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Rooftop Decks for Coastal Homes, continued

Deck Frame and Decking

I build my deck frames with pressure-treated southern yellow pine (Figure 6). We fasten the ledger to the parapet wall with lag screws sized to more than handle the load.

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Figure 6.The crew first bevels the back of the ledger (bottom) to promote free drainage around the edges of the deck. Because the roof surface slopes, the ledger board has to be leveled — the drainage space gets wider toward the front of the roof.

We predrill for the lags, and before we send in the screws, we fill the drilled holes with a manufacturer-recommended caulking that's compatible with the EPDM membrane (Figure 7). That completely seals those holes, so water won't get into the wall structure where the screw penetrates the rubber. We caulk the top edge of the ledger to keep water from getting behind it.

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Figure 7.A heavy bead of sealant keeps water from getting behind the ledger (top). The crew also puts sealant on the back of the ledger (bottom) before screwing it into place. Depending on the loads, lag screws driven into predrilled, precaulked holes may be added for structural support.

For decking boards, I prefer to use a heavy, durable, rot-resistant tropical hardwood. I've used mahogany on some of my older decks, and currently I like to use Ipe. Alternatively, I'll use a plastic composite decking material such as Trex. I don't ever use treated southern yellow pine for decking. It doesn't deteriorate, but I've found that the juvenile wood, with its flat grain, twists, checks, and splinters, just isn't suitable for my projects.

One caution to keep in mind: Coatings commonly used on wood decks may contain solvents that will eat through EPDM. I always warn my clients that they can't apply an oil-based stain to their deck, unless they want to burn big holes into their roof. I prefer to use lumber that's naturally resistant to weather and rot and needs no stain, but if the customer wants a deck stain, we prestain the wood before we install it.

Also, I often assemble my deck frames in smaller panels, or modules, that can be unfastened and lifted out in sections if the deck needs recoating, or, more important, if the roof membrane has to be repaired or modified. Sometimes we fasten our deck hangers to the ledger, or even send the fasteners for the hangers all the way into the framing of the parapet wall; but instead of fastening the joists into the hangers in the usual way, we just set the joists into the stirrup of the hanger and put a hold-down strip across the top. That way, someone can remove the strip later and lift the joists up out of the hangers, if need be — the deck comes free in panels.

Once we installed a large deck over the entire top of a mansard-roofed house in a hurricane zone; in that case, I let a beefy center shearwall extend up through the roof to support the center of the deck, suspending the ends of the deck from the sides of parapet walls in the more usual way. That deck was assembled in panels as I've described, with the decking nailed on in a herringbone pattern. The panels were set into their joist hanger cradles, and a wide deck board placed across the support wall covered the joist ends on both sides and held the whole assembly in place. If necessary, the panels from one side can be lifted out and set down on the other half of the deck, to provide access to the roof membrane

Railing

If I'm putting a deck over a rectangular room, I'll suspend the deck frame between two half-wall side parapets and run a railing across the whole front opening. More commonly, though, the deck rests above an 8-sided or 12-sided room. In that case, we frame up the parapet walls around almost the whole circumference of the polygon, as we did in the example shown here, and we frame the deck with joists running from the main house wall out to the parapet half-walls. With the roof itself pitched gently toward the outside, the angling polygon half-walls naturally catch the rainwater and funnel it toward the outermost segment of the polygon. There, we leave a gap and install a railing across the opening, letting the rainwater drain out over a copper drip-edge and into a gutter (Figure 8). Or, sometimes, I will close in that wall as well, installing just one or two scuppers for drainage.

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Figure 8.Deck joists are first screwed into place, with hangers added after.

Scuppers and Gutters

We have copper scuppers custom-bent at a metal shop for each job, and we let the roofer attach the membrane to the metal. Then we drop the metal right into a fir gutter. I like to use wood gutters, because they look like a piece of the woodwork — almost like a cornice or a piece of crown molding. You can get fir gutter stock at most lumberyards, but if appearance is critical, I order it from a specialty supplier who carries high-quality material. We always preprime and prepaint the gutter before installing it, and we line it with copper. The big key is to make sure that you space the gutter away from the fascia when you attach it (Figure 9).

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Figure 9.A cedar rail will finish off the opening at the front of the deck (top). The copper flashing will drain runoff from the roof surface into a copper-lined wooden gutter. The author sometimes uses scuppers to carry the runoff into a gutter, as in this small deck with a plexiglass rail (bottom). Note the height of the scuppers, which are at roof level, relative to the decking.

We use stainless-steel screws to fasten the gutter to the fascia, but we attach it through predrilled spacer blocks of pressure-treated plywood (sawn lumber would be more likely to split).


Andrew P. DiGiammois a design-build contractor and a partner in an architectural firm in Assonet, Mass.