Even if the deck doesn't sit high enough off the ground for you to use the area beneath as living space, you can turn that square footage into an ideal storage room for lawn and garden equipment - and you won't have to sacrifice a single square foot of lawn or garden to do it.
Although your under-deck enclosure will likely differ from ours in overall dimension, any project of this nature will share the same basic steps: addressing water management, establishing a solid foundation and building walls and doors.
To create a dry space beneath the deck, you’ll need to install a below-deck water barrier that diverts the rainwater that normally flows between the deck boards. Then you’ll build a foundation of 6x6 pressure-treated timbers that will rest on a bed of compacted gravel.
Next, you’ll frame the walls and doors with pressure-treated 2x4s and cover them with 19/32-in.-thick grooved plywood siding (commonly known as T1-11) and 1x6 pressure-treated trim. Finally, you can install treated diagonal lattice to encourage climbing plants that will camouflage the finished enclosure.
The costs of building an under-deck enclosure will vary. For a room the size of our project (about 12 x 14 ft.), expect to spend $550 to $650 for the building materials, which include the framing lumber, plywood, lattice, trim, stain and hardware. The cost of the water-diversion system will depend on the brand you purchase and whether it’s contractor-installed. DIY systems can cost as little as $6 a square foot, whereas contractor-installed systems can cost as much as $12 a square foot.
A variety of products is available to divert the water that flows between deck boards. Some are dealer-installed and some are for the DIY market, but not all are appropriate for every deck. We were limited by 24-in.-OC deck joists (not an uncommon situation, though 16 in. OC is more prevalent in newer construction). As a result, we chose a system from Underdeck (see SOURCES) that offered panels sized for our situation (photos 1-4).
Regardless of the brand, it’s important to understand the details of your deck construction before you order a kit or hire a contractor. Take careful measurements, note the placement of support posts or beams and study the requirements of each brand to ensure that the system is right for your deck.
Collecting the water is only half of the battle — you still have to direct it somewhere. For our project, I connected the gutters for the Underdeck system (via vinyl downspouts, photos 5-7) to 4-in. corrugated drain lines buried beneath the 6x6 timbers that form the foundation. (Be sure to install any buried drain lines before you lay the base for the foundation.)
The drain lines slope away from the structure and lead to pop-up bubblers, which rise above grade only when water pressure builds in the drain line, such as during a rainstorm. When it’s not raining, the pop-ups lie flat, slightly below sod level. Corrugated lines, pop-up bubblers and a variety of adapters, elbows and fittings are available at most home centers. By combining fittings, you should have little trouble assembling a drain line layout that works for your deck and yard elevation.
Building the Structure
To establish a stable foundation, start by marking the perimeter of the enclosure. Drop a plumb line from the corner points of the deck. Dig 9-in.-deep x 9-in.-wide trenches centered on the perimeter line; then fill and level the trench two-thirds full with gravel. Use a short length of 6x6 post to compact the gravel once it is level.
Next, set the 6x6 foundation timbers — I used treated lumber— on top of the compacted gravel. Fasten them together with 12-in.-long landscaping timber screws.
Drop the corner plumb lines a second time to mark the location on the 6x6 foundation for the sill plates; then cut the sills to length and nail them to the timber foundation (photo 8). Nail top plates to the underside of the deck’s rim joists, making sure the plates are flush with the exterior edges of the rims (photo 9). Measure, cut and toe-nail into place the wall studs, setting them 16 in. OC. Don’t be tempted to cut all of the studs to length at once, as most decks slope away from the house to allow for drainage, and you’ll need to size the studs to follow that pitch.
Cover the framework by cutting the plywood siding to size (again, remember to allow for the deck’s pitch) and attaching it to the studs (photo 10). T1-11 comes with a shiplap edge, so cut the panels so that the edges meet correctly. Nail 1x6 trim to the top, bottom and sides of the walls, and then cut the lattice panels to fit into the openings. Because lattice comes in 4-ft.-wide sheets, you’ll need to nail 1x6 separators between sections to obtain the cleanest look (see drawing, above, and photo 11).
