A couple of years ago, a landscaper asked us to work a deck into the landscape design for one of his clients. But after the initial budgets for the landscaping and deck came in a little too high, the customer did the right thing and tabled the whole project for a later date. Instead of compromising on their goals, they decided to save up while they refined their design. The result – a very thoughtful, useable outdoor living space that we recently completed – was worth their wait.
The design features a full patio below an elevated deck, so the clients wanted us to use as few posts as possible to support the structure. Fewer posts mean longer spans, which require bigger beams and deeper joists. Another request was an underdeck drainage system, to keep the patio dry. Because the drainage system’s rubber membrane would also keep the joists and beams dry, this seemed like a perfect opportunity to take advantage of I-joists and LVLs to frame the deck.
Engineered Framing
Even if we hadn’t planned on waterproofing the deck, we could have incorporated either Alaskan cedar glulams or PT glulams (probably in about a 5 1/4-inch-by-16-inch size) into the framing. But Boise Cascade, our supplier, uses exterior-grade adhesives in their engineered wood products and warrants them for open but protected applications, including decks that have been covered with a rubber membrane from above and fascia from the sides. With full water protection for the deck, we had the option of using Boise's standard 1.75-inch-by-16-inch Versa-Lam LVLs to build up a 19-foot-6-inch three-ply beam, which was cheaper and easier to install than a single big beam.
Typically, our supplier sizes the beams and provides the fastening schedule. Usually, we nail the plies together as we install them per the schedule (in this case, six 16D nails at 12 inches on-center, both sides), then use structural screws as needed to pull the plies tightly together.
One portion of the deck extended 18 feet out from the house, which – with a flush beam – would have required 2x12 PT joists. In general, we prefer to use uniformly sized joists in a deck, even with varying spans, since different-sized joists tend to dry out at different rates. This can cause a wavy surface in the decking, a problem that we combat by installing midspan blocking. It’s also more difficult to install under-deck finishes when the joists aren’t uniformly sized.
On this project, we used standard 9 ½-inch BCI 6000-series I-joists for the joists, and 1.75-by-9 ½-inch LVLs for the rim joists and ledger (where there was a ledger – more on that below). Since we weren’t waterproofing the stair landing, we framed this conventionally with 2x10 PT joists and beams.
Minimum footing depth in our area is 42 inches, so - as on most projects - we used helical piles to support the framing, subcontracting their installation to a local Techno-Metal Post installer.
Solving the Cantilever/Ledger Connection Problem
The existing deck that we removed was directly attached to a cantilevered section of the house, but this cantilever was never designed to carry a deck load. Our clients didn’t want to interrupt their new below-deck patio with additional posts next to the house to support the cantilever, however, so we needed a different solution. We decided to bypass the cantilever entirely and rest the new deck joists on top of the basement exterior wall, which bears directly on the foundation.
The existing first-floor system was framed with 11 7/8-inch I-joists, which would allow us to slide our 9 ½-inch deck I-joists alongside them until they rested on top of the wall plate. First, though, we had to remove the soffit below the cantilever and then cut away some of the spray foam used to insulate the joist bays. Luckily, the original insulation contractor didn’t do a very good job insulating the cantilever, so there were a number of voids that were missing foam. This made removing all of the foam in the cantilever a bit easier.
Once the foam was gone, we cut out channels in the exterior rim where we could slide our new joists in. To make it easier to re-insulate the assembly with foam later on, we were careful to cut these very accurately and only slightly oversize them.
The existing house joists were spaced 19.2 inches on-center, while we were installing the new deck joists 12 inches on-center. So every 8 feet along the length of the 22-foot-long cantilever, a deck joist is aligned with a house joist. There were also a few oddly-spaced joists where this occurred. At all of these locations, we installed Simpson Strong-Tie TP37 tie plates on the underside of the joists to connect the house joist to the deck joist where they overlapped. This was done to meet lateral load connection requirements.
Where the new deck I-joists simply rested on top of the wall, we fastened them with three 16D nails toenailed through the bottom chord into the top plate.
The original deck didn’t extend past the cantilever, so here we had to remove the siding and relocate a bathroom exhaust vent in order to install a new ledger. After bolting a 2x10 PT ledger to the house framing, we built out the ledger with an additional two-ply LVL beam that extends past the house, and is supported at one end by a 6x6 column.
We started to see the benefits of using I-Joists over standard green treated lumber almost immediately. For one thing, the joists are consistently sized, so the top-flange I-joist hangers we used could all be set to the same height, making them much quicker and easier to install than standard joist hangers. We were able to lay out and install all of the hangers on both the ledger and the beam and then simply drop each joist into place.
Compare that with sawn framing, where we typically toenail the joists flush with the top of the ledger and beam and then come back afterward to install the hangers. This technique leads to some precarious balancing on top of scaffolding to hold a 100-pound joist flush while toenailing it in place. Weighing only about 20 pounds or so, an 18-foot-long 9 ¼-inch I-joist can be easily handled by one guy.
