(William J. Decker, CMI, IL. Lic. 450.002240)
I see a great deal of roof decks, especially in new constuction, in my area.
What I mean by roof decks is wooden decks (2 x 4 joists with planking) laying, directly, upon a modified bitumen roof covering. Sometimes, they put another layer of membrane under some points of the joists.
Putting aside the questions about proper structure and loading, it has been my experience that the deck will a) move, rub and tear the roof covering and b) increase the cost of replacing the roof (you have the added cost of having to remove the deck first).
Does anyone have any reference, code or authority to cite when calling this out?
I also understand that there is a membrane roof covering that allows this?
I looked quickly to find you something, but did not find much other than what I have seen and known over the years.
Modified bitumen or built-up roof systems always had statigecally spaced blocking pre-planned for the deck and incorporated in the roof system at the initial install.
If done after the fact, blocks of wood or sleepers were adhered to the roof membrane and water proofed and became part of the roof system.
On rubber membranes, an addittional layer of rubber is adhere to the membrane where sleepers will be for the deck framing. Usually, this is .060 rubber, whether it is Carlile, Firestone, or Goodyear product.
I have seen at times where walking pads were installed to protect the membrane of the roof system, regardless of the type of roof.
As far as finding some back up for what I am saying, I can’t find any at the moment.
I did find this if it helps.
Building a Flat Roof Deck
Occasionally a project is presented where a deck will be built over a flat roof of a house or a garage. First you must make sure the roof is strong enough to support 55 lbs per square foot. Next you will want to install a watertight EPMD rubber membrane over the top of the roof deck for protection against water infiltration in place of roofing materials. You will then install “sleepers” or flat 2x6’s laid on their profile every 16” on top of the roof surface. This will provide the surface for you to attach the decking. In most cases your “sleepers” will float on the surface of the roof with only the weight of the deck to hold it in place.
Make sure the deck slopes slightly to promote drainage towards gutters or at least away from the house. Always check with the local building inspectors and obtain a permit before building a rooftop deck.
Decks over roofs are a bad idea, because leaves and other organic stuff gets through the deck onto the roof, where it can retain moisture, and cause deterioration of ordinary roof membranes. A better solution would be fiv=berglass or vinyl roofing which are designed to also act as decks and walking surfaces, and they also have the advantage of being self-flashing.