Repairing Rotten Trim

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Repairing Rotten Trim

Don’t replace the whole board; cut out the damage and make a perfect patch

Posted on Jan 7 2016 by John Michael Davis

If I look hard enough at any house here in New Orleans, I’m sure to see one: a length of casing, fascia or corner board, with a hideous scarf joint only a foot or two from the end. This joint wasn’t put there by the builder; it was added years later to repair a rotten section of trim.

We get a lot of rot down here, and the ends of the boards are often the first to go. When they do, the standard repair is to cut back to undamaged wood at a 45º angle (what’s known as a scarf joint), then attach a new section of trim using yellow glue and finish nails. Sometimes it looks good—for a while.

After a year or two of seasonal movement, however, the joint separates, the rot sets in again, and the whole thing stands out like a sore thumb. On numerous occasions, I’ve been called in to repair the repair.

In some circumstances, the proper treatment is to tear out the entire length of trim and replace it with new. But if the patient is an 18-ft. long, old-growth red-cypress fascia board that has stood up to a century’s worth of abuse with only a few inches of rot to show for it, I refuse to replace it with an inferior grade of lumber. Even when the injured party is not a valuable trim element, it may still be difficult to remove or expensive to replicate.

Whatever the reason might be, when I decide to repair rather than replace a rotten trim board, I surgically remove the damaged section by making a square cut using a speed square and a circular saw. (Whenever possible, I use my 41⁄2-in. Porter-Cable trim saw for this job because it’s lighter and easier to control than a full-size circular saw.)

I prefer to fashion a new piece of trim out of the same species of wood as the old piece; I save old-growth lumber from demolition projects just for this purpose. To ensure an invisible and permanent repair, I use high-strength marine epoxy and a plastic biscuit to fasten the new section of trim to the old. Even in this pressure cooker that we New Orleans residents call a climate, I’ve yet to see one of my patches fail.


Cut back to sound wood
A simple 90º cut with a circular saw and a speed square readies damaged trim for repair.

Router doubles as biscuit joiner
In tight spaces, a standard router equipped with a specially designed slot-cutting bit (inset photo) clears space for the biscuit that connects the patch to the existing trim.When using a router, the author makes a single cut across the end of each board, stopping about 1⁄4 in. from the edges.

Dry-fit, then prime all surfaces
After inserting a biscuit and making sure that the patch fits perfectly, the author gives every surface of the board (especially end grain) a liberal coating of primer. To maximize rot prevention and to minimize delays, he uses a fast-curing epoxy primer that dries clear.

For this job, the author uses a plastic biscuit that will not be affected by rot should water ever penetrate the joint.

Epoxies demand good housekeeping
Set up far enough from the job site to be free of airborne debris, a sheet of plywood over sawhorses provides a clean place to mix epoxy. The canvas bag in the background keeps all the epoxy supplies close at hand.

After wetting the joint surfaces with liquid epoxy, the author adds enough thickening agent (West System’s #403 Microfibers) to give the epoxy a peanut-butter-like consistency.

Thickened to the correct consistency, a high-strength epoxy adhesive fills gaps but remains flexible enough to expand and contract with the wood.

A pry bar draws the joint together while the author fastens a scrap across it. As the epoxy cures, this splint will hold the joint tight and ensure that the patch stays in plane with the old section of trim. To make sure the splint doesn’t stick to the epoxy, its back side is covered with plastic tape.

Sand smooth, then paint
After allowing the epoxy to cure overnight, the author removes the splint and polishes the joint using a