Flat roof repair options

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I have a flat roof on a rowhome/townhome-style building that I want to do a repair/fix on. I am looking for some suggestions or options for how to do it, what materials to use, etc.
Basically, the roof is almost flat with just a slight pitch from the from to the back. The pitch is maybe something like a 1 inch drop in every 10 feet. Along the back edge of the building, the water runs off and over the edge into a gutter and the into a downspout to the ground. There is a problem with the way the gutter is done -- primarily that there is no real drip edge to speak of. Instead, the water runs over the edge of the roof and some of it goes into the gutter and some of it runs down the fascia behind the gutter and down the side of the building. I pretty much know what to do to fix the drip edge, gutter, and fascia issues, so I am all set with that part. That part will involve replacing the gutter and fascia and creating a drip edge so that all of the water runoff goes into the gutter like it is supposed to do.
But, there is a secondary issue that I would like to address and fix if I can at the same time. The secondary issues is that over the years part of the roof has sagged just enough to cause a large puddle to form across the whole roof starting at the back edge and going about 6 to 8 feet toward the front of the property. The puddle is about 1/2 to 3/4 inch deep along a line that is about 3 or 4 feet from the back edge of the roof. In other words, along the back section of the roof, it actually pitches slightly in the wrong direct causing the water to run back onto the roof rather than off of the back edge of the roof and into the gutter.
The existing roof -- strange as it may seem -- is made up of two 1/2-inch layers of sheetrock attached to the top of the roof rafters, and then about 1-1/4 inch of asphalt sheeting or asphalt board of some type with what looks like just asphalt coating on top of that. NOTE: I posted earlier about this roof being weird and literally having a sheetrock decking; and the property was built in or around 1945 (long story, but true).
Nevertheless, here is what I am thinking about doing: Remove the existing roof down to the rafters along a path about 4 feet wide that goes all the way across the last 4 feet of the roof along the back edge. Then add new 1/2 inch 4-ply plywood decking along that back 4 feet across the back of the roof. That will enable me to put a new roof over the new plywood decking, and because that new decking and roofing will be thinner and lower than the original 1 inch sheetrock plus 1-1/4 inch asphalt, I will be able to have the roof slope slightly in the correct direction and run off the back edge of the roof and eliminate the puddle that now forms across the back section of the roof.
My question is, "What type of roofing material could I use for the 4-foot wide new decking that I will be creating along the back of the roof? The original roofing material is asphalt of some type, so I need to figure out what goes on top of the new plywood decking along the back, and how I will tie that into where that section meets the existing asphalt roofing. In other words, "What do I use for the new roofing material on this almost flat new section of roof decking that I will be creating?"
I assume there is not enough slope to use roll roofing. I have seen some roofing called Lo-Slope (or something like that) but I'm not sure that is the correct approach. I also know there is glue-down rubber roofing, or TPO vinyl roofing, or EPDM roofing. But I don't know if any of those can be tied into the existing asphalt roofing that will be remaining on the front section of the roof.
It may just turn out that I will have to have a regular roofing company come out and give me a price to do what I want done and just let them do it. But, if I could figure out what they would be doing, or what materials they would use to do this, I may be able to do this on my own.
Any suggestions or ideas would be appreciated. Thanks.
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On 11/8/2011 9:28 PM, Ron wrote:

Pro roof (probably membrane) on bare deck, and depending on how the front and rear parapets are set up, they can insulate and reslope roof at same time with foam panels. But before you get that far, need to find cause and extent of saggy spot. Leak rotting decking and a couple joists? Was a wall removed underneath? (They did that on a flat roof of one wing at work. Oops. Didn't realize it till 18 months later when an 20"+ snowfall collapsed half the roof into occupied space.)
IMHO, some things are not WORTH doing yourself. Specialized roofs are one of them.
--
aem sends...

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Thanks. It may turn out that I will end up using a professional roofing company as you suggested. But if there is a way that I can do what I need, I would like to just do that and get it done in the next day or two. I say "I", but I have a reliable construction person who is on site doing a lot of other work, so he and I (mostly him) would be doing the roof fix if we do it ourselves. He has done a number of shingle roofs before, but not flat roofing.
I had already taken out the entire ceiling underneath, so I was able to see the whole roof from that angle. The roof is actually above a second floor apartment that I am completely redoing. The sag appears to have just been a slight bow in the 2x8 rafters over many years. They are all in very good shape with no rotting etc.
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The only effective repair of a flat roof is a pitched roof.
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Ron wrote:

So, do I have this right? You want to eliminate the puddle?
Why?
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Yes, that is correct.

