How many Layers of Tarpaper on a Flat Roof?

Hi, on my house I've got a flat roof covered with tarpaper. The top layer is cracked "alligatored" and needs another layer of paper on top of it. How many layers of paper can a flat roof take? At what point do all of the layers need to be torn off? What about city codes limiting the number of layers of paper for houses in baltimore City, MD?
Thanks
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Tarpaper? What they call tarpaper here goes under shingles, not as a flat roof finishing system.
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It's on a flat roof on a row house. I'm trying to find out the maximum number of layers of roofing covering are allowed on a flat roof.
Is there a roofing newsgroup?
Thanks
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I think you are asking the wrong question. tarpaper is an underlayment used under a layer of either shingles or roll roofing. It is not, itself, a roof covering. Is what you have, roll roofing? If so, you never want more than two layers. Anything more than one layer shortens the lifespan of added layers. Two is the maximum before you need to tear off and start over. By that time, there is usually decking and flashing that needs to be uncovered and repaired anyway.
Flat roofs with roll roofing last about 10 years before they start failing. Sometimes they can go a few more years than that, but that's about the average for a properly installed roll roof. If you own the building, you really should investigate either a builtup roof, or as someone else suggests, building a pitched roof over what's there. You may get 25 years out of a built up roof or a pitched roof withy shingles. There are no real shortcuts. When you compare the cost of different options, compare them against years of service. Re-doing a roll roof every 10 years gets expensive and annoying. If a 25-30 year solution costs 3 times as much as a 10 year solution it's a no brainer. Just do it. Even at 4 times the cost it's got a lot of advantages. Heck, with adding a pitched roof, you have an opportunity to add a lot of insulation, too.
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Not entirely true.
For a hot tar roof, the crew often puts down 90 lb tar paper, and then hot mops tar over it. The tarpaper is then part of the roofing layer and you could count the layers by counting how many layers of tarpaper you tear through as you take it off.
JK
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On Sun, 9 Nov 2008 15:48:13 -0800 (PST), Big_Jake

Oh, you mean an underlayment!
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I'd like to do that but I'd have to deal with the City bureaucracy and building codes, which over the years have become very burdensome and mind numbing. Guess I'll drop 5k for a complete tearoff.
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If you can't build a pitched roof, you might still be able to improve things by re-roofing with a built up composite, or a torch down membrane, both of which will be more expensive initially than roll roofing, but will last much longer. Worth getting some comparative estimates, and talking to some roofers about it. Roll roofing is really the low end of roofing materials. It's the northern equivelent of a thatched roof made from palm fronds and grass. <g>
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Flat roof buildings dont have enough ceiling insulation, consider a foam roof, you get a roof and whatever R value you need to save on utilities.
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