DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Roofing and Siding > Flat/Built up roof repair and options




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Old 04-02-2010, 10:24 PM  
CityLimit
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Default Flat/Built up roof repair and options

I am trying to buy a house with a flat roof. I initially saw the house a year ago and there was only slight problems at the time. It was clear the water was not draining properly but since it was not dealt with (vacant) it has since gotten much worse. It is now dripping from the ceiling but not yet caused massive damage. There is a crack in the living room maybe 24 inches (no bowing or water stains the water is just draining through the crack in drops). There is also now a small chunk missing in the master bath maybe the size of two fists.

I had the house inspector out today and he seems to think it may need to be replaced. I don't know if there is a possible patch for this or if they can fix it as its only one section of the house (the master bath and living room are next to each other).

Does anyone know what options/costs are? I have until the 11th to get all inspections done and I am having a roof inspector come out tomorrow but the house inspector is a contractor and said to be careful as most roofers want to replace an entire roof instead of just fix sections. If the roof needs to be replaced (possible I supposed but the damage doesnt seem that drastic yet). What is the cost and what are the options to replace a flat roof.



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Old 04-03-2010, 01:19 AM  
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City Limit:

Generally, you can't count on more than a 25 year life out of a flat tar and gravel roof. If your roof is close to that age, it DOES make more sense to replace the entire membrane than to keep spending money on repairs until you finally cut your losses and have it replaced.

The cost of the work will depend on what you have done. Typically, a flat roof will involve stripping off the old gravel and built up membrane. Then stripping it right down to the roof deck, or at least stripping off the top layer of insulation board to ensure they have a clean dry surface to start with.

Typically, when you install a flat roof, you will strip the old roof down to the wooden roof deck and replace any rotted wood,

Then you put your extruded or expanded polystyrene insulation down on top of the roof deck. It's actually a good idea to opt for a "sloped roof" rather than a "flat roof". On a flat roof, then they just put down the thickness of polystyrene insulation you specify. On a sloped roof, they make a map of the roof with any roof drains on it. Then they feed slabs of insulation into a computer controlled cutting machine that cuts the tops off those slabs in two dimensions at the same time with a hot wire. When the cut slabs of insulation are installed in the correct order, you end up with a 1/4 inch per foot slope in the roof to the roof drains.

The main advantage of a sloped roof is that if there ever is a roof leak, the damage will be minimal because the water will drain off the roof into the roof drains rather than puddle on top of it.

Over the insulation you put fiberboard panels. You want at least two layers of fiberboard panels and have them staggered at the seams so that the 300 deg. F tar won't seep through at the joints and melt the polystyrene insulation below.

Over top of the fiberboard panels they "build up" a roof membrane. This is done by rolling out 4 foot wide rolls of roofing felt paper into hot tar, which bonds each layer of paper together. On my building's sloped roof, they used 4 layers of paper in the membrane, plus another 4 layers around the perimeter of the building. I don't know why they used the additional layers around the perimeter.

Finally, once the membrane has been built up, they mop hot tar onto the paper/tar membrane and broadcast clean pea gravel (typically 1/4 or 1/2 inch clean) onto the roof membrane. The purpose of the pea gravel is to protect the membrane from the intense UV light from the Sun which would deteriorate the membrane over time.

In my opinion, you get the most bang for your buck if you're intending to keep the house a long time by going with the sloped roof rather than a flat roof. On my building, which is in an "L" shape with outside dimensions of 100 feet by 100 feet, I paid about $25,000 for a new sloped tar and gravel roof in 1991. I have had no problems with that roof so far, but it is nearing the end of it's lifespan.



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Old 04-03-2010, 01:27 AM  
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What you can do for the time being is repair the leak. The way you would do this is as follows:

1. Get up on the roof and clear away the gravel over the area where you think the leak is. The leak will look like a crack in the roof membrane and will most commonly occur at penetrations through the membrane, like vent pipes.

2. Apply black roofing cement over the crack with a bricklayer's pointing trowel.

3. Now apply a layer of roofing fiberglass mesh. This stuff is similar to the fiberglass drywall joint tape, but it's about 10 inches or so wide. It's purpose is to carry any tension forces across the crack in the membrane so that the crack in the membrane doesn't open any more than it already is.

4. Repeat steps #2 and #3 until you have three layers of black roofing cement and two layers of fiberglass mesh.

5. spread the pea gravel you cleared out of the area back into the fresh top layer of black roofing cement.

The repair is now complete, and is waterproof immediately.

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Old 04-03-2010, 02:11 PM  
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Thank you for your response it has definitely helped me a lot. I am not sure the roof is a tar and gravel flat roof. I didnt get to see the roof myself as the inspector had already done that when I arrived. Its a white roof with a pool of water where the issue is stemming from. The roof inspector rescheduled for Monday so I can still get an estimate to negotiate with the bank about at least a patch if not replacement. I know they wont cover the replacement but I am going to readjust my offer to cover the costs. I am slightly worried they might try and back out but state law requires them to now disclose the bad roof to all buyers so they would have to drop the price a lot if they chose to do so.

Unfortunately I am not allowed to fix anything until its mine so right now its sitting there with the water dripping and nothing covering the areas that need patches. Hopefully we will close before it rains again.

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Old 04-03-2010, 02:44 PM  
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Wood is a natural material and WILL bend under it's own weight with time. That may explain some of the puddling that's going on over the roof, but you want to be sure that none of the roof joists have started to rot as this would require the replacement of the rotted section.

An experienced flat roofer can tell if the roof decking is rotted just by walking over it.



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