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
Photo 1: Hang the purlins
Rune screwed the 2×4 purlins to spacer blocks. The blocks are incrementally thicker as they approach the outside edge of the deck so the new under deck ceiling will drain toward the outside.
How to Build a Deck Roof
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 1/4 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 1/4 = 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 (Photo 1).
Screw the panels to the purlins
Photo 2: Attach the fiberglass panels
Attach the fiberglass panels with special screws that have a neoprene washer under the head to seal the holes. The panels overlap at the seams to serve as an under deck ceiling.
Starting at one end, attach the first fiberglass panel to the purlin with the roofing screws (Photo 2). Place the screws in every other valley. Snug the screws enough to compress the washer slightly. Overlap the next panel onto the one you just installed and attach it the same way, this will make waterproof deck systems. When you get to the end, the last panel may be too long. You can just overlap it a bit more or cut it to fit. It’s simple to cut panels to length or width with a circular saw and a carbide blade.
Install gutters and downspouts
Photo 3: Finish up with a gutter
To keep rainwater from splashing onto the floor, Rune installed gutters along the beams. The gutters slope to a downspout at the end.
If you want to install gutters for your under deck ceiling, plan ahead and leave space for them. You may have to get creative to come up with an attachment method, depending on how your deck is built. Since the in-stock 12-ft. panels didn’t quite reach the beams on Rune’s deck, he nailed vertical 2x4s to every other joist to provide an attachment point for the gutters. Then he screwed the gutter hangers to the 2x4s. Slope the gutters toward the downspout for drainage.
3 Ways to keep dry under your deck
If you want to buy a manufactured system rather than build your own as we show here, you have a lot of choices.
1. One option is to cover the deck boards with a watertight membrane. DeckRite makes a deck floor covering that has the added advantage of creating a dry space below the deck. For more information, visit
2. If you’re building a new deck or replacing your deck boards, you can use a system like Trex RainEscape, which installs in the joist spaces before you install the decking. The advantage of this system is that it allows you to easily add lighting or other wiring in the joists, and cover the bottom of the joists with bead board or any attractive ceiling finish you choose. Learn more at
3. If your deck is already built, you can cover the bottom of the joists with a system like TimberTech’s DrySpace. One advantage these waterproof deck systems have over simply screwing roof panels to the bottom of the joists is that you can remove a section to gain access to the joists if necessary. To learn more, visit
These are just a few examples of what’s available. Search online or visit your local lumberyard or home center to find other options.
Required Tools for this Under Deck Ceiling Project
Have the necessary tools for this DIY how to build a deck roof project lined up before you start—you’ll save time and frustration.
- Circular saw
- Cordless drill
- Drill bit set
- Safety glasses
Required Materials for this Under Deck Ceiling Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
- Fiberglass panels
- Gutter and downspout
- Pressure treated 2x4s
- Roofing screws