Because the drainage troughs run between the joists and can be
made to fit any space, this below-deck drainage system works with
any deck, including custom-shaped ones.
As a deck builder in the Pacific Northwest, I often get requests to
waterproof second-story decks to make the space below useful for
leisure or storage. My standard approach used to be to build the
deck with a slope, put plywood decking down, and apply a textured
coating system over the plywood. Not only did this look bad and get
dirty quickly, it puddled easily and leaked in a few short years.
So I started looking for a long-lasting, problem-free product I
could install easily and cover with good-looking decking, for any
shape deck I dreamed up. I also wanted a great profit margin.
After doing some research, I found a geomembrane that was
waterproof, inexpensive, and flexible, and rated to last 25 years
in direct sun (Figure 1). I now use this membrane to create
drainage troughs between the joists, and with some off-the-shelf
aluminum or plastic gutter, I form the system I describe
Figure 1. Caulked and stapled to the joists,
polypropylene geomembrane forms troughs that drain into a
It works like a champ — and generally costs less than 50
cents per square foot for the membrane system and $5 per lineal
foot for gutter and downspout. As added benefits, the membrane
protects the joists from moisture, so the deck framing will last a
lifetime; and the deck surface can be built level, because the
slope for drainage is between the joists.
While I designed the system for new construction, it also works
fine on pre-existing decks, with just a few alterations.
In brief, the process goes like this. After framing the deck but
before installing the decking, I cut the membrane into pieces long
enough to cover the length of the joists and lap over the ledger
board and up the wall; and wide enough to droop down between two
joists to create troughs.
The slope is created by increasing the amount of "droop" in the
membrane from one end to the other. Lay the upper end of the
membrane (usually next to the house wall) in place with only a
small droop in the middle of the membrane. Droop the lower end of
the membrane all the way to the bottom of the joist. A second
installation of membrane starts at the outer rim joist, draining
back over the support beam and into the first run of
The troughs drain into a gutter, which I place against the outside
edge of the support beam, tight against the bottom of the
Cutting the Membrane
I use a 20-mil polypropylene membrane (Layfield Plastics,
800/796-6868, www.layfieldfabrics.com), which commonly comes in
10-foot-4-inch-wide rolls of just about any length you want. I
chose this product because it's strong enough to span a 16-inch or
24-inch joist layout and it's durable. Readily available and
inexpensive, it costs around 30 cents per square foot at a local
To determine how much membrane you need, drape a string over two
joists (Figure 2). At the low end of the drainage system, the
middle of the string should droop to the level of the bottom of the
joists; at the high end of the system, the string should droop
about an inch down from the tops of the joists. Overhang the ends
of the string about 1 inch down the outside of each joist so you
have a little extra to play with. Simply measure the strings to get
the widths of the two ends of the membrane.
Figure 2. Use a string to measure the span of
the membrane. On the gutter end, it should droop almost to the
bottom of the joists. On the house end, it should droop down only
an inch or so.
Find a flat, solid surface, such as a concrete slab, on which to
lay out and cut the trapezoidal-shaped pieces of membrane. Then
determine how many pieces you need and lay them out —
alternating the wide and narrow ends — across the
10-foot-4-inch-wide sheets of membrane (Figure 3). I can usually
get about five pieces out of a roll. The length can be as long as
you need, although 20 feet is the longest I've ever needed. Be sure
to allow for enough excess membrane on one end to go up the house
wall 3 inches, and on the other end to run to the center of the
Figure 3. The string measurements are
transferred to a layout, alternating wide and narrow ends across
the width of an appropriate length of membrane. Cut lines are
snapped with a chalkline.
I use a lumber crayon to mark the measurements on the membrane, and
a chalk line to mark the cut lines. I score the membrane along the
chalk lines using a utility knife; by not cutting all the way
through, I save the point of my knife blade from being worn off by
the concrete under the membrane. Once scored, the membrane easily
rips along the score lines (Figure 4). I fold and roll my pieces
for easy handling and set different sizes into different
Figure 4. After the polypropylene membrane is
laid out on a concrete slab and scored with a utility knife, the
membrane tears easily. It's best not to cut all the way through, to
preserve the knife's sharpness.
Installing the Gutter
Before I install any membrane, I install the gutter; this sequence
gives good access to the top of the gutter. If the gutter is short
enough to be carried on my truck, I have it made to length at the
gutter supplier's shop and install it myself. I also supply the
shop with the location and measurement of the downspout, and have
the hole for it punched. I usually have no problem matching the
gutter to the ones already on the house, and this approach is much
cheaper than having the gutter supplier come to the job site.
To make gutters and spouts less noticeable, I place the gutter
against the outside edge of the support beam, tight against the
bottom of the joists (Figure 5), and I run the downspouts down the
support posts. If you build a deck with a flush beam that's on the
same level as the joists, you may need to install a board under the
beam to mount the gutter to, unless the beam is thicker than the
Figure 5. Traditional aluminum or plastic
gutter affixed to the main beam carries away the water from the
Installing the Membrane
At the ends of troughs that are more than about 6 feet long, you'll
need to install baffles to catch any water that may shoot out past
the gutter. Baffles are simply scraps of membrane that span between
the joists, running several inches higher than the trough, and
lapping into the gutter. I attach the baffles to the joists using
stainless steel staples.
One person can install the membrane, but it's much easier with two.
Start the installation of the main troughs by running a bead of
caulk along the top of the first joist (Figure 6). I usually use
black Dap (Dap Products, 800/543-3840, www.dap.com) or Vulkem
(Tremco, 800/321-7906, www.tremcosealants.com) urethane caulk to
match the color of the membrane. Both brands are good quality, flow
well in hot or cold, and are readily available. They adhere to
polypropylene well enough to produce an effective seal.