Best of all, our deck was perfectly flat right away, compared with one framed with conventional PT lumber, where getting the joists installed is just the first step in framing it. Next, we would have to install midspan blocking every 6 feet on-center to help straighten out any cups in the joists. What we couldn’t fix with midspan blocking, we would then have to plane down to get everything lined up, a tedious and time-consuming – but absolutely critical – step.
Waterproofing
Our water protection system started out as a 100-foot-by-20-foot roll of 50-mil EPDM rubber. Using the on-center distance between each set of joists as well as the distance from the ledger on the house to the front rim (plus a couple of extra calculations), we laid out our troughs on the full sheets of EPDM. The resulting strips had a tapered shape that is roughly 4 inches wider at the beam end.
After cutting and labeling the troughs, we cut out the bibs, which were 14-inch-by-18-inch rectangles installed at the front of the beam to direct water flowing down the trough into the gutter. We also cut perimeter strips from the membrane, which varied in size and were used to cover the front rim and any flat blocks that direct water from the front edge back into the main troughs.
Installation started with the bibs. These were stapled tight to the front rim and flush with the top of the joists, so that they wrapped around the each side of the joist bay a few inches. The bibs hung below the joists by about 4 or 5 inches, and were trimmed to length once the gutter is installed (the main thing here was to make sure the bib hung into the gutter).
Next, we installed the troughs, starting at the house. We tucked each trough up behind the housewrap 6 inches and sealed it to the sheathing with neoprene flashing cement. We then stapled the troughs down along one entire joist before starting over at the house on the other side. Since the troughs were tapered, installing one edge of the rubber along the edge of the joist automatically created a pitch away from the house as the trough started tight and loosened up at the front rim. To ensure that water will flow into the bib and gutter, we trimmed away any excess trough material.
As we installed each trough, we ran a bead of neoprene flashing cement over the previous trough on top of the joist, where the two troughs would overlap. When we were done installing the troughs, we taped the seams on top of the joists with 4-inch-wide Vycor, a self-sealing flashing tape that added another layer of protection to the assembly.
Finally, we taped any other seams in the membrane, as well as where the Tyvek housewrap overlapped the rubber membrane. This ensured any water running down the Tyvek will be directed into the trough system.
Sleepers
We installed the decking over a sleeper system, which consisted of treated 2x4s installed 12 inches on-center perpendicular to the main joist framing. The sleepers allowed us to complete the initial framing quicker since we didn’t have to consider where we might have seam boards in the decking or where we might need tail catchers for diagonal decking, and so on. The sleepers also simplified water protection under the perimeter picture-framed border, which would have otherwise required additional blocking. Where the border ran parallel with the sleepers across the front of the deck, we installed short 2x4 blocks directly on top of the joists and beam.
At all of our railing post locations, we screwed 2x10 flat blocks directly into the joists below, and toe-screwed them into any adjacent sleepers. We put a dab of neoprene sealent between any 2-by sleeper or blocking and the rubber and fastened them to the framing with pairs of #10 2 ½-inch structural screws. The sleepers and blocking help lock the entire system together, making for a very rigid structure. Afterward, we hit everything with a quick coat of black paint so that the sleepers and blocking wouldn’t be visible between the gaps in the deck boards.
Before installing any decking, we always thoroughly water-test an underdeck drainage system by flooding it, especially in all the corners along the house. If we find any leaks, we figure out the cause and repair that defect, rather than just blindly filling the area with more caulk.
Finishing Up
On this project, we installed Azek PVC decking using TigerClaw clips and guns. Around the perimeter, we fastened the picture-frame border with 2-inch (rather than 2 ½-inch) Cortex screws and plugs so that we didn’t run all the way through our sleepers and into the rubber below. Since 2-inch screws have a little less bite than 2 ½-inch screws, we spaced them more closely together.
We installed a Westbury aluminum railing system with top-mounted posts. Because we had installed flat blocking between the joists at all post locations, as well as additional blocking as part of the sleeper system, we had a solid 3 inches of framing and one inch of decking for fastening our posts. After predrilling the holes for the GRK RSS lag screws, we squirted some neoprene sealant into them prior to installing the post fasteners.
After one more water test, we moved on to finishing the underside of the deck. First, we installed an aluminum gutter system across the front edge to direct the water from the bibs out into the landscaping. After the electrician installed the recessed lighting cans and 110-volt wiring, we finished the ceiling with 5 ½-inch WP4 tongue-and-groove cedar.
To match the house, we trimmed out the posts and beams with LP SmartTrim. After we were done, our painter came in and stained the cedar and painted all the trim. He was followed by the landscaping crew, who installed a concrete sport court, boulder retaining walls, and the paver patio underneath the deck (to see a timelapse video of the entire project compressed into one minute, click here).