Use ring-shank nails, screws or staples to attach the lattice to the paneling; then cut a second layer of 1x6 trim and overlap it 2 in. past the edges of the lattice panels. That way, the weight of future climbing plants won’t pull the lattice free.
Assembling the doors
To provide easy access (and to avoid having to allow space for door swing), I built twin sliding doors rather than installing traditional hinged ones. L.E. Johnson’s wall-mount pocket-door track makes this type of installation easy — once you level the aluminum track, simply screw it to the exterior face of the front of the structure (photos 12-13). As an added benefit, eliminating traditional hinges means you don’t have to worry that the doors will develop sags and fail to shut properly.
To build the doors, first determine their size by measuring the height and width of the overall opening. Divide the width by 2 (as there are two doors), and subtract 1/2 in. from the height (to allow for sliding clearance). Build the door frames from 2x4s cut to match the dimensions you’ve determined; then attach a skin of T1-11 panels to the door frames flush with the edges (photo 14). Trim the perimeter of the door fronts with 1x6 strips, and add decorative diagonal battens. Attach the hanging hardware to the top edge of each door following the manufacturer’s instructions; then slide the doors into their tracks (photo 15).
For the interior floor, use a material that will provide good drainage in the event of an accidental water leak. The ground beneath my deck was already covered with a layer of granite landscape rock (which works fine for drainage), but you could also use compacted gravel, brick pavers or 2x2 concrete patio pavers.
Once the flooring is in place, stain the structure (I used ZAR’s Rain Stain semitransparent deck stain), install a few battery-powered tap lights at strategic points inside and attach any remaining hardware such as handles, hasps and locks. All that’s left is to haul your gardening tools to their secluded new home.
Step 1: Begin installing the Underdeck system by snapping a pitch line. Start the line against the house, no higher than 3-1/2 in. from the bottom of the joist, and allow a drop in pitch of1 in. for every 10 ft. of joist length.
Step 2: Following the chalk line, use 1-1/2-in. galvanized roofing nails to fasten the joist rails to the joists. Start 1 in. from the ledger board and allow a 2-in. gap between rails for drainage.
Step 3: Starting at the ledger board, snap the collector panels into the joist rails. First insert one long edge between the upper and lower flanges of the joist rails; then gently push the other edge into place.
Step 4: Use a utility knife or hacksaw to cut the joist gutters to length; then firmly snap them into place onto the joist rails.
Step 5: Attach the main collection gutter (into which the joist gutters discharge) to the support member of the deck. For every foot of gutter, allow a 1/8-in. drop in pitch.
Step 6: Before laying the foundation, bury corrugated flexible drain line and pop-up diverters to carry the runoff away.
Step 7: Install downspouts from the main collection gutter to the underground diverter.
Step 8: Start construction of the walls by first nailing treated 2x4s to the treated 6x6 foundation members.
Stpe 9: Nail the top plates to the deck framing structure; then toe-nail the 2x4 studs in place.
Step 10: Cut exterior-rated plywood siding to size and nail it in place. Orient the panel sections so they properly overlap.
Step 11: Attach the first layer of 1x6 trim; then cut the lattice to size and nail it to the structure. Cut the second layer of 1x6 trim to size and fasten it so that it
overlaps the first layer and traps the lattice in position.
Step 12: Position and temporarily clamp the door track to the structure and check that it is level.
Step 13: Drill pilot holes through the track’s upper flange, and fasten the track to the structure using 2-in. stainless steel panhead screws.
Step 14: Assemble the doors by first building 2x4 frames. Cover the frames flush with T1-11 panels; then use stainless ring-shank nails to attach 1x6 trim and battens.
Step 15: asten the hanging hardware to the top edge of the doors; then slide them into the tracks. If necessary, make adjustments to the hangers, following the manufacturer’s instructions.