Composite Deck Installation on Cape Cod: Built for Salt Air

Composite deck installation on Cape Cod is a framing job first and a decking job second. The boards in the photo are the part that will not wear out. What decides whether the deck under them is still level, still tight to the house and still safe to stand on is the part nobody sees: footings that reach below frost in loose sand, a ledger flashed into cedar shingles the right way, joists spaced for a board that bends more than wood, and hardware that does not rust within sight of salt water.

That photo is one of ours, a composite deck going down on the back of a shingle style house on the Cape, boards on, posts set, railing next. Coast Carpentry Home Improvement builds composite, PVC and wood decks across Barnstable County from our shop at 14 Parker Rd, Unit A in Osterville. Fagner Rodrigues holds the Massachusetts Construction Supervisor license every Coast job runs under, and he walks the yard and the house framing before we price a deck. This is how we build one so it lasts here.

Why does a composite deck on Cape Cod come down to the frame, not the boards?

When we get called to look at a tired deck in Falmouth or Yarmouth, the owner usually points at the boards. The boards are rarely the problem. The ledger is soft where water ran behind it for ten years. The post bases are buried in sand and have been wet since the day they went in. The joist hangers have rust bleeding down the lumber. Pull the deck boards off and you find the deck itself is done.

Composite changes that math in one direction only. The walking surface stops being the thing that wears out, so the frame becomes the whole story. A composite deck built on a quick frame is a very expensive way to find out the frame was quick. We build the frame as if it were the finished product, because once the boards go on it is.

How deep do the footings go in Cape Cod sand?

Dig a hole anywhere between Sandwich and Dennis and you are in sand within a shovel or two. That sand is why Cape basements stay dry and why a lot of Cape decks sit low on one corner. Water moves through it so fast that frost is not the main enemy. Settlement is. A pier poured in a loose hole sits on sand that keeps shifting under it for years.

So we overdig, compact the bottom of the hole with a hand tamper or a plate, and set a footing form with a wide base before the pier goes on it. Where the yard is tight, where the grade drops away toward the water, or where Conservation wants no excavation inside a buffer, we use helical piles screwed to a torque reading instead. Either way the post never touches earth. It sits on a standoff base bolted to the top of the pier, an inch above the concrete, so the end grain can dry.

Posts are six by six, not four by four, on anything more than a step off grade. They are notched and through bolted to a doubled beam, never just toe screwed to its face. Those two choices are what keep a deck from racking in a nor’easter.

How do we attach a deck ledger to a cedar shingle house?

The ledger is where most Cape decks die, and the reason is almost always the same. Someone lag screwed a ledger straight through the cedar shingles. Shingles are not flat, so the ledger stood off the wall on their butts, and water ran down the siding, behind the ledger and into the rim joist for a decade. By the time the deck felt soft, the house framing behind it was soft too.

On a white cedar shingle house, which is most of the houses we work on, the ledger goes against the sheathing, not the siding. We strip two or three courses, lay membrane up the wall and over the rim, set the ledger with half inch hot dipped galvanized or stainless through bolts or structural screws in the staggered pattern the code tables call for, and bend a cap flashing that tucks behind the house wrap above and laps over the ledger below. Then the shingles go back on, cut to clear the decking by a finger width so they can dry. We cover the same flashing details in our guide to white cedar shingle siding on Cape Cod, because a bad ledger ruins good siding just as fast as it ruins a deck.

Some houses should not get a ledger at all. A cantilevered floor, a rim we cannot verify, brick veneer, or an old Cape with a sill we do not trust all call for a freestanding deck on its own beam a few inches off the wall. It costs a row of footings and saves a wall.

What joist spacing does composite decking need?

This is the first thing we check when someone asks us to put composite over an existing wood frame. Most decks built on the Cape before composite was common were framed at 24 inches on center for two by six treated boards. Composite on 24 inch centers feels like a trampoline and sags between joists within a couple of summers. The frame either gets sistered to 16 inches or it gets replaced.

Spacing is only half of it. Every composite board needs solid bearing wherever it ends or changes direction, which means blocking under every seam, doubled joists under a picture frame border, and a flat plane across the whole frame. Composite shows every high joist as a hump. We string the frame and plane the crowns before a single board goes down.

We also tape the tops of the joists and the beam with a self adhered flashing tape. Composite boards shed water into the grooves and onto the joist below, and a treated joist that stays wet under a board that never dries out is how a composite deck ends up with a rotten frame under perfect decking.

Which fasteners and hardware survive salt air?

