We recently completed a project that we only half-jokingly referred to as the M.O.A.D., or mother-of-all-decks. While the size—more than 700 square feet with all the bells and whistles—was impressive, there were a couple of structural challenges that helped earn the deck our nickname. The first was that the homeowners wanted the 37-foot-wide elevated deck to be supported by a minimal number of columns in order to preserve the views from the living space underneath the deck. The second challenge was that the deck would be built in a seismic zone.

Living and working in California southeast of Los Angeles, we have a lot of experience building earthquake-proof decks. Often the decks are located on the side of a hill, because that’s where the views are, so they require substantial engineered footings and plenty of diagonal bracing.

In this case, however, the deck would be located on flat ground on top of an existing concrete patio. Instead of breaking up the patio to dig holes for individual footings that were deep and wide, we planned to install a continuous grade-beam footing that would allow us to tie the steel support posts to the foundation. The lateral support for the deck framing would come from the grade-beam footing and structural connections between the footing, the columns, and the deck frame, eliminating the need for diagonal braces.

Grade-Beam Details

Working from the architectural plans drawn by the engineer of record, we laid out the foundation directly on the concrete patio. We contracted with a saw cutting company to open up a one-foot-wide slot for the grade beam around the patio perimeter and to clean up the broken concrete afterward.

To begin the project, workers laid out and cut a 12-inch-wide slot through the concrete patio for new engineered grade-beam footings, which were needed to provide lateral support for the freestanding deck.

Before excavating through the slot in the patio for the footings with a backhoe, I checked with the homeowner about utilities. He brought out a pair of dowsing rods and immediately identified the location of water lines for the sprinkler system. Skeptical, we carefully dug down in the location that he had marked, and sure enough, the water line was right where he indicated it would be. Still skeptical, I tried the rods myself, and when I put them over the pipe, they closed, and when I pulled them away from the pipe, they opened. They worked without a doubt. I always had laughed off dowsing rods, but even though I’m baffled as to how they actually work, I’m now a believer.

Partially assembled rebar cages were delivered to the job site. Workers finished tying them together after the steel reinforcement was placed in the 24-inch-deep trenches.
Then the crew carefully formed up the piers for the deck’s steel support columns to ensure that the deck ended up at the proper elevation. Steel reinforcement is continuous through the piers and grade beam.
Prior to pouring concrete, rebar dowels were added to the cut edges of the patio that would tie the existing concrete slab to the new grade beam and prevent the edges from settling.
The completed grade-beam footing. Nuts were threaded onto each pier's four anchor bolts, which would later be used to fine-tune the height of the columns and adjust them for plumb.

After cleaning up the 24-inch-deep trenches for the grade-beam footing, we turned our attention to the pad footings for the steel support columns. The columns would be bolted to the footings, which in turn would be tied into the grade beam, a seismic design that would allow us to eliminate knee bracing.

We spent the better part of a day fine-tuning the trench for the footing and chipping away any concrete that was in the holes. Meanwhile, because we would need rebar cages to reinforce the grade beam, WC Rebar (our local rod busting company) fabricated as much of the rebar cages as they could in their shop, then delivered them to the site. The rebar plan had an overlapping splicing detail that didn’t require any field welding, and we were able to complete the assembly on site when we placed the cages in the trench.

To provide support for the cut edges of the concrete patio, we doweled the footing and slab together, drilling holes for rebar in the slab every 16 inches and alternating sides every 8 inches.

Once we had placed the steel for the grade beam and built the forms for the pad footings, we tied off the connections between the rebar cages and footing reinforcements per the engineering plans. Then we called the building department for our foundation inspection and ordered 16 yards of 4,500-psi concrete for the pour, which was delivered by pump truck.

I wanted to wait until the grade beam was poured and we had stripped the forms from the piers to determine an exact height for the steel columns that would support the deck framing. Fortunately, the tops of all of the piers were within 1/8 inch of each other, well within dry-packing tolerances, so all of the columns could be specced at the same height.

While we waited for the concrete to cure and the fabricator to assemble and deliver the steel columns, we stripped off the stucco cladding from the house wall where the deck would be located to expose the sheathing and weather-resistive barrier. Though the deck would be freestanding without a structural ledger, we needed to tie the deck’s under-deck waterproofing to the house’s WRB, and we needed to prep the wall for the deck-to-wall flashing detail. In addition, the bottom of the beam along the house would be acting as a stucco-covered soffit, and we needed to be able to tie that finish to the wall stucco.

Steel Columns

The steel columns that support the deck’s carrying beams are the key to the seismic design. Weighing about 250 pounds each, they were fabricated, from the base plates to the special top brackets, by certified welders at Schorr Metal. Their overall height was critical to accommodate a spiral staircase that I had already ordered.

