A New Roof
After being sold on a membrane roof on my house in Atlanta (this is often referred to as a "torch-down modified" roof - it's applied in layers using a MAP gas torch), only to have it fail within 6 months, I can give you quite a bit of qualified advice.
First, if you have a flat roof, it was probably installed as a "built-up" roof, using an underlayment of felt, some type of insulative barrier (my roof had 1 inch styro board - these days they use perlite and possibly ISO board, which is actually for insulation with perlite). These boards were typically nailed - note that if your ceiling is also the roof (in my case the roof boards are the same as the 2x6 tongue-and-groove ceiling) and any wiring for lights is typically nailed to the roof under the boards (be careful and mark where the wires are) with special fasteners - then a layer of hot asphalt is applied with a mop (called hot-mopping). Over this is laid sheets of 15 or 30 pound fiberglass/asphalt felt. This is repeated, possibly twice (for 2 to 3 layers), before a cap of asphalt is hot-mopped. Onto this is spread pea gravel as ballast. The gravel also plays an important part in continuing the life of your roof - the rock reflects light away from any exposed fiberglass underneath - it's actually the UV rays of the sun that causes the most damage to this type of roof, and not the weather per se.
There are usually four avenues for a replacement/repair for this type of roof, depending on the condition (in order of cost - least to most):
1. Rolled touch up using some type of liquid barrier/sealer - the cheapest method, the gravel is brushed to the side and the sealant is applied with a roller - then the old gravel plus any extra for coverage is spread back over the roof. This is usually only good for very temporary repairs - say you need a year to get your money together for a real repair. I would actually not recommend this method but it could get you dry for a little while.
2. Torch-down Modified - with this method, a roll of thick roofing material is melted onto the existing roof using a MAP torch - can be risky as the whole roof can literally catch on fire (asphalt burns!). To meet manufacteror's specifications, the roofer should actually fasten a layer of perlite to the existing roof after brushing off the gravel (using approved fasteners), the roof edges should be built up with a nailer board, the material applied (hopefully melted onto the substrate) and new flashing installed. Typically the old flashing is left in place but cut-off where it extends to the eaves. This method works well if done correctly but understand what is really happening is that the roof will "float" on top of the existing roof. The roofer will tell you this is a 10 year roof or some such. I would consider this temporary at best - you might get 5 years without issues - but if the membrane is ever penetrated you will get wet. Also, because you are not removing the existing roof, any existing wet spots can stay saturated against the original asphalt (I added below what happened to me using this method so read on).
3. Asphalt Built-up - this is the original technique used by the builders of these homes - however it requires the removal of the original roof which can add quite a bit to the expense. Since you are pulling everything off, this is the time to replace any rotted or twisted boards. This is also a good time to locate any additional lighting and shore up any rotted beams, etc. The original flashing will be removed along with everything else. If you don't mind the look, raise the roof slightly by going to 1.5-2" of ISO board - it will add an R-value of 10-14 and improve your utility bills. You will also need to add a wood nailer around the perimeter to raise the flashing to the height of the ISO board. I've described the methods used above and this is still the most common method used on commercial buildings. This type of roof can last 20-25 years or more (manufacturer usually states 10-15, but the reality is that this last a long time).
4. Asphalt built-up with a Modified cap - this is the method that I actually chose (the most expensive method but worth it if you plan to living in your house forever). This method is similar to the Asphalt built-up above, only a hot mop-down modified membrane roof is attached to the top of the asphalt layers. This can be doubled up to extend the roof even longer.
The initial Torch-down modified roof I had put down was improperly installed. The roofer (who was supposed to be qualified with experience in this type of installation and was also highly recommended) gave me a very reasonable bid of $5k do do the roof (the roof is approximately 40 squares or 4000 square feet - I should have figured it was a scam and too good to be true) to place a membrane roof over the existing asphalt "built-up" roof. As the other bids I received were over $10k for the roof, we went for the cheap, thinking that we would get 5 or so years out of the roof - the plan was to eventually get a better roof through a complete tear-off and rebuild. We actually got about 6 months before things started to go wrong.
First, the roofer used a shovel to scrape off as much gravel as possible - this turned out to be stupid as everywhere a stone was pulled from the existing asphalt, a gap appeared in the existing roof as a potential leak. Second, the roofer did not cut off or replace the existing flashing. Third, because the substrate was not reinforced with some type of nailer, the new roof was not truly melted onto the old - the roof basically was sitting as a flat sheet on top of a sieve. Because the flashing was not replaced, the edges were not sealed - this made this worse as the roof "crawled" across its expanse, water crept under the edges and found the missing rock holes to penetrate - the result was massive leaks, with water coming down almost every wall. Every attempt to get the roofer back to fix the issue was ignored - ultimately we had to litigate (we won, but have as yet received no money on the judgement, nor do we anticipate we will).
When reality hit we knew we needed to do something quickly. Not wanting to get bitten twice, we heavily researched roofing methods and applications. That was when I discovered that 90% of the roofs for our type of house in the area are incorrectly installed - meaning that even if we had a claim against the manufacturer of the roof with a 10 warranty, etc., the claim would have been invalid as the guarantee is only good if the roof was properly applied. We ended up getting 12 estimates, ranging from $7500 to $40k (ouch!). We decided to go with a real commercial roofer who did a phenomenal job. The original estimate was for $14k, however we went with 1.5 inches of ISO board and replaced several hundred feet of rot, including the ends of two beams. This brought the price up to about $17K plus an extra thousand for additional wood materials.
I think the greatest lesson learned is to research the roof options, then really question the roofers to make sure they know what they are doing. Our final roofer actually had the GAF rep come out and do a core sample so that the warranty was registered with them. The cores they cut exceeded the manufacturer's specifications. I'm happy to report that three years have gone by on the new roof without any issues.
(Originally published 2005.01.10 in the thread: "Flat Roofs Don't Like Rain" on the Lotta Living forums)