Flat roofs good for decks but tricky in rain

Bill Burnett, Kevin Burnett
 Q: We added on to the back of our house on the lower level about 15 years ago, and this

created the opportunity for a roof deck that is accessible from our second floor.
The roof is covered with tar and gravel, and we placed wooden 2-foot-square decking tiles over it. The plans originally called for ceramic tile, but we ran out of money.
Unfortunately, the roof on the addition was built with the incorrect slope and the builders put in only one of the two drains and scuppers that were called for in the plans.
Rainwater collects on the roof, and we have to use a pump to clear it off several times a day when it rains. We are now planning a remodel of another area and want to get this problem fixed. The plan calls for adding the second drain and installing tile or slate in such a manner that it slopes away from the house, then down to both drains.
I am concerned that slate or tile may fail or leak and that we should be considering some other kind of surface. Any suggestions you have would be appreciated.
A: Flat roofs are the bane of builders. There’s a reason you don’t see that many of them. Unless they are constructed perfectly, they are prone to leaks. And even if they were built right from the get-go, the ravages of time and movement of materials can make an adequately sloped roof prone to puddles.
You have the added issue of increased weight, called load, of people and whatever equipment and furniture you might have on your second-floor retreat.
From time to time we mention the Second Empire Marcus and Remmel Victorian house Kevin owned in Alameda. Second Empire means a mansard roof, and mansard often means flat.
Legend has it that 17th century French architect Francois Mansard developed this architectural style as a tax dodge. Seems that taxes in Paris at that time were assessed on a per-floor basis, with the attic being exempt. Because the top-floor living area is really the attic, the mansard design allowed for tax-free living space in an area equal to the footprint of the house.
The roof on Kevin’s house was flat, and although built well enough to have lasted more than 100 years, it still collected puddles of water in the rainy season. It also leaked.
Adding the scupper and roof drain left out during your remodel 15 years ago is the right thing to do. While you’re at it, see if you can correct the slope on the roof so water drains correctly.
Because you’re removing the old tar and gravel anyway, this is an ideal time to remove the roof sheeting and adjust the framing for the proper slope. At the same time, make sure that the framing is sized properly to take the added load of people and furniture on the deck. Deflection caused by the added weight will cause the framing members to flex, putting stress on the roof membrane. Eventually, cracks will develop and the roof will leak.
Don’t replace your existing wood decking with ceramic tile or slate. It will add significant extra weight to the roof. The cement grout joints probably won’t withstand the expansion and contraction that occurs with changes in the weather and normal use. Eventually the deck will look lousy, and the compromised joints are a leak waiting to happen.
You might be surprised to know that, for our money, the solution you’ve had for the past 15 years is the best one. Early in Kevin’s career he had the opportunity to work on a custom home being built in the Hayward hills. The architect designed a roof deck off the master bedroom.
The roof was tar and gravel and the deck was a series of 4-by-4-foot redwood panels. The panels were made of 2-by-2 redwood strips screwed to 2-by-4-inch redwood boards. Four 2-by-2s formed the perimeter of each panel and 2-by-2s were screwed with a 1/2-inch gap between them in the field.
When Kevin asked the architect why he did it this way, the response was that gaps allowed rainwater to drain from the deck to the roof, and the panel construction allowed for easy removal should the roof fail. Still makes sense today.
E-mail the Burnett brothers at sweatequity@sfchronicle.com