Expert: Richard Taylor, AIA - 4/26/2010

Question
My home has a mansard style roof, it was built in about 1975 and the current roof has been re-shingled twice to date.  The current roof is approximately 15 years old and has begun leaking.  I am currently preparing to have it re-shingled and have a wide range of estimates.  It was mentioned that I need a "membrane" added to the roof before re-shingling to eliminate the leaking.  They have also mentioned a "felt sheet". Is this one in the same, or do they serve different purposes?  And is it required over the entire roof or just edges?  Some of the estimates say it is not required and some say it is mandatory to prevent further leakage?  What's the truth?  I want to ensure I do it right.

Your expert opinion is GREATLY appreciated.
Isabelle

Answer
Good evening Isabelle - thanks for your question!

A proper shingle application always requires underlayment; that is most often "roofing felt" or "tar paper" which is a paper sheet impregnated with with coal tar to make it waterproof.  Roofing felt is required by most building codes, but even if it isn't required, it is good construction practice to use it.  Roofing felt is typical "15#", which means 15 lbs. per 100 square feet.  Two layers of 15# felt is common practice in some areas.

Roofing felt comes in rolls, and it is rolled out (overlapping each layer) over the plywood decking.  The shingles are nailed on over top of it.  Roofing felt works best on more steeply-pitched roofs.

A relatively new product is membrane underlayment, which is a rubber or plastic sheet that takes the place of the tar paper and has a sticky side that holds it to the roof.  It's commonly known as "ice and water shield" - a trade name of the Grace Construction Products Company.  It's much more expensive than tar paper, but more effective, too.  Membrane underlayment can be used on lower-pitched roofs.

Now, on to your situation.  Mansard roofs have a very steeply pitched part, and a very low-pitched part.  You should probably put the membrane underlayment on the upper, lower-pitched areas and the roofing felt on the lower, steeply-pitched area (and save a few bucks).

To complicate things just a little, some building codes requires membrane underlayment at the edges of the roof (usually the first 24" or so) to help to deal with ice-damming.

To be sure you're getting the right roof, call your local building department and ask what they require.  Make sure that the bids you get all call for the same products in the same places so you can compare apples to apples.

Don't skimp on underlayment - it does more than just help waterproof, it helps extend the life of the shingles, too.  Use the heaviest shingles you can afford, and be sure the valleys are flashed with metal.

Hope this helps, and good luck!

Richard Taylor, AIA
www.rtastudio.com
www.rtastudio.blogspot.com
www.rtaplans.com