Under deck waterproofing options

[OneStaple]

Hey,

We're moving soon to a new house (unfortunately, that doesn't mean my shop gets any bigger). The house has a second floor deck that's roughly 350 sq ft, and we'd like to use the space underneath for storage (bikes, lawn mower, etc.). Ideally, I'd really like to keep the space dry, which means adding some sort of waterproofing or way to redirect rain.

From what I can tell, the systems that are installed when the deck is built are the best approach, such as Trex RainEscape, ones by Dek Drain, or similar DIY versions using EPDM rubber hung between the joists. They protect the joists and don't get in the way below. But, short of pulling up the entire walking surface, it doesn't appear that I can install a system like this right now.

So what *good* options are available for an already-built deck? I've seen lots examples online of people installing some sort of corrugated plastic or fiberglass sheets on a slope from the bottoms of the joists to direct the water into a gutter. My two big concerns with that are 1) you lose headroom in a space that is already a little short on this house and 2) that might cause water to be trapped a little bit and not dry out as quickly after rains. I'm more concerned about #2, given that we live in a relatively humid area (outside DC), and I certainly don't want to encourage decay in the joists/framing.

Am I overthinking this? Should I just go with the corrugated sheets, or steer clear of them? Is it worth the effort of pulling up the walking surface to add waterproofing? Or is there another option out there, perhaps that can be fit between joists from underneath?

My guess is that the deck has another 5-10 years of life to it. Whenever we redo it, I'll probably add waterproofing on the top side. But I need a decent solution for now.

Thanks,
Tyler

[Snipe Hunter]

You can use corrugated galvanized sheet metal roofing (holds up better than fiberglass) and cut it just a hair wider than the distance between joists. Bow it like a "U" so it holds tight to the joists. Run it over the beam maintaining about a 1/4" per foot slope. Then cut 2x2 or 2x4 blocking and install it under the sheet metal, between the joists to hold it up. If you make good straight cuts, you'll have very little leakage. Seal the end where it hits the ledger board with spray foam. Some people terminate it just past the beam and install a rain gutter on the beam. But you can go all the way to the end of the deck and let it drain off the end or rig up something to hang a cutter. It isn't very noticeable if it terminates at the beam. The problem with letting it drain off the end and not into a gutter is that the water really beats up the ground where it drains.

This is a good opportunity to buy a pneumatic or electric shear. The harbor freight one is only about $15

[Willyou]

Based on my experience, don't put anything under the deck between the joists as it will quickly collect debris that, in turn, holds water which, in turn, attracts bugs and promotes rot. The best solution is some sort of surface water proofing system.

[Gary G™]

It’s only 350 Sq Ft; pull up the decking and do it right.

[Snipe Hunter]

It’s only 350 Sq Ft; pull up the decking and do it right.

Ya know what? I think you're right.

[Willyou]

Can you tolerate some dripping under the deck so long as it isn't steady multiple streams? If you can, make sure your deck boards are running in the direction of the slope and make sure the slope is adequate for positive drainage (at least 1/4" fall per foot of run). Assuming you are using PT 2x lumber, place the boards as close together as you can and still allow for expansion and shrinkage. If the boards are newly purchased, you can usually butt them together as they will shrink as they dry. Also, use the widest boards you can afford as this gives you the most surface area relative to the spacing between boards. Using this method will shed more water as it will mostly run down the length of the boards and very little will actually go through the between board spaces.

[OneStaple]

It’s only 350 Sq Ft; pull up the decking and do it right.

I had a suspicion that this might be the right approach, of course with some extra labor. If that's the way to go, is there any recommendation for one system over another? It seems like just cutting my own trapezoids out of EPDM rubber is probably the cheapest, with a bit of extra work on my part of course. That's normally not an issue, but we'll be moving, so there are a thousand new projects to tackle.

Given that I'd plan to reuse the walking boards, it might be a little bit of a challenge to line up the screw holes again once there's a layer of rubber on the joists.

Can you tolerate some dripping under the deck so long as it isn't steady multiple streams? If you can, make sure your deck boards are running in the direction of the slope and make sure the slope is adequate for positive drainage (at least 1/4" fall per foot of run). Assuming you are using PT 2x lumber, place the boards as close together as you can and still allow for expansion and shrinkage. If the boards are newly purchased, you can usually butt them together as they will shrink as they dry. Also, use the widest boards you can afford as this gives you the most surface area relative to the spacing between boards. Using this method will shed more water as it will mostly run down the length of the boards and very little will actually go through the between board spaces.

Willyou - The deck boards are perpendicular to the slope direction, and they've been installed (and painted) for a while. They're in decent shape, so I'm not really looking to replace them.

Thanks,
Tyler

[GaryMc]

I did roughly what you were originally thinking about 10 years ago, under our Azek deck, with some additional tricks.  First, I excavated underneath about 6 inches to provide headroom and a fairly flat surface.  Then I installed a "floor" using recycled flagstone from an old walkway.  Corrugated sheeting from HD was installed on 6-ft. wide frames, which abutted the house header on one end and sloped to the parallel deck joists.  Sheeting between frames overlapped by two ridges. Bolted together the frames and installed a gutter and downspout at the lower end of the sheets.  Completed the job by enclosing the underneath using recycled cedar siding (with one open side and a window) when Hardie siding was installed on the house.

In 10 years, there has been no evidence of mold, rot or debris collection, and the floor stays quite dry except for the flagstone becoming slightly moist when it is very humid.  The only downside is Robins love to build nests down there
.

[Willyou]

I had a suspicion that this might be the right approach, of course with some extra labor. If that's the way to go, is there any recommendation for one system over another? It seems like just cutting my own trapezoids out of EPDM rubber is probably the cheapest, with a bit of extra work on my part of course. That's normally not an issue, but we'll be moving, so there are a thousand new projects to tackle.

Given that I'd plan to reuse the walking boards, it might be a little bit of a challenge to line up the screw holes again once there's a layer of rubber on the joists.


Willyou - The deck boards are perpendicular to the slope direction, and they've been installed (and painted) for a while. They're in decent shape, so I'm not really looking to replace them.

Thanks,
Tyler

I thought you were considering a re-build. Hope you find a solution.

[DieselDennis]

Kind of hard to say without seeing a picture of the underside of the deck or some given dimensions.  

My first thought would be to build a shed slightly smaller than the deck underneath.  

A deck is an outdoor walking surface.  It's not a roof.