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Question DetailsAsked on 2/20/2015

I have a very new composite deck but now I realized I also want a dry under deck area. I'm thinking of covering the

The entire deck area 1000 sq. ft. with Grace ice and water shield and then resurface the deck with Quick Cap composite decking.

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3 Answers


PLEASE - Noooooo. For one thing, Grace Ice and Water Shield is not designed for long-term exposure to the environment and will degrade fairly quickly exposeed to ultraviolet light. It is designed as a backup concealed water barrier for use under singles and around windows and doors, not as a primary water repellant layer nor for exposed use for any more time than needed for construction to cover it- typically days or at worst a month or two, not years.

Secondly, even if you did use a long-term exposure-rated liner like Hypalon or HDPE for this, you would always have little dimples and sags and flat spots in it that would hold water, so your deck would smell like a stagnant pond in short order - not to mention all the dirt and debris going down through the gaps between the QuickCap board would accumulate, so you would in a few years (assuming you have some trees around your area) end up with essentially a mini compost pile in each gap between the boards. Plus most synthetic composite deck boards will soak up water over time (they are basically just plastic-resin particle board, after all), so they would likely start degrading and falling apart in a few years. Heck - an awful lot of them do that just exposed to normal weather in deck and railing use, without even sitting in water and muck.

Overlaying a deck like this is a BIG mistake in the first place - for above reasons - traps dirt and water between overlay and underlying boards - plus the underlying boards are usually being covered because they have already started going south, so you ware basically generally covering up a decaying material with an overlay - NOT good practice. Just replace the decking if the old decking is worn out !

To get a dry underdeck - in general waterproof concrete would be the normal first choice, but requires a lot more substantial substructure. Second choice would normally be a heavy duty roof liner overlain by a floating deck - basically a wood deck build on pedestals on-through the roofing liner, with hoseable gap underneath, and usually built in panels that can be removed easily for inspection, cleaning, and roof liner or deck repair. Typically used for patios and deck on flat urban rooftops. Another possibility would be an IRMA type roof construction - basically a roofing membrane under foam board insulation with a walkable stone or paver (usually) surface on top. However, unless deck surface could be tilted a bit for drainage you would not get away from the stangant water issue, plus existing deck structure again is likely way too light to support it.

One other but expensive solution is interlocking metal decking - steel or aluminum decking with joint seals so basically water tight, but is subject to icing and traction issues unless it comes with an aggressive antislip coating, which generally has to be replaced if you repaint 10-20 years down the road, which gets expensive too.

I would ask HOW dry you want it underneath - bone dry, or just pretty dry - because there are under-deck tray drain systems that can be retrofitted to most typical deck designs to capture and run off most of the water. Do a fair job, though always have a few leaks here and there.

Depending on headroom, it is also possible to build removeable sloping panels made of roofing materials, pond liner, or greenhouse plastic or similar on a frame screwed up under the deck, which will drain the water off to under the front of the deck - though does reduce headroom a bit, mostly because of the need for drainage slope. Should be removeable to allow for full cleaning off of accumulated debris, because generally washing from above (through board cracks) is not real effective - though one can make the first row or two of deck boards (assuming parallel to house, not angled) at the high end of it removeable (using high strength screws or lag bolts) to provide access for pressure washer or hose with jet nozzle to get in under and hose it out. But that is asking a lot of the homeowner to do that every year or so. Could be hinged to tilt down for washing - but that adds cost and complexity. Also, if you have a substantial and effective roof overhang and can tolerate a bit of wetness on the area under the deck along the house during driving wind or snow, the catch trays do not have to reach all the way to the house, thereby leaving the uphill end open to hose access for cleaning.

I have seen a couple where they just ran high-strength pond liner under the deck, sloping off to just under the outer edge support beam, using a wood stretcher arrangement on the outer edge hooked to the support posts, which could just be unhooked to let it down for cleaning - sort of like a dining fly or awning, and with large liner sheetsw (and a couple of people to put up and down) can be one-piece for quite a large deck. Or course, just a free-standing sloping anwing under the deck can serve the same function, at a somewhat greater loss of headroom.

The most important thing to remember with this type of system, which many of the commerical ones do NOT do, especially for after-the-fact installation (as opposed to installing on exposed joists) is you do not want to be trapping the dirt and water alongside the joists or keep the wood wet and rot them out.

I would question WHY you want the dry space under the deck - might well be cheaper to provide equivalent full house length dry storage by building plywood storage cabinets with sloping waterproof roofs along the house wall under the deck like I have, a yard shed (under deck or elsewhere), awning over deck, drape-over covers for what you are trying to keep dry, better garage storage shelving/racks, crawlspace racks, etc. Also - consider how often you will need or use this dry spacde - don't spent more on the dry space (or on a larger one than needed) than it is really worth to you.

I would really look at why and where the underspace dry area can be replaced economically - maybe even a Gazebo or similar in the yard if considering it as a recreational space, as opposed to storage space.

Answered 5 years ago by LCD


okay so what if I use ice and water shield and then go over that with Gorilla water shedding deck system or Duxxbak system?

Answered 5 years ago by Pizza


Looks a lot like Nexdeck or GeoDry - which have had some noted failures reported in the press. Saw a demo of Gorilla at a Contractor's day at a building supply place, where they laid out a sample run of maybe 10 feet - even without any end joints (can't see how they handle them without leaks) it leaked in the demo, and the material was already getting a soft fuzzy surface after only an hour or so of demonstration.

Here is a blog that talks about the duxxbak product -

All I can say is any paper or wood fiber / resin product like this is not going to get any thnumbs up from me - have been seeing them come out and then disappear for about 40 years now and have never seen on that showed promise of lasting any longer than untreated, unpainted pine. My recommendation - put a separate waterproof covering over the desired dry space and don't count on the deck to do it for you.

If you want to try making the deck waterproof, I would at least remove the existing decking otherwise you are likely to get degradation and progressive rot in it and then loss of support and puddling in the surface product, and put on either a walk-on roofing system (which without slope will pond and you unlikely to be happy with in the long run), or go with a metal "waterproof" decking like - oh, what is it called - LokDry and VersaDri or something close to that are two I remember. Course, since you say the exiswting composite is quite new, you might be OK putting another surface over it - but pretty much ANY surface you put on will have a fw leaks eventually, so you may eventually get some roten points.

However, unless they are slightly pitched to drain, you will get puddling. Also, they generally have to be laid down without end splice joints (I have never seen a joint system that would not leak) so unless your deck is short (not at 1000SF, probably) the planks will probably have to go crosswise to the deck. And the house end has to be sealed permanently somehow to ensure the water does not drain out of the joints at the house face. Course, metal decking costs a lot more, and many people do not like it for appearance or walking purposes.

If I was in your place, on a budget, I am not sure what I would do myself - depends on aesthetics, how visible the underside of the deck will be, how critical keeping the underlying areas dry is, etc - but I would probably keep the existing decking so no tearoff or overlay costs, and install a seamless tensioned reinforced geoliner (basically settling pond or lagoon liner suitable to permanent exposure) from near the house (to leave that edge accessible to getting a hose in there to clean it) sloping down to the posts with tension adjustment there on a rolled-edge reinforcing rod or wood strip, with that edge removeable to slope it down for easier washing every few years if needed. Of course, if you have a lot of wet snow or freezing rain that will go through the deck boards (as opposed to dryer or cohesive show that will stay mostly on top for shoveling) that could mean having to snow rake it too. Also, if you commonly have high winds, that could be a problem - both with respect to liner life and flapping noise in high winds.

Answered 5 years ago by LCD

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