Expert: Richard Taylor, AIA - 8/11/2004

Question
Richard,
I am working on designing an addition to my house. I have an 85-year-old colonial in the suburbs of Philadelphia. I want to put an addition across the back of the house. The first floor will be the entire width of the house. On the second the addition will only be on a little less than half of the width of the house. My concern is the new lower roof. I do not want a shed roof because it would conflict with the existing second floor windows. I am considering a flat roof that would be a deck accessible from the second floor addition.
My question is what to do structurally. Would concrete or lightweight concrete be a good option. I want to do this right the first time. The addition is going to be 16' deep.  What would you recommend.
Thanks,
Ed Brady


Answer
Mr. Brady:

Thanks for your question!

You've got better options than concrete in this case.  I'd suggest you use dimensional lumber, but detailed in a way that'll let water drain off the deck.  Here's how we do it:

Install flat "rafters" running front to back, with the top ripped to slope down at 1/8" per foot to drain.  Over a 16' span, that'll drop the the rafters 2" at the low end.  For a 16' span, I'd start with 2 x 12s - you'll end up with 10" of lumber at the low end (but check structural codes in your area first).

On top of the sloped rafters, install a plywood deck as you normally would.  Over this place a rubber roofing membrane system - the manufacturer will help you with information on placement and detailing of the membrane (try Carlisle or Firestone's websites).

Now cut "sleepers" to install over the rubber.  Sleepers aren't nailed to the decking (that would compromise the membrane); instead they're just set loose - the deck boards will hold them in place, and they can be secured at the ends.  Your sleepers will be 2 x 6s, ripped on the bottom the same 1/8" per foot as the rafters; that makes the top of the sleepers level.

Before you install the sleepers, cut small strips of rubber roofing and glue them to the bottom of the sleepers for an extra measure of protection against puncturing the membrane below.

With the sleepers in place, install wood deck boards on top just as you normally would.  Now you've got a level deck over a sloped roof - without using concrete.

You can detail the sleepers and deck boards such that you can remove it in sections as needed for cleaning and maintenance of the rubber roof below if you like.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com