What should I do about the fascia and the open space under my deck?

Fascia: what it covers and what it should be

The fascia is the board that runs along the outside of a deck and covers the rim joist, usually standing a little proud of the deck surface or tucked just under the overhanging board ends. Its jobs are to hide the pressure-treated rim and any exposed board ends, to give the deck a crisp edge line, and to take the weather so the structure behind it does not. On a composite deck the fascia is typically the manufacturer's matching fascia board; on a wood deck it is a clear, stable board in the same species as the decking or a painted trim board on a painted railing scheme.

Composite fascia moves a lot with heat because it is thin and wide, so it must be fastened with the manufacturer's screws in slotted or oversized holes at the spacing they call for, and never glued or driven tight. Wood fascia wants its back primed or sealed, a small gap at the top so water drains out rather than behind it, and stainless or coated fasteners. Either way, a fascia pressed tight to a wet rim joist with no drainage is a rot sandwich.

Why skirting goes wrong

Homeowners want to hide the dark, dusty space under a deck. Fair enough. The mistake is sealing it. Composite and PVC makers require open ventilation under the deck for their warranties, and wood framing needs it to dry after every marine-layer morning. Tight lattice with the backer board behind it, a solid tongue-and-groove skirt, or a planter bed mounded against a skirt all turn the underside into a humid box. Mildew shows up first, then soft joist ends.

Skirting also has to let someone get under the deck. Hose bibs, irrigation valves and the frame itself all need to be reachable, so at least one panel should be removable, and the whole thing should be designed knowing that a person with a flashlight will want to crawl in there once a year.

Skirting that looks closed and still breathes

On the porch stairs we rebuilt in North County, the porch skirt was closed in with the grade corrected to run away from the structure, so the skirt defines the porch visually while water and air both have somewhere to go.

• Horizontal boards with a consistent gap, often three-quarters of an inch to an inch and a half, run on a simple frame; this is our favorite because it matches a modern deck and ventilates on every row.
• Louvered panels, angled down and out, that read as solid from eye level but pass air.
• Open lattice set off the posts with no backer, and with a gap at the top and bottom.
• Vertical pickets or slats echoing the railing above, spaced for airflow.
• Planting in front of an open skirt, kept a foot or more away from the wood so the plants do not become the seal.

Keeping critters out without sealing it

An open skirt invites skunks, possums and the occasional neighborhood cat. The answer is hardware cloth, not solid boards: a quarter- or half-inch galvanized mesh fastened to the back of the skirt frame and buried a few inches at grade or bent outward in an L stops animals and still passes air. Mesh also keeps leaves from blowing under the deck, where they hold moisture against the frame.

Design notes

Fascia and skirting are where a deck meets the yard, and they carry more of the look than people expect. Keep the fascia line level and continuous, run skirting boards parallel to the decking, and align skirt panel joints with the posts above. Where the deck is low, a single deep fascia board with a gravel band beneath often looks better than any skirt. Where the deck is high, treat the skirt as a wall of the garden and give it the same care as a fence. Our fence and custom woodwork service grew partly out of exactly this kind of work.

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