How to Build an Under-Deck Roof

The Under Deck Ceiling Design

One of our readers, Rune Eriksen*, sent us photos of his design for a home-built deck drainage system, and we were impressed enough to check it out. His low-cost, easy-to-build under deck drainage system catches the water that drips through the deck boards and redirects it to the outside of the deck. Now Rune can use the area under the deck as a covered patio, where he can enjoy warm summer rainstorms without getting wet. Several commercial waterproof deck systems are available to create a dry space under a deck, but Rune’stactic of adding a roof to a deck seems to be just as effective. And it’s inexpensive and easy to build using materials found at any home center. Here are the details in case you’d like to build your own building a deck with a roof version. Thanks, Rune!

*Rune Eriksen is a retired electrician who has spent the past 20 years building his dream lakeshore cabin, garage and shop.

Figure A: Roof Details

Attach 2×4 pressure-treated purlins to spacer blocks sized to create a slope from front to back. Then screw the panels to the purlins.

Round Up the Materials for an Under Deck Drainage System

Rune bought corrugated fiberglass panels at a home center to use for his under-deck roof. The panels he used are 26 in. wide and 12 ft. long. He attached the panels with special roofing screws that have hex heads and neoprene washers for sealing. You’ll find these screws where steel or fiberglass roofing is sold. You could also use galvanized steel or plastic roofing panels and install them the same way. Rune screwed treated 2×4 purlins to spacer blocks to support the panels and provide the necessary slope (Figure A).

Gutters aren’t required, especially if the water drains onto your lawn. But if the water falls onto the patio, gutters can prevent splashing. Rune chose PVC gutters because they’re inexpensive and easy to cut and install. If you decide to install gutters, you’ll need lengths of the gutter, gutter straps, end caps, a downspout outlet, downspouts and special glue to join the sections.

How to Build a Deck Roof: Attach the Purlins

Plan to space the 2×4 purlins parallel to the house and 3 ft. on center. To provide drainage, the panels should slope toward the outside edge of the deck about ¼ in. per foot. If the span under your deck is 12 ft., for example, the purlin at the outer end of the deck should be 3 in. lower than the purlin along the house (12 x ¼ = 3).

First, mark all the purlin locations on the deck joists. Install the purlin along the house and the outer purlin. Then stretch a string between them. Measure down from the deck joists to the string at the other purlin locations. Those measurements (minus the thickness of the purlins) will give you the widths of the spacer blocks.

Cut the spacer blocks and screw them to the bottom of the joists at the marks. Then attach the remaining 2×4 purlins by screwing them to the spacers.

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