Since we will be doing the repair on the drip edge issue anyway, I thought there may be an opportunity here to eliminate the puddle at the same time. Eliminating the puddle would cause the water to run off rather than just sit up there in a big puddle until it dries out. Not having water just sitting there seems like it would make sense to me in terms of preventing future leaks.
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Remove the entire roof, restructure as needed to be level, then build a modern roof with a better pitch towards the gutter... Sheetrock and asphalt get very fickle as they age in an environment of extreme temperature changes like a roof...
Do the whole roof all at once or leave it alone, as when it leaks it won't leak near where you have "fixed it" and your insurer will be wondering why you only "fixed" such a small spot on the back of such an old roof just to get rid of a puddle...
~~ Evan
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On 11/9/2011 5:08 AM, Ron wrote:

again, why? the puddle isn't leaking now. perhaps it won't in the future. it isn't hurting anything. ever hear 'leave sleeping dogs lie'?
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Ron wrote:

Oh, I see.
For what it's worth, I have two, 2-car garages with flat roofs. Each has, after a rain, a ten-to-fifteen foot puddle that's as much as 1/2" deep. It's been like that for 26 years (at least).
YMMV.
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I grew up in a row house with a flat roof in NYC. Lived there for 25years.
I recall that we, and just about every other house on the block, had puddles on our roofs after a rain ever since I was able to climb the ladder in my closet and get out onto the roof to hang out.
I don't recall it ever being a problem.
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wrote:

Some hotels have swimming pools on the roof, too. ;-)
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wrote:

...
The wife and I recently spent a few nights at the Hilton Bonaventure in Montreal.
Not only do they have a year-round heated pool on the roof, but gardens, streams and a "duck pond".
A bird's eye view:
http://tinyurl.com/Bonaventure-Roof-Hotel
A view of the garden walk:
http://tinyurl.com/Rooftop-gardengarden-walk
The pool in winter:
http://tinyurl.com/winter-pool
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Some of the "best" flat roof systems are "wet" roofs. I don't like flat roofs, period - but some membrane roofs that hold water apparently last longer as they are kept "cool'? by holding water.
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Ron wrote:

Thanks again to everyone. The overwhelming consensus seems to be to forget about the puddle that forms because it's been that way for a long time and it doesn't leak -- so don't worry about it. Okie dokie, will do -- sort of.
We still had to deal with the drip edge and fascia problem. It turned out that to do that we had to cut back the roof down to the rafters about 18 inches back from the edge of the roof. We replaced that section with 1/2-inch 4-ply plywood decking, created about a 1 inch overhang with the plywood, and then fixed the drip edge so the water runoff now will go into the gutter the way it is supposed to. The new 18-inch plywood section is now on a slight pitch -- it pitches down about 1 inch over the 18-inch length toward the gutter. So, now we have a new plywood deck across the whole back edge of the roof, 18 inches wide and 20 feet across.
The question now is what type of roofing material to put on the 18 inch by 20 feet plywood decking that is on a slight pitch?
After cutting back the original roof, it turned out that it has: two 1/2-inch layers of sheetrock on top of the rafters which are on 16-inch centers; then two layers of stiff asphalt decking(?) that are each about 1/4-inch thick; then 3/8-inch plywood; then the final roof which looks like it may be some type of roll roofing or asphalt(?) that is about 3/16-inch thick. That last layer is easy to separate from the 3/8-inch plywood, so new roofing can be slid underneath that if needed.
When we added the new plywood decking along the back edge of the roof, we lined it up so it matches with the existing 3/8-inch plywood decking. So, we have the option of cutting back just the top 3/16-inch layer of the old roof about 3 inches back from where the two plywoods meet -- then create new roofing over both plywoods -- meaning the 18-inch new plywood and the 3 inches of exposed old plywood. Then, do whatever new roofing we choose to cover that 21-inch span of old and new plywood decking.
One option is to use a GAF Liberty self-adhering roll roofing system that is supposed to be for low-slope roofs. But, I am not sure about that.
Is there some other option for this small area that is about 21 inches wide and 20 feet long of bare plywood decking? Would it be too weird to just spray the plywood with asphalt primer (such as Karnak Quick Dry 108AF Asphalt Primer), then just use roof cement and fiber mesh to create the new roof for that 21-inch strip?
Or, any other suggestions?
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the transistion areas you are creating will be the area of future leaks:(. you will be far better off replacing the entire roof, if your planning on keeping the home more than a few years.
and even if your selling the repair and puddle will complicate home resale. and no doubt decrease your homes value:(
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bob haller wrote:

Not if done correctly.

Sell the home when the sun shines.
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IN PA all such not normal issues MUST BE DISCLOSED at home resale or the seller can be sued for the entire amount of repair, and even lost revenue on a rental if the repair disrups renting or tenants use of home.
a neighbor lady sold her home with a bad sewer line, and got sued for 10 grand in repairs. she failed to disclose she knew it was defective
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Disclosure is a joke. "Yep, saw problem. Fixed same.": Disclosed.

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Right. They would have to prove that you know. That's certainly possible, particularly if you ask stupid questions. ;-)

I've seen plenty who admit it freely here. ;-)

Right. Don't ask questions where you might not like the answers.
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bob haller wrote:

Sure she should have disclosed, she knew the sewer line was defective. A puddle on a flat roof is NOT a defect. It causes no damage nor ever will cause any damage. At worst it might cause uncontrollable twitching on the part of a fuddy-duddie owner.
And I dispute your claim that "not normals" must be disclosed. If the house was painted in vivid colors would you have to disclose that? How about mismatched brick on the outside veneer? A double-ugly wrought-iron sculpture in the patio? Substantial hooks in the ceiling of the master bedroom, whose purpose shall not be mentioned?
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