We have opened up decks in Hyannis where the boards were fine, the joists were fine, and the hangers had rusted to lace. Standard hardware fails early here. Hardware rated for treated lumber and salt exposure loses it slowly enough that the deck outlives you.

For grooved composite we use the manufacturer’s hidden clip system in stainless, which fastens the board at the groove and leaves a clean face with no screw heads to collect water. For square edge boards and the border we use color matched stainless composite screws, and we predrill the ends so the cap does not mushroom. The clip spacing follows the board, one per joist, no skipping. Every connector in the frame, hangers, hurricane ties and the tension ties that hold the rail posts, is hot dipped galvanized or stainless and matched so two different metals never touch each other in the wet.

How do we gap composite boards for heat and cold?

Composite barely cares about moisture and moves with heat instead, mostly along the length of the board. A twenty foot board installed at forty degrees and butted tight against the next one has nowhere to go when the deck is a hundred and thirty degrees in the sun in August. It lifts, or it pushes the border off the frame.

So we read the thermometer before we set end gaps, follow the manufacturer’s chart for that temperature, and leave a gap at every house wall, post and fascia the board will grow into. The fascia gets its own rule. Composite fascia is thin and moves the most, so it is screwed with fascia screws that allow movement, never nailed tight, and it is gapped at the corners.

What are picture framing and breaker boards, and why do we use them?

The cut end of a composite board is the one place the core is exposed, and it is also the end that soaks up sun and grows. On a deck longer than the boards we carry, the old way was to stagger butt joints across the field. The joints open and close with the seasons and every one of them is a line of exposed ends. We run a breaker board across the deck instead, with a doubled joist under it, so every field board ends on a full bearing and no two ends meet in the middle of nowhere.

The picture frame does the same job at the perimeter, and it is what makes a deck look finished instead of cut off. Underneath it there is a doubled rim and blocking every sixteen inches, because a border board with half its width hanging off a single joist will cup.

Which railing holds up on a Cape deck?

Any deck more than thirty inches off grade needs a guard. Code sets the height, the spacing so a four inch ball cannot pass through, and the load the top rail must take. The part the code cannot see is how the post is anchored. We bolt rail posts through the rim and into blocking with a tension tie on the inside, so the load goes into the frame and not into two lag screws in the end grain.

What the rail is made of is mostly about the view and the upkeep. The cable rail on our 84 Alden Drive deck disappears in front of the water. Aluminum and composite rail kits keep their finish without paint. Wood rail on a composite deck is the one combination we talk people out of, because the rail becomes the only thing on the deck that needs a brush every other year. The low maintenance deck guide goes through those trade offs in more detail.

Does a composite deck need airflow underneath?

A deck twelve inches off the lawn with tight lattice skirting and bare dirt under it is a terrarium. Nothing dries. The composite does not rot, but the green film on a north facing deck under the pitch pines is as real on composite as it is on cedar, and the frame below it is sitting in wet air all season. We pitch a low deck slightly away from the house, strip the sod under it, lay fabric and stone so weeds and splash stay down, and skirt it with something that breathes.

The ground side matters most when a deck sits on a slope or near the water. We build those on taller piers and let the air do the work, the same approach we take on a deck in Mashpee where the lot drops toward the Sound.

When is pressure treated or mahogany the better call?

We install a lot of composite because it fits how people use a Cape house. Many owners are here in summer and gone by Columbus Day, and a deck that needs sanding and sealing every other spring is a deck that does not get sanded. It has two honest drawbacks. A dark board in full sun gets too hot to walk on barefoot in July, and it never looks quite like wood up close.

That is why the deck at 84 Alden Drive is mahogany. The owner wanted real wood, the deck faces the water with a cable rail, and they are set up to oil it. Mahogany stays cooler underfoot, cuts and holds a fastener beautifully, and silvers to the same gray as the cedar shingles if you let it. The frame under it is identical to the frame under a composite deck. Only the top layer changed. Pressure treated boards still make sense on a utility deck, a stair landing or a structure you expect to rebuild, and the new deck cost guide covers how each material changes what you pay over time.

Do you need a permit for a composite deck on Cape Cod?

We pull the permit, draw the deck to the code tables, and schedule the footing inspection before concrete goes in, because the inspector wants to see the depth and the base, not a photo of them. In a wetland buffer or a flood zone the Conservation filing comes first and can change the footing type, which is one reason the site visit happens before the quote. An unpermitted deck does not go away. It turns up in a home inspection at the worst possible time.

A deck is often the first step in a bigger exterior project. If siding, windows or a door onto the deck are in the plan, the sequence matters, and our guide to exterior remodeling on Cape Cod walks through what goes first.

Frequently asked questions