_Building a Deck on a Flat Roof

[Jack Goldstein]

Hi gang,

I need to build a recreational deck on a newly-rebuilt flat roof that has a 2-ply "rubber" torch-welded membrane. The new roof is framed with 3/4" plywood screwed down on to 2x8's, 12" o.c., and is *very* stiff.

I was originally planning on doing the following:

1. Lay 2x4 P.T. "sleepers" on the membrane, 16" o.c.
2. Screw 2x6 P.T. boards on to the sleepers perpendicularly, using three #8x2.5" green deck screws at each "intersection".
3. Leave a "nail's width" between each decking board.
4. Wait a year, then stain the P.T. decking with an oil-based stain.

Recently, I have received several suggestions from friends and am not sure which suggestions are good and which aren't. Here they are:

1. Glue down the sleepers on the membrane.
2. Glue down 4" strips of 1" thick rigid foam insulation on the membrane then glue the sleepers on to the foam.
3. Use 2x6x5/4" decking instead of 2x6 lumber.
4. Use 2x4 lumber instead of 2x6 lumber.
5. Don't leave any space between the decking boards.
6. Don't *ever* stain the P.T. boards.
7. Use cedar decking boards, not P.T.

Does anyone have any ideas regarding any or all of the above

7 suggestions? Many thanks.

[Mark Jerde]

I suggest you also post this in alt.home.repair. Lots of knowledgeable people post there.

-- Mark

[Wood Butcher]

You have a better handle on things than most of your friends do. First check with the mfgr of the membrane for their recommendations.

I wouldn't screw or nail anything to the membrane as any hole is an invitation to a leak. Nor would I glue anything to it as things move with temperature differences and there may be a risk of tearing the membrane.

Consider making the entire deck of 4x4 or 4x8 sections which can be removed for maintenance of the roof or the deck. This will allow you to use chemical cleaners, stain, oil, whatever on the deck and not get it on the membrane where it may cause damage.

2x4 PT sleepers on edge should be fine. Trex is better as it will never rot. Be sure to run them in the direction of the drainage.

Don't use PT for the deck surface as any splinters could be poisonous. Use cedar, redwood, trex or other material. Use either 2x4 or 2x6, let your taste/style dictate. Space it for your climate (usually 1/8" or so) for expansion and to allow rain to drain through. 3/4" thickness is fine for 16" OC sleepers.

Art

[Wood Butcher]

Bah! I meant to say 1 1/2" thickness.

[Ron]

This is what I've heard being recommended quite often, however,... I did just that two years ago when I built my deck out of P.T., this past summer some of the gaps were nearly 1/2" wide... My sister had a contractor build her a deck early last year, they installed the decking tight against one another, by the end of the summer the P.T. had dried and there were some nice 1/8 to 1/4" gaps between the boards.

Consider this and how 'green' your P.T. is when you install it.

Maybe consider buying the decking several months early and let it dry & shrink.

HTH, Ron

[Eric Ryder]

I'd recommend a slip sheet on top of the roofing - a loose piece of rubber EDPM roofing would do the trick.

[Tom]

There's gotta be some fall to the roof so it will drain. If you do glue them down somehow, remember the pitch and make sure you don't restrict free drainage. Me, I'da rather'd 4x4 posts sistered to the joists, resting on the sill plate(except the center posts), extended through the decking. Then roof and flash the deck and posts. Build your new deck on the 4x4's sticking out of the new roof. Tom Someday, it'll all be over....

[Bridger]

one approach I like is to lift the deck off of the roof with risers consisting of about 16" long black iron or galvanized nipples with a bolt on flange at either end. make sure the ends attached to the roof are bolted somewhere that can take the point load. bolt down the flange, screw in the nipple and slip a roof jack over that. seal the roof jack with the appropriate roofing mastics for your roof. screw the other flange onto the top of the nipple and bolt your beams to that.

this substructure has minimal restriction to the drainage of the roof. Bridger