Fabricated by a certified welding shop, each of the engineered steel columns weighed about 250 pounds; a portable contractor’s lift was used to hoist them into position on top of the concrete piers.
To plumb the columns and adjust their height, workers turned the nuts supporting the columns with a box wrench. Then they packed the voids between the column base plates and piers with non-shrinking grout.

Before setting each column, I sprayed its bottom face with red rust primer. Because of their weight, we lifted the columns into position using a Sumner portable contractor lift, which can lift loads of as much as 650 pounds almost 13 feet high. The lift is heavy, weighing more than 250 pounds, so it’s a bit of work to transport it to and from a jobsite. But on this job, it proved invaluable, as we also used it to lift the heavy beams into position once the columns were bolted in place.

The columns sit on nuts threaded onto the 3/4-inch-diameter bolts embedded in the piers, allowing us to use a wrench to fine-tune the column heights and make sure they were plumb. Afterward, we packed the bases with non-shrinking grout.


The next step was to lift the heavy beams into place, through-bolt them to their brackets with 3/4-inch-diameter bolts, and begin framing the deck. Most of the deck framing is non-PT Doug fir, with solid-sawn No. 1 Doug fir 6x12s for the seismic beams, but we used 7x14 PSLs (parallel strand lumber) to assemble the main carrying beams. The center post is actually off-center, to maintain views from one of the house windows under the deck, so the longest PSL span is 19 feet. With a very dry climate, the deck protected by an under-deck drainage system, and the beams completely boxed in and 100% impervious to the elements, treated lumber wasn’t required.

With the columns bolted to the piers, workers used the portable lift to hoist the heavy 7x14 PSL beams into place.
The 7x14 PSLs also provide support for the freestanding deck at the house. The inside and outside carrying beams were connected to each other with 6x12 Doug fir beams, which were fastened with 3/4-inch-diameter through-bolts to brackets welded to the steel columns.

Typically, we install the joists first using toenails, and then go back later to install the hangers. That way we can ensure that the tops of the joists are flush with the tops of the beams, regardless of any dimensional discrepancies in joist depth. Also, we like to wait a few days to allow for any shrinkage that might occur. In the past, we’ve installed joist hangers as we’ve installed the joists, only to return after a week or so to find that the joists have shrunk and left 1/8‑inch gaps or more between the seat of the hanger and the joist, which is unacceptable both to us and to our building inspector.

After the joists were installed, we blocked out the outer flush beam and seismic beams with 2x12s, to which we through-bolted the 4x4 guard posts. Then we blocked out the base of the posts with another 2x12, which created a rock-solid post-to-framing connection and plenty of bearing for the decking under the post, and ensured that the post sleeves that we used to finish the posts sat properly on the decking.

2x12 Doug fir joists were hung from the carrying beams. Because the entire deck would be protected by a waterproofing system, non-PT-treated material and engineered lumber was used in its construction.
4x4 deck posts were bolted to the framing, then blocked out with 2x12s sandwiched around the posts, which reinforce the posts and provide support for the outermost course of decking.

One detail that threw a major monkey wrench into the project was a support column that the engineer had located too close to a doorway into the house. While the problem wasn’t apparent on the plans, the homeowner felt that the column would unacceptably crowd the entry area and hoped that the column could be moved or eliminated altogether.

The solution that my engineer and I came up with was to open up the wall of an angled bump-out next to the column location and install a new 4x12 header. Then the 7x14 PSL could be extended a bit so that it could bear on that header.

Flashing that detail proved to be a challenge, but my stucco guy did an outstanding job with the metal around that area.

Installing a steel column to support the carrying beam at the house here at the door would have blocked views from inside the house. Instead, workers opened up the bump-out wall and framed a new header to support the beam.

Water Management

When we laid out the mounting bolts for one of the corner columns, we offset them to accommodate the downspout that connects to the gutter that drains the water collected by the Trex RainEscape under-deck drainage system. The downspout runs down inside the OSB box we built around the column, the substrate for the eventual stucco finish. Because I was concerned about the system’s ability to handle runoff from the roof above, as well as from the deck, I consulted with several gutter experts for sizing help and to make sure our installation would be able to handle the volume of water that could be expected to run off the deck and from the existing roof above the deck.

With 2x12 joists over a 14-foot span, the pitch of the RainEscape troughs is less than 4 degrees. That’s fairly shallow, which means that water drains very slowly off the deck, into the one-piece gutter, and down the 3-inch-diameter round downspout. We always water-test our waterproofing systems before installing the decking, and our initial testing showed that this one was able to handle rapidly dumped 15-gallon loads, with more than enough gutter and downspout capacity for that volume of water.

