Dec 7, 2008

Q & A: Installing A Deck Over A Roof

We have a 71 year old (stone) house with an attached porch. The porch floor is slate with wooden columns. The columns have wood encased beams that support a roof.

This roof has a decorative railing and there is a door that leads out of our master bedroom onto the roof. The columns (and we suspect the beams) are rotting. We are going to replace them. We would like to make the "deck" above the porch a usable deck space. We will replace the wooden columns and beams, but are concerned with what kind of roofing to use. We would like to put some sort of an engineered decking and railing on the roof.

We are concerned that by putting anything on whatever roof we choose it will void the warranty (and will leak). We have talked about no nailing through the roof and only attaching the decking at the edges, but are concerned about high winds, etc that would make this unstable. Do you have any recommendations?


Thanks for your question!

You have a couple of issues to address here; the first is structural. The loading requirements for a deck are much higher than for a roof alone; be sure the structure is designed to accommodate that load. That means the roof structure, the connection of the structure to the house (very important) and the columns. It also means the foundations below the columns. Have this all checked out by an experienced pro.

You'll also have to meet code with the railings. This is a critical design issue, as you don't want to change the character of what you have too much.

The roof you'll be building has three basic parts. The first is the structure itself; the second is the waterproof membrane, and the third is the deck surface.

There are a couple of ways to build this, I'll describe the one we use most frequently. The roof structure should slope from the house to the edge (where the gutter should be) at a rate of about 1/8" per foot. You can achieve that by cutting a slope into the top of the rafters.

Plywood decking goes over the rafters, which is then covered by a single-ply roofing membrane - typically a "rubber roof" - which is glued to the decking (check with the manufacturer on the proper substate). The posts for the railing should already be in place; use the prefabricated rubber "boots" provided by the membrane manufacturer to flash around them.

Now you have a waterproof roof, how about the deck surface? If you install it directly on the membrane it will be sloped and you'll punch holes in the rubber. Instead, use "sleepers"; sleepers are sloped 2 x members that sit on top of the rubber roof. They are cut with the opposite slope to the rafters, so that the bottom edge follows the slope while the top edge is level.

The sleepers are the nailing base for the decking...but here's the important part...the sleepers are NOT nailed to the roof. The sleepers just sit there, held in place by the weight of the decking boards. No need to nail at the edges; there's plenty of weight to hold the deck down even in strong winds. Some designs even build the deck in sections, so that they can be lifted up for cleaning the rubber roof below.

I also like to glue a small strip of rubber roofing to the bottom of each sleeper for extra protection of the membrane below.

The details are the important part; how you handle the posts, the railings, the gutter, the flashing to the existing wall, etc. Do these right to keep the project looking good and working properly. You can get a lot of help on the details from the rubber roof manufacturers (the big ones are Carlisle and Firestone).

The above is a generic description; you'll need to do a little more homework on this to get it planned just right. You might try for more info.

Hope this helps!

Richard Taylor, AIA