Though installed following the manufacturer’s recommendations, the composite fascia trim on this deck is pulling away from the framing in several locations, including this mitered corner.
Jason Russell Though installed following the manufacturer’s recommendations, the composite fascia trim on this deck is pulling away from the framing in several locations, including this mitered corner.
Most deck builders install blocking on the flat for picture-frame borders, but installing it vertically (as shown) promotes quick drying and helps prevent popped deck screws.
Clemens Jellema Most deck builders install blocking on the flat for picture-frame borders, but installing it vertically (as shown) promotes quick drying and helps prevent popped deck screws.
Shimming the fascia away from the framing, as shown in the illustration, will allow water to drain away from the assembly and permit differential movement between the trim and the substrate.
Clemens Jellema Shimming the fascia away from the framing, as shown in the illustration, will allow water to drain away from the assembly and permit differential movement between the trim and the substrate.

Q: On a recent deck project, I installed 1-inch by 8-inch Trex Transcend fascia boards according to the manufacturer’s installation guidelines, which called for fastening the fascia to 2x8 framing with a pair of Starborn DeckFast fascia screws a maximum of 18 inches apart (actually, we secured the fascia to the framing with screws 12 inches on-center). We also followed the manufacturer’s gapping recommendations of leaving a 1/8-inch end-to-end gap when installing the boards in temperatures above 40°F. Still, the fascia boards are pulling away from the framing and popping away from some of the fasteners, especially at the miter joints (see photo, right). What did I do wrong, and how can I fix the problem?

A: Clemens Jellema, a deck builder in Owings, Md., responds: The problem with composite and cellular PVC trim is that these synthetic materials react differently to humidity and temperature changes than does pressure-treated wood framing. Wood tends to expand and contract across its grain with changes in humidity, while PVC and composite materials tend to expand and contract along their length with changes in temperature. So not only do the trim and the substrate move in different directions, they are affected by different weather conditions.

The weather also matters when the trim is installed. If it’s cool and moist—as in the Northwest—when you build the deck, and then the weather becomes hot and dry, your fascia is going to expand lengthwise, while the pressure-treated wood framing is going to contract across its width. Even if you secure the fascia to the framing on 12-inch rather than 16-inch or 18-inch centers, you won’t be able to force the two materials to move in concert by screwing them together. Eventually, the two components will separate and break the screws.

A simple solution that I use to tackle this problem is to space the fascia board away from the framing with small, 1/16-inch-thick treated shims, which I rip from scrap PT stock. Using stainless brads or finish nails, I install the shims vertically around the perimeter of the deck every 16 inches on-center (12 inches on-center is fine, too) in preparation for the fascia. Then I install the fascia with fasteners driven through the shims following the manufacturer’s directions, which allows both materials to move somewhat more independently. I use the same approach with horizontal decking borders, which I install over 2-by blocking oriented vertically (like joists), rather than horizontally, to keep the deck screws from popping.

I would also recommend using a border and tucking the fascia up underneath, rather than using a flush fascia style. This will help prevent moisture and debris from accumulating between the fascia and the framing, which makes the problem worse.

Even if you use shims, however, it’s possible that you won’t be able to totally eliminate the problem. I know some deck builders who feel that this is more of a product issue with synthetic fascia trim than an installation issue. If you follow the manufacturer’s installation guidelines, shimming the fascia away from the framing as described above, and still encounter problems, probably your best recourse is to contact the manufacturer directly.