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Old 09-21-2010, 09:26 AM   #1
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Best way to replace/insulate a flat tar-gravel roof


Hi Guys,
I have a 1965 one-story bunglow house, attached garage, total area is about 1500 sq.ft. The current roof line is very flat, about 1/12 pitch, with tar and gravel roofing. The over hang is about 2 feet.

The walls are 2x4 construction, so I finished adding 1.5" rigid foam insulation to the exterior walls and replaced all windows and doors. So the weak point now is the roof. I think I have 8" tall truss, above the ceiling drywall I have: vapor barrier, then about 4" thick fiber glass butt insulation, air space, roof plywood, then roof.

So the R-Value of my roof is quite limited, maybe 6~9? And the tar and gravel roofing is quite old (we bought the house in 2002, and it was about 10 years old then), although there is no big leaks, I have been patching the roof here and there.

I am thinking to re-roof and add insulation at the same time. I had a roofer came yesterday and he did an estimate, what he can do is a 2-ply system, he will:
strip the tar and gravel down to the plywood
install the base ply
install the 250 top ply,
install all the accessories, flushing, gutters and etc.

This is CAD$15000. He can also install 1", 2", 2.5" or 3" rigid foam insulation over the existing plywood, the price ranges from 18K to 20K depending on thickness of foam I choose.

I asked if he should blow in insulation between my current fiber glass butt insulation and the roof plywood, since it is an open space to the outside, he said it should NOT be done like this. But I fear with this 3-4" air space, the rigid foam insulation he adds on top of the plywood is wasted.

What do you guy think of his method?

After reading a little bit, I can think of 2 options.

A. continue with the flat roof
1. strip the current tar and gravel
2a. strip the ply wood and fill fiber glass insulation to the open space above the existing insulation, or
2b. blow in insulation between the space of current insulation and plywood.
3. nail 4" rigid foam insulation to the rafters.
4. add a new sheet of plywood
5. install new roofing material (I need to read more about this)

But since I am a little "afraid" of open flames on my own roof, I am not sure how to do the flat roofing in step 5, yet.

Option B: adding pitch to the flat roof, to maybe 4/12 or 5/12, so that I can install shingles. I can then add simple/cheap fiber glass insulation over the existing insulation at ease.

What do you guys suggest that I can do in my situation? Option C is to sell the house and buy one with high pitch roofing already built.


Thanks,
Hex

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Old 09-21-2010, 10:59 AM   #2
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Best way to replace/insulate a flat tar-gravel roof


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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Old 09-22-2010, 03:01 PM   #3
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Best way to replace/insulate a flat tar-gravel roof


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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Old 09-22-2010, 03:55 PM   #4
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Best way to replace/insulate a flat tar-gravel roof


federer, are you doing it yourself or you're hiring a professional roofer?
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Old 09-22-2010, 04:30 PM   #5
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Best way to replace/insulate a flat tar-gravel roof


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, , 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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Old 09-22-2010, 08:03 PM   #6
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Best way to replace/insulate a flat tar-gravel roof


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Originally Posted by Hexar View Post
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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Old 09-22-2010, 11:01 PM   #7
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Best way to replace/insulate a flat tar-gravel roof


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Originally Posted by federer View Post
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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Old 09-22-2010, 11:02 PM   #8
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Best way to replace/insulate a flat tar-gravel roof


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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Old 09-23-2010, 01:19 AM   #9
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Originally Posted by Hexar View Post
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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Old 09-23-2010, 12:12 PM   #10
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Best way to replace/insulate a flat tar-gravel roof


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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Old 09-23-2010, 09:22 PM   #11
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Best way to replace/insulate a flat tar-gravel roof


Quote:
Originally Posted by Grumpy View Post
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.
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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Old 09-23-2010, 10:02 PM   #12
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Best way to replace/insulate a flat tar-gravel roof


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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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Old 09-23-2010, 10:10 PM   #13
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Best way to replace/insulate a flat tar-gravel roof


Quote:
Originally Posted by Grumpy View Post
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.
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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Old 09-26-2010, 10:42 PM   #14
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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?
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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Old 09-27-2010, 01:31 PM   #15
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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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