A worker installs the one-piece gutter that will drain water collected by the under-deck drain system as it flows toward the outside edge of the deck (above).The gutter slopes toward a corner column, where it is drained by a 3-inch-diameter downspout concealed within the boxed column (right).
Trex RainEscape downspouts were installed in each joist bay to empty water collected by the troughs that are draped over the tops of the joists into the gutter.
At the house, a custom-fabricated flashing and weep screed tie the WRB behind the stucco to the deck waterproofing system.

To raise the underside of the decking up away from the troughs near the house and promote drainage, we had installed ground-contact-rated pressure-treated 1-by furring over the joists after the RainEscape system was installed. The 1-1/2-inch-wide rippings were nailed to the joists through the butyl seam tape used to seal the troughs, which self-sealed around the fasteners. Finally, we laid down sheathing over the joists and covered everything with a protective plastic wrap to prep the site for the stucco guys and then the painters. We’ve learned the hard way that cleaning stucco overspray off composite decking is no fun.

Unfortunately for me—but fortunately for the job—it was during this stage in the project when I had an unrelated ladder accident (stringing Christmas lights up with my family). This had the effect of stalling our work on the project while I recuperated. Southern California isn’t known for its wet weather, but several winter downpours while the decking installation was on hold indicated that everything was leakproof.


We subcontracted out the stucco work, which including patching the wall where we had tied the deck waterproofing to the house. Here, we had installed 2-by framing on the flat to bridge the gap between the structural beam next to the house and the house sheathing. Covered with self-adhered flashing, this provided a solid backing for the custom metal flashing and weep screed that integrated the home’s WRB with the deck drainage system.

On the underside of the deck, we had framed out a soffit next to the house and boxed in the gutter and columns with OSB sheathing before starting the ceiling installation and turning the project over to the stucco guys and painters.

Underneath the perimeter of the deck, workers framed a soffit and covered it with OSB in preparation for a stucco finish.
After covering the soffit and boxed columns with building paper and wire lath for the stucco, workers installed the tongue-and-groove wood plank ceiling.
J trim was installed at the corners where the ceiling will meet the stucco to give the installation a clean look.

Because the ceiling height is 9 feet, the space underneath the deck doesn’t feel like a cave. For the owners, the space offers welcome shade from the sun, which hammers that spot in the summer.

Decking ... and a Slow Leak

After we resumed work on the project and my crew had almost completed the decking install, we discovered a small drip (one drop every 20 seconds or so) coming from a knot hole in the wood ceiling. To track down and fix the leak, we needed to remove almost half of the decking to redo a complete bay with a new trough and butyl tape. The culprit? A screw through metal flashing around a hose bib that we had installed on the second level. The metal flashing had mushroomed up around the screw, preventing the butyl flashing tape from self-sealing around the screw. Lesson learned.

Workers fastened the Trex decking down with FusionLoc fasteners, installed with FastenMaster’s FiveShot collated nail gun. To provide extra clearance between the underside of the decking and the RainEscape troughs and to improve drainage, 1-by PT rippings were installed on top of the butyl tape used to seal the troughs where they lapped over the joists.
A seam board in the middle of the 37-foot-wide deck provides a transition point for the decking.

Finish Details

Besides the decking and railing installation, our final major task was to install the Trex spiral stair, which had already been fabricated and delivered to the site. We were anxious that we had nailed our elevation for the stair landing, as there is no adjusting of the rise or run after the stairs have been fabricated. Happily, the landing elevation was spot on and the stairs installed without a hitch, except for the tricky landing detail at the top of the stairs (we wish that Trex made wider tread stock, so that we could have more cleanly finessed the picture-frame detail with the rest of the decking). Next time, I’ll run the decking wild at the landing and allow the ends to come all the way to the landing.

While the spiral stair assembly was straightforward, accurately calculating the bottom landing elevation when placing the foundation was critical because the stair parts were ordered in advance and fabricated off-site.
The stair rail came in a coil, and had to be carefully stretched out to avoid kinking it prior to installation.
Sections of drink rail were fabricated from the decking and fastened to the top rail with screws that thread into inserts installed on the underside of the drink rail.
The stairs came with precut treads to match the decking, but the upper landing was picture-framed on site; because of the deck board dimensions, a narrow strip was needed between the picture-frame detail and the edge of the decking.
The new stucco finish on the columns and soffit was painted to match the existing house. Though not shown in the photo, a new door that provides access to the upper level was installed in a follow-up project.
To wrap up the project, the old concrete patio was prepped to receive new plank-style patio tiles patterned to match the deck’s wood ceiling.

After that, our tile installer finished the patio with a plank-style wood-grain tile, and our job was done.

Photos by Michael Walter.