[Editor's note: Jim Finlay's comprehensive feature article, Building a Deck on Grade, is coming in next month's JLC. For an appetizer, here's a sneak preview of Finlay's bench seating detail, with a few extra photos. Take a look.]

Our customer’s requirements were simple: Build an interesting deck and patio off the back of the house. The deck would be low enough to make rails unnecessary. Building the patio was straightforward, but building the deck became challenging. Here in New England, we like to set the height of our decks one riser below the interior floor (a step down of 6 to 7 1/2 inches), to help keep rain and snow out of the house. The area for our 15-by-17-foot deck was mostly flat and about 14 1/2 inches below the kitchen floor. So we would set the deck 6 inches below the floor, leaving 8 1/2 inches for the deck.

A specialty contractor drives a helical pier into the soil until the torque required for driving the screw reaches the level that indicates sufficient bearing capacity (above left). At right, a worker installs one of the brackets that will support the deck's carrying beam. Photos by Jim Finlay
A specialty contractor drives a helical pier into the soil until the torque required for driving the screw reaches the level that indicates sufficient bearing capacity (above left). At right, a worker installs one of the brackets that will support the deck's carrying beam. Photos by Jim Finlay

We opted for post-and-beam framing: we would locate a carrying beam about 2 feet back from the deck’s front edge, under the joists. Because the structure of joists on top of a beam would occupy 16 1/2 vertical inches, the beam would need to be set into the ground, with its top just about at grade level. We used helical footings—long galvanized pipes with a large screw at the end. We then attached adjustable brackets to support our beam.

Finlay's crew placed crushed stone around the beam for free drainage and frost protection, not for load-bearing purposes: the steel brackets of the helical piers, driven to below the frost line, are the deck's structural support.
Finlay's crew placed crushed stone around the beam for free drainage and frost protection, not for load-bearing purposes: the steel brackets of the helical piers, driven to below the frost line, are the deck's structural support.

As planned, the step down from the deck to the patio was about 8 1/2 inches, but sloping ground on the right side increased that drop to almost 16 inches—too tall to comfortably or safely step off. We addressed this hazard by building a bench along the edge of the deck. A standard bench with pairs of legs might have looked too massive for this small deck. We opted instead for a “floating bench,” which employs single 4x4 legs that extend down into the deck frame.

The key to making the bench solid and stable is to through-bolt each leg between two joists below and between pairs of cross supports on the upper frame (see slideshow, above). We clad the bench frame with Fiberon’s synthetic ProTect fascia, ripped to height, and softened the bench edges to improve sitting comfort by holding that trim 1/4 inch below the seat. Treated wood shrinks considerably as it dries during the first months of its exposure, and the initially tight joints in our bench may become loose and wobbly in six or nine months. Fortunately, this shrinkage stops and the wood remains moderately stable thereafter. So we’ll return within a year and tighten our connections.

For long-term durability in service, Finlay's ground-level beam and the deck it supports are framed with preservative treated wood labeled for use in ground contact, as called for in the 2016 American Wood Protection Association standard U1.
For long-term durability in service, Finlay's ground-level beam and the deck it supports are framed with preservative treated wood labeled for use in ground contact, as called for in the 2016 American Wood Protection Association standard U1.