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 systems that use this approach or you can fashion a system yourself.
Creating Drainage Troughs
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). This type of system needs to be installed before the decking.
Retrofit Drainage Systems
Below-deck systems designed more for retrofits tend to be more expensive and complicated. Most include a finished ceiling of some sort if that is important to you. Also, most require wood or plastic members attached perpendicular to the joists for fastening and drainage. A number of these systems are listed below.
I have built below-deck storage sheds using plastic corrugated roofing, attached with rubber-gasketed roofing screws. In one case, I fastened the corrugated plastic roofing to strapping nailed perpendicular to the joists above, using tapered spacers to create slope. For good drainage, you want a minimum of 1/8 in. per foot of slope away from the house, preferably 1/4 in. per ft.
In another case, I fastened the corrugated plastic to the roof rafters of the below-deck shed as there was plenty of headroom. Both of these were retrofits to existing decks and worked pretty well, keeping out all but a few drips. 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.
Cleaning & Maintenance
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:
Certainteed UnderShield
Deck Drain Underside
Dry- B-Low
RainTight Underdeck System
Timber Tech DrySpace
UnderDeck Oasis
ZipUp Ceilings
Read more about Deck Construction.
Rick says
Mold & Mildew Problems With Underdeck Ceiling?
Hi. I’m planning on putting up an under-deck ceiling made from steel roofing panels. I am concerned about ventilation. I know that over the joist systems protect the joists from mold and mildew, but I will be constructing mine under-deck. I’ve seen some videos of moisture problems, but am not sure if that is common or not. Do you have any thoughts on the likelihood of problems, or the amount of ventilation needed to avoid problems or techniques to allow enough ventilation?
buildingadvisor says
A well-designed underdeck draining system, with gutters and conductors to move water away from the deck should not lead to problems with mold and mildew. However, systems with pooling water or water contacting wood structural members will likely promote mold and mildew growth.
Any deck surface, top or bottom, wood or composite, is subject to mold growth and will need to be cleaning periodically to keep the decking clean. Of course, inspecting and cleaning the underside of the decking is rarely done with an underdeck ceiling, so any mold will most likely go undetected. Unless the wood stays saturated with water for extended periods of time, any mold growth will be superficial and will not lead to wood decay. So I would not be overly concerned.
Ventilation of the underdeck cavity will definitely help reduce mold growth. I’m not aware of any studies that have looked at this issue, but — based on roof ventilation guidelines, would want at least a two-inch air space between any decking or structural wood members and the ceiling panels with openings at both ends of the vent channel to promote airflow.
steve patterson says
What About DuxxBak No-Drip Composite Decking?
I am thinking about using DuxxBak composite decking, which is a no-drip-thru design. It’s pricey at about $18/sf, but it sure seems like it would save lots of work compared to an under-deck solution. Has anyone tried it or something similar?
buildingadvisor says
DuxxBak decking is a a hollow-core composite decking product without a capstock (dense plastic surface wrap). Similar products have not fared well over the years.
DuxxBak claims to have a unique composition, which sounds good on paper, but without much of a track record you would be taking a risk.
There are similar aluminum decking products with built-in drainage. These have a good track record, but not everyone likes to looks and feel of metal decking. The price tag is high, but in the same ballpark as you are budgeting for DuxxBak.
For a contractor’s perspective, check out this link.
Andy Loughridge says
Underdeck Drainage Missing Gutter, Staining Trim
Hello, This is a helpful post. I have had issues with the under-deck drainage system at a spec home I purchased about 2 years ago. Water seems to pool under composite deck boards for several days after rain.
The system is designed to drain water along the front fascia board with no gutters right into the mulch. The moisture in the front of the house has caused artillery fungus. Also, fascia board is stained with green and rust-colored streaks on the board where the water drains.
I had it professionally power washed but most of the staining remained. The builder claims it’s working as designed. There are no trees nearby but I do plan regular maintenance to avoid debris in the drainage troughs. Is my best course of action to install a gutter? Any suggestions would be appreciated!
buildingadvisor says
While these systems all sound great in the marketing brochures, few are installed perfectly so some degree of leakage and puddling of water in the troughs is the norm. The more careful the installation, the more effectively the system will perform.
In you case, it sounds like there is a low point or not enough slope where the water is pooling in the troughs. The build-up of debris in the troughs will also trap moisture and can develop quickly if there are a lot of trees overhead.
It also sounds like the water is draining directly over a trim board, which a prescription for water stains and probable mold if the area stays wet for long. A gutter would definitely be an improvement. A gutter will also help direct the runoff to a safe place away from the house. You could also try using a plastic or gravel mulch next the foundation if it is causing problems with mold on the house.
