As the owner of an Archadeck franchise in suburban Boston, I've been building residential decks full-time for 18 years. Usually we fasten the deck to the house with a structurally attached ledger, and the house wall provides lateral and horizontal bracing as well as vertical support. But sometimes circumstances force us to build a self-supporting freestanding deck. I don't mean that the deck is all alone out in the yard - just that it doesn't rely on the house for bracing or support. Typically, it touches the house and appears to be connected, but the framing underneath is structurally independent. In this article, I'll discuss the methods we use to stabilize a freestanding deck.
Why a Freestanding Deck?
The house shown on this page is clad with brick veneer. You can't make a structural attachment to brick veneer, and going through the brick to attach a ledger to the framing behind it is not a practical option. In such a case, a freestanding deck makes sense. But there are other reasons we might choose not to tie a deck to a house. Take, for example, a garrison colonial, where the second-floor framing cantilevers a foot or two past the wall below. Those overhangs are almost never designed to support the additional weight of a deck. We have in fact attached a deck to such a cantilever - we did so for a job just last year. But that was a rare case where the house had been engineered from the beginning to support a future deck, and the house framers had configured the joists, the band joist, and the framing connections for that purpose. Interestingly, they attached the house band to the floor joists with upside-down hangers - since the cantilevered joists supported the band, and not vice versa. But with a conventionally framed garrison house, attaching a deck to the cantilever would be downright dangerous.
I also hesitate to attach decks to concrete block foundations. Concrete is very strong in compression, which makes it good for supporting vertical weight. But it's weak in tension, and bolts set into the 1-inch-thick concrete of a hollow-core block have little withdrawal strength. We've attached decks to block walls on occasion, though; the trick is to install a horizontal 2x6 cleat on the inside, then run bolts all the way through the ledger, the blocks, and the cleat, to tie them together. The cleat acts like a big washer that spreads the withdrawal load across 10 or 12 feet of wall. But we can't always access the back of the block wall; sometimes it's a frost wall that's backfilled on the inside, or often there's a finished basement we'd rather not disturb.
In such situations, it's safer and easier for us to support the deck with another set of braced columns and a beam outside the house, resting on footings set below the frost line. Doing this may add cost, but it avoids some structural traps and gives us total control. We can pre-engineer our structure, and we don't have to guess about the house wall's construction or its capacity.
When you separate a deck from the house, you lose some inherent stability - you can feel the difference while you're framing the deck. Sometimes my carpenters will put a few screws into the side of the house to temporarily stiffen the framing while we're working. But for long-term structural stability, we can't rely on a few screws; we have to build the stiffness into the deck's own structure.
Every freestanding deck needs a system of connections strong enough to prevent it from collapsing or breaking apart. But avoiding catastrophic failure isn't enough - beyond being safe, the deck should also feel safe. Walking on a deck should feel like walking on the ground. The connections should be tight enough to make the deck act and feel like a single, rigid structure.
Decks can move in three directions: vertically up or down; horizontally parallel to the house wall; and laterally away from the house. Our goal is to provide support and bracing in all three directions.
We support our decks on 6x6 columns, which rest on footings set 4 feet below grade (the frost depth in our area). Built-up pressure-treated lumber beams sit in notches in the tops of the columns and are through-bolted into place. In the past we used standard galvanized bolts for this connection, but recently we've switched over to FastenMaster ThruLoks, which self-tap like screws but have a nut on the end that cinches the joint tight.
Screw piers. A while ago I began using a helical pier system from TechnoMetal Post (technometalpostusa.com). These piers have a number of advantages over the more traditional poured concrete Sonotube piers. First, there's almost no disturbance of the soil with this approach - which can be very important to the mason if there's a stone patio under the deck. So, for example, before we switched over to helical piers, we would have dug a dozen 4-foot-deep holes for concrete footings for the project shown on the first page of this article. Whether we used a mini-excavator or a Bobcat with a backhoe - or even dug by hand - we would have been left with a lot of large holes to deal with (the hole for a 12-inch Sonotube is at least 18 inches across). We would have had to backfill them with good soil or gravel, then attempt to compact around the piers. A mechanical compactor can compact 6 to 10 inches of material at a time, but it's not going to successfully compact a 4-foot-deep hole. The fill around the footings would inevitably settle over time, which would not make the stone mason happy.
Helical piers are a great solution because there's simply no excavation to disturb the soil. On the same project, the mason came in before we started, dug out the topsoil, put in a 6-inch-thick aggregate base, and compacted it. There was no deck in the way, so he could easily prep the entire 30-by-50-foot area before the pier driver came in. Each pier has a 2-inch-diameter galvanized steel shaft with a screw at the end; the pier driver, powered by an 8-horse motor and hydraulic pump, screws the piers into the earth to a depth of 4 feet or more, disturbing only the 6-inch-diameter spots where the piers enter the ground. At that point we can come in and build our deck, or the mason can lay the patio - the sequence doesn't matter.
Another advantage is that helical piers provide verified support for the load-bearing columns. When I draw a footing plan, I calculate the design load for each footing and give this data to the helical footing subcontractor. The pier machine has a dial that measures the torque as the screw helix spins into the earth. There's a direct relationship between this torque and the soil's bearing capacity. Depending on the helix diameter and the soil characteristics, the screw footings can support from 2,482 pounds to more than 8,000 pounds. The installers always exceed my support requirements because it's so easy to do: They just keep turning the helix in until they reach the torque associated with the bearing capacity that I need, then go a little beyond. On completion, they give me a signed report that specifies the guaranteed load-bearing capacity of each pier they install. I don't even need a footing inspection.
The installers can angle the piers as needed to avoid rocks; occasionally they hit solid ledge, which is fine as it provides great support. Often they'll go all the way down to the full 7-foot length of the tube; sometimes the soil is so dense they stop at 4 feet. They cut off the excess shaft length with a small portable band saw, then either bolt or weld a steel post-bracket to the end.
Uplift protection. We attach the columns to the steel brackets with four HeadLok screws, creating a continuous load path that not only supports the weight of the deck but also resists wind uplift.