks Federer, but I am bit confused, for my roof, I don't have an attic, my roof is pretty much flat, with 8" rafters.


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## federer (Aug 20, 2010)

Hexar said:


> Thanks Federer, but I am bit confused, for my roof, I don't have an attic, my roof is pretty much flat, with 8" rafters.


thats how mine is too. basically with us there is not enough attic space for regular insulation, because we can only fit like 5in of fiberglass or whatever. so for us spray foam works better because what they do is spray it right to the decking so it seals the roof off completely from the fake attic, so to speak. just be sure you do open cell in case roof leaks you can tell. do you have drywall for your ceilings?


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## Hexar (Mar 5, 2010)

federer said:


> thats how mine is too. basically with us there is not enough attic space for regular insulation, because we can only fit like 5in of fiberglass or whatever. so for us spray foam works better because what they do is spray it right to the decking so it seals the roof off completely from the fake attic, so to speak. just be sure you do open cell in case roof leaks you can tell. do you have drywall for your ceilings?


I have drywall for my ceilings, that is the one reason I don't want to insulate from within the house -- if I do that, I will have to put another sheets of ceiling drywalls.

What do you mean "do open cell" in your reply "just be sure you do open cell in case roof leaks you can tell"?

Thanks again!

Oh, if you don't mind, could you tell me how much, roughly of course, that spray foam your house would cost? I assume you will have the tar-n-garvel removed, roof plywood removed, spray foam, install new plywood and new roof materials?


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## federer (Aug 20, 2010)

yea with your case spray foam is probably best. because they can just spray right down to the attic floor on top of the ceiling and it will expand against the drywall. open cell refers to the spray foam used. there are 2 types. open means its permeable, so if roof leaks you can tell. for my house the first estimate is at least 5k


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## Hexar (Mar 5, 2010)

federer said:


> yea with your case spray foam is probably best. because they can just spray right down to the attic floor on top of the ceiling and it will expand against the drywall. open cell refers to the spray foam used. there are 2 types. open means its permeable, so if roof leaks you can tell. for my house the first estimate is at least 5k


Thanks Federer for the valuable information!


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## federer (Aug 20, 2010)

i got a better estimate-3k only.


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