Is the trim board stained, painted, unfinished, or something else. If power washing did not remove the stains, they are probably pretty deep in the wood. The deep stains will stay even if the surface mold has been removed or killed. But the mold will return if the area gets wet for prolonged periods. Basically, mold spores are everywhere, waiting for the right conditions – wet and warm.
You can either replace the trim board or try sealing the board with a stain-blocking primer (or shellac) and then painting or staining with an opaque (solid or semi-solid) stain.
Cleaning mold is usually pretty easy with a smooth surface. Scrubbing with soap and water and a scrub brush often does the trick unless the mold has penetrated the surface. If that doesn’t work, you can try spraying first with a 50/50 mix of vinegar and water, borax and water (best to wear gloves and goggles). A step-up from there is a TSP solution, which requires gloves, safety glasses, and a respirator. TSP can also hurt plantings if they are not protected.
You can read more on cleaning mold at our sister site Inspectapedia.
Andy Loughridge says
Steve, Thank you for your insights. These are very helpful.
Here are a few pics of the front of the house. I think the trim is some kind of composite material. The deck boards and trim are Envision decking products but I am still trying to find out from the builder what system they used for the drainage. Am I safe to assume the builder has no obligation to fix these issues since my 1-year warranty is over? It seems like this will involve pulling up the decking, fixing the slope, and installing a gutter.
buildingadvisor says
Thanks for the pics. It does appear that the water is draining directly over the horizontal trim board (fascia or skirting), a prescription for water staining and mold.
Since the trim board is a composite product with a smooth finish, there’s a pretty good chance you can clean the stains with soap and water or one of the products mentioned earlier. I would try using a non-abrasive scrubber like the type used on Teflon pans.
Also, I’ve had good luck using Magic Eraser sponges for all sorts of stubborn stains on smooth surfaces. These use a micro-abrasive that is safe to use on nearly any surface. But avoid using highly abrasive cleaning pads which will scratch the surface, making it attract dirt and resist cleaning in the future. Also check with the decking manufacturer for more specific cleaning tips.
In most systems, the gutter is installed inside behind the rim joist, so it is hidden and will not stain the trim board. Whether this would be covered depends on the language of your warranty as well as state law. But since the builder does not even acknowledge there is a problem, it sounds like you have an uphill battle.
If the builder does not respond, you might want to check with your state’s consumer protection office about new home warranties in your state.
Brian Pierce says
I agree that a regular maintenance and inspection is essential, especially after bad weather.
Ginny Burr says
Simple Under-Deck Drains With Waterproof Fabric?
I would like to use waterproof fabric under my deck to keep water from dripping through the deck boards. Not much water drops through in a light rain but still, one gets wet if trying to sit outside. Is there an easy way to do this?
buildingadvisor says
There are waterproof sheeting systems, as described above. However, these must be installed before the decking. They use a heavy-duty materials, such as rubber roofing membrane rather than a “fabric”. Strips of the sheeting are draped over the joists to create troughs. The strips are cut in a tapered pattern to create a slope along the bottom to drain water away from the house.
The troughs should slope at least ¼ in. per foot away from the house for effective drainage.
These systems include a gutter along the lower edge to direct the water to a drain. They also include accessories to seal around posts and other framing pieces that interfere with the troughs.
You could try fashioning one of these systems with a less expensive material, but unless it is very durable, it is probably not worth the effort.
I would not describe this approach as “easy” due to a lot of fussy details. Also it sounds like your decking is already in place, so unless you want to remove and install it, you will need another approach.
For retrofits like yours, a better approach is using corrugated plastic roofing panels, which you can purchase at your local home center. The corrugations create natural drainage channels. Assuming the joists run perpendicular to the house, you will first need to install 1x3strapping across the joist bottoms at 24 in (or less) on-center. Then install the roofing with gasketed roofing nails or screws.
Some decks are built to slope away from the house. However, to create adequate slope, you will probably need to shim the strapping pieces with thicker shims as you move away from the house. If you want to get fancy, you can add a gutter at the bottom and seal miscellaneous leaks with flashing membrane.
Finally, don’t forget maintenance. If any trees are overhead or nearby, you will collect leaf debris in the under-deck drains. The debris needs to be flushed out periodically or it will build up into a wet mess like you find clogging many gutters.
To flush the drains with a hose, you will need to leave a gap where the corrugated roofing meets the house, or leave a couple of removable deck boards above. With periodic flushing, this system should provide years of almost leak-free service.