Figure 6. Polyurethane caulk seals the
membrane to the joists.
With one person on each end of the joist bay to be covered, place
the membrane over the caulk flush with the outside edge of the
first joist. Leave enough excess membrane on the house end to go up
the wall 3 inches (Figure 7), and to run to the center of the
gutter on the outer end.
Figure 7. The siding and tarpaper on the wall
of the house is peeled back to allow the membrane to be integrated
into the ledger flashing system.
Staple the membrane along the first joist about every 2 feet to
hold the membrane in place until the decking is screwed down over
it. I slide the stapler over the membrane on top of the caulk to
flatten it out and get rid of any lumps (Figure 8). Tighten the
membrane as needed to create a smooth trough for the water —
make sure, though, there is adequate slack at the wall end to allow
the membrane to lay flat over the ledger board — and staple
the other side of the membrane to the top of the second
Figure 8. A stapler is used both to attach the
membrane and to smooth out the caulk under the layers of
The steeper the slope, the more easily rainwater can carry out any
debris that falls through the gaps between the decking boards.
Also, placing the low end of the membrane at the top of the gutter
Next, run a bead of caulk along the second joist on top of the
membrane you just stapled in place. I always take care to cover the
staples with caulk so there is no chance of a leak there (Figure
9). Put the next piece of membrane on top of the caulk, stretching
it tight and placing it as before. Overlapping and caulking the
pieces on top of each joist prevents leakage caused by wicking
between the layers. Staple it in place, locating the staples over
the caulk so it will fill the staple holes.
Figure 9. A bead of caulk atop the first piece
of membrane seals the joint. Be sure to caulk over the staples to
Stretch the membrane into a nice trough, paying attention to the
wall junction, and staple the other side of the membrane in
position just as you did the first one. Repeat this procedure with
all the remaining pieces.
The final touch is to cover each membrane seam over the joists with
a self-sealing tape such as 4-inch Vycor Deck Protector (Grace
Construction Products, www.grace
construction.com). This tape seals around the decking screws to
prevent leaks from occurring where the screws penetrate the
membrane (Figure 10).
Figure 10. Grace Deck Protector caps the
joists and the polypropylene membrane, adding a layer of protection
that self-seals around the decking screws. Note the Deck Protector
continuing up the wall, lapping the joint between the pieces of
One important note is that this system may occasionally need
maintenance in the form of debris removal. Usually, flushing the
troughs with a garden hose is enough, but you need access — a
strong argument for screwing down the decking. When the gutter
needs to be cleaned out, access is easy from the underside of the
deck. If you've installed soffit under your deck, it's wise to
install the last board with screws, so it can be easily removed for
A return trough is required from the support beam, whether the
joists hang from it or are cantilevered beyond it (Figure 11).
Typically, the main trough from the house drains to a gutter
mounted on the outer edge of the beam. To drain the cantilevered
portion of such decks, I install a shallower return trough from the
outer edge of the deck to a point overlapping the main trough by
about 6 inches (Figure 12). The water from the return trough will
simply drop into the deeper trough and flow into the gutter.
Putting Water in Its Place
Figure 11. Main and return troughs work in
tandem to drain the deck. The main return feeds directly into the
gutter, aided by a baffle stapled to the main girder to prevent
overshooting runoff from soaking the beam. The return trough passes
over the main beam and drains to the main trough.
Figure 12. Just a slit between the troughs and
the baffle shows water's path into the gutter below. There are a
couple of inches of clearance between the main trough and the
return to prevent debris from hanging up and clogging.
The return trough is the secret to doing angled, round, and
cantilevered decks. Return troughs can be as short as a few inches
or as long as needed. Just be sure to leave at least a couple of
inches of clearance between the return trough and the main trough
so that debris in the main trough doesn't hang up on the return
trough. Install the return trough the same way you did the main
trough, with caulk, staples, and seam tape.
Joining to the House
The junction of the deck to the house wall is a delicate and
important area. It's a common place for leaks to occur, and if the
seal isn't tight, the house can suffer a lot of damage. You will
have to get the membrane over the ledger and at least 3 inches up
under the siding and housewrap or tarpaper to insure it will catch
all the water blown against the wall in a driving rain. You may
need to cut into the siding of the house to do this (Figure 13). I
use Deck Protector seam tape to seal the membrane to the sheathing.
Place a trim board or replace siding as appropriate after the
membrane is run up the wall.
Details Keep Framing Dry
Figure 13. To insure a watertight
deck-to-house connection, the trough membrane and Deck Protector
tape integrates with the ledger flashing.
Deck Protector is also great for detailing around posts and steps
The final step is to screw the decking down to the joists over the
membrane. It is harder to find the joists when they are covered
with the tape and membrane, but to avoid leaks, care must be taken
to be sure the deck screws hit the joists. If you miss, fill the
screw hole you just made with a little caulk so a leak doesn't
Figure 14. To waterproof around details such
as posts (right) and steps (below), the author stacks the layers
with the upper over the lower, just like roofing, and uses Deck
Protector as flashing.
You can install soffit under the deck now, and even can lighting.
If you install can lights, try to get the shorter versions and tell
your customer to use only fluorescent bulbs, to keep the cans cool
and not melt the membrane. I buy IC-rated lights, and place double
foil-faced insulation over them to provide a heat barrier —
in case someone forgets and puts a flood light in the can.
Scott Smith is a deck contractor in Bonney Lake,