David writes: My deck will be about 4 ft. high with steps leading down to the ground and will have quite a large storage area underneath. My problem will be how to keep this under-deck area as dry as possible. I will be using Trex Contour with joists at 16 in. spacing. Could I put silicone caulking between the boards so rain runs to the edges or put a waterproof fabric. such as roofing felt, over the joists and under the boards again so that rain runs to the edges? Any suggestions would be much appreciated. Thank you.
Answer: I would not recommend the silicone approach for a number of reasons. It would be messy to install, difficult to clean, hard to maintain, and leaky. Draping a waterproof material between the joists is a much better approach. There are a couple of commercial s systems that use this approach or you can fashion a system yourself.
The best material would be light-gauge (.45 in.) EPDM, a synthetic rubber used for roofing. It is very durable, UV-resistant, and easy to work with. Roofing felt would be difficult to fashion into troughs and prone to tearing. Some of the newer waterproof synthetic roofing underlayments might offer a cheaper alternative than EPDM, but you would need to experiment. I built one system like this with EPDM, which has held up well.
Basically, you need to cut tapered strips of material that drape between the joists, with a slope from the house to the outside edge of the deck (hence, the taper in each strip). Staple these to the sides of the joist near the top edge. Then cover the top of each joist with a 3” to 4” strip of material to protect the joist tops and direct the water in the troughs. A flashing tape such as Grace Vycor Deck Protector is ideal if you want a waterproof seal where the deck screws penetrate the material. Using hidden fasteners with this type of system is difficult and may result in more leakage than you want.
You will also need to create a flashing where the deck joins the house, above the deck ledger, to direct water into the troughs. And you will a need a metal or plastic gutter at the outer edge of the deck (inside the band joist) to catch the water from the troughs. A piece of material stapled on the inside of the band joist in each joist bay is also a good idea to protect the band joist from water and to direct water into the gutter.
If all this sounds like too much to jury-rig, you can use a manufactured system with all the parts pre-cut and designed to work as a system. I know of two: Trex Rain Escape and Dek Drain Topside (see photo).
The below-deck systems are designed more for retrofits and tend to be more expensive and complicated. Most, however, include a finished ceiling of some sort if that is important to you. A number of these are listed below.
I have built below-deck storage sheds using plastic corrugated roofing, attached with rubber-gasketed roofing screws. One time I fastened the corrugated roofing to the joists above, with tapered spacers to create a slope, and another time fastened to the roof rafters of the shed below the deck as there was plenty of headroom. Both of these were retrofits to existing decks and worked pretty well. Because of the rigid materials and larger troughs, these are easier to flush out and keep free of debris than systems that use a membrane hung over the joists.
Whichever approach you use, be aware that any system will require maintenance, and none is 100% leakproof. For the most part, maintenance means flushing the system with a hose periodically to clean pine needles and leaf debris out of the system – not much different from cleaning your gutters except that access is very limited. Make sure the first decking board, on the house side, is removable for hosing out the troughs.
In a wooded area, a lot of debris can build up pretty fast below the decking and create a mess if not flushed regularly. Over time, the wet organic debris turns into wet mulch. The same is true for rooftop decks over membrane roofs, which also need periodic cleaning.
How water-tight the system will be is a function of the materials you choose, how carefully you assemble the system, and how well you seal all the holes made in the membrane by the decking screws. It is better to consider this a gutter system than a true waterproof roof. It will keep most water off your head, except for the occasional drip here and there.
Here’s a list of manufactured systems that install under the deck:
Read more about Deck Construction.