Robert H Thompson says
Post Detail In Middle of Drainage System
I am installing corrugated polycarb panels under my deck, but it will need to have a gap to go around a deck post in the middle of the deck. I am concerned that just putting flashing around the post opening will allow water to collect in the valley of the corrugated, causing problems. I am thinking about putting some kind of angled ridge cap over the corrugations upstream from the post to divert water into the valleys on either side of the post. Does this make sense? Any better ideas? Thanks
buildingadvisor says
If I understand you correctly, you are concerned about water getting trapped in the troughs of the corrugated panels upstream from the post. Posts, blocking, braces, and other obstructions are always a challenge with underdeck drainage systems. Most prepackaged systems offer workarounds for common problems, but you often need to jury-rig your own solution if you are creating a system from scratch.
Most systems include some type of post flashing kit to direct water running down the posts into the drainage system. Support posts underneath present a different problem. On large decks with a central beam and posts, some people direct the drainage in two directions, with the central beam as the high point.
For drainage in one direction, your idea sounds like a good one. If you keep water out of the troughs above the post, then you won’t need to direct drainage water around the post. Seems like a ridge cap flashing could do the trick as long as it gets the water beyond the post. You should probably cap the troughs that terminate at the posts in case any water collects in them. Directing those troughs into a downspout is another option, but you probably don’t want one in the middle of your deck.
Best of luck!
Jo says
Is DEK Drain Durable and Leakproof?
Hello,
I have an existing deck and having a finished ceiling is important since we will be entertaining. Which of the systems,from experience, do you believe to be the most durable, and have little to no leak issues? I am seriously considering doing the DEK Drain system, even if it means taking apart the existing deck boards.
buildingadvisor says
As I mentioned above, I have built a couple of underdeck drainage systems using corrugated plastic roofing. This is much simpler than creating drainage troughs, but not suitable for a finished ceiling below.
I have seen a few installations using EPDM troughs, a couple site-built, and one using the DEK Drain Topside product, which also uses EPDM. For durability, you cannot do much better than EPDM. It is made to be exposed to sunlight for decades, so I believe that under a deck, it will outlast the decking (and probably me).
Installing these systems is fairly tedious, but simple enough on a standard rectangular deck. If you have unusual shapes, intermediate flush beams that block the troughs, or other access issues, then it can get more complicated in a hurry.
It is difficult, but not impossible, to make these systems 100% waterproof. It’s important to use a Vycor type product over the tops of the joists to seal where the decking screws penetrate. If you back out a screw, you may need to reseal the screw with some sealant. You will also need to carefully seal the bottoms of railing posts if they are inside the drainage area.
The other major leakage areas are where the water drains into the gutter. You can get splashback where the water hits the “bibs” – the individual pieces of membrane stapled to the inside of the band joist, at the low end of each trough. Also, if you have a cantilevered deck, you will need to install short reverse troughs to conduct water from the cantilevered area to the main troughs (see diagram below). These short pieces can also leak if not done carefully.
DEK Drain Topside uses EPDM troughs to conduct water into a gutter. With cantilevers, a short reverse trough is needed on the outboard edge. Courtesy of DEK Drain
First time out, DEK Drain will be a lot easier than buying EPDM, cutting all the tapered strips, and figure out all the details. They pre-cut all the pieces to fit your specific deck and provide detailed instructions. After testing and troubleshooting with Vycor and sealant, you can probably catch all leaks.
But I would still not install a wood ceiling below as leaks may develop over time. I would use a PVC or other plastic/waterproof material for the ceiling. Remember, this is a gutter system, not a waterproof roof deck.
If you want a truly waterproof deck, with living space below, you really need to build a sloped roof, cover it with EPDM or another single-ply membrane, then install the decking over sleepers. This is clearly a more expensive approach, but worth considering. Done correctly, this is a permanent solution. See details for a rooftop deck here.
Finally, don’t forget about maintenance. An awful lot of organic crud can collect under a wood deck – either in the drainage troughs or on the rubber roof. This is mainly a problem in wooded areas with pine needles or leaf debris. Depending on the rate of buildup, you’ll need to hose these out every 1-3 years to keep the drains fully functional.
David says
Hi Steve, many thanks for your comprehensive reply – I’m deciding on the waterproof sheet material over the joists but am taken with your idea of corrugated roofing under the joists. As the area will only be for general storage then appearance doesn’t matter and it doesn’t have to be 100% watertight. I have sat through videos of the Trex Rainescape system – what I would call “Belt and Braces” – OK where you have a living space underneath and money is no object, so really it’s horses for courses.
buildingadvisor says
Sounds good. You can either attach the corrugated roofing panels to the roof of the shed or to the bottoms of the deck joists. In either case, the more slope the better in terms of drainage – a minimum of ¼ inch per foot. If you want attach upward to the joists, you will probably need to attach tapered wood strips to the joists.
You will also need to attach furring across the deck joists or shed roof to attach the roofing to. Use special screws with neoprene washers made for corrugated roofing. Make sure you drill oversized holes in the roofing to allow for thermal movement.
Read more on different types of corrugated plastic and metal roofing.