Q: In 1988, we had a new roof installed with 40-year limited warranty fiberglass shingles put over our gravel roof. While the roofers were working, dry rot was discovered in the kitchen, and we had it repaired.

Since 1997, there has been a widening area on the ceiling (where the dry rot was repaired) showing yellowing and the start of crumbling plaster. We were told by the contractor who repaired the dry rot that the yellowing was salts leaching out of the plaster. We contacted the roofer and were advised to caulk around the outside of the building where wind might be blowing in moisture.

In January 2007, the roofer said the yellow color of the plaster means water is present. I also noticed the wallpaper bubbling near our fireplace. The roofer said the nails holding the shingles had rusted through because of fog and dampness and water pooling to low places on the roof. He recommended a new roof and would use copper nails and what he said was an "Alaskan" shingle rated for 100 mph winds. The roofer bid $1,685 to repair the damaged area.

All this is background. Now I have three questions for you guys:

  • The roofing company gave a five-year workmanship guarantee. The roof has been on for 20 years. Is it usual for roofing nails to give out before the shingles do?
  • The roofer wants to put a new shingle roof over the old one. I am worried about the weight; is this safe to do?
  • Because the leak is where the dry rot was previously, how can I make sure the leak will be solved with a new roof?

A: You have a difficult decision to make. On the one hand you can have the 20-year-old roof repaired at a cost of around $1,600, or you can bite the bullet and install a new roof that is more compatible with its low slope.

If you repair, you risk throwing good money after bad. If you opt for a new roof, it will cost more today but will eliminate the chance of leaks in the nearer term and probably will save you money in the long run. For that reason, we would opt for the new roof rather than the repair.

You’ve been lucky to get 20 years out of the 40-year shingles you have. Composition shingles are designed for installation on roofs pitched 4 in 12 or greater. This means that for every 12 inches of horizontal run, the roof increases 4 inches in height. The steeper the peak, the greater the pitch.

A "flat roof" often will have pitch of 2 in 12. A standard 1950s tract house will have a roof with a 4 in 12 pitch. On the other hand, the roof of a 1930s Tudor cottage will pitch 8 in 12 or even 10 in 12. Simply put, when installed on roofs with a pitch of 4 in 12 or greater, composition shingles shed water. Installed on roofs with a pitch less than 4 in 12, they might not perform.

You’ve provided several clues that tell us that the pitch of your roof is less than 4 in 12. The original roof was tar and gravel, the roof covering of choice in 1988 for low-pitched roofs. You mention that the nails have rusted out. It is unusual, and it’s evidence that water is pooling on the roof — another characteristic of low-pitched roofs.

Also, the problems you’ve had with leaks, evidenced by the yellowed and crumbling plaster, is proof that the roofing material installed in 1988 is not right for the roof structure you’ve got.

The roofer’s proposal is reasonable for a repair, but not for a new roof. We share your concern about the weight of the material. Tearing off the area that is to be repaired is a better option in our book. We’re not quite sure what an "Alaskan" shingle is, but any shingle is not such a good idea.

It seems to us that you’re jumping through a lot of hoops with special shingles and copper nails to try to save the existing roof. We suggest that you tear off the two existing roofs and start anew. Look into roofing material made especially for low-sloped roofs. Built-up tar and gravel roofing is the classic.

A more modern alternative is a single-ply membrane consisting of plastic, modified bitumen or a synthetic rubber sheeting that is laid over the roof deck, usually in a single ply and often with a top coating to protect it from ultraviolet light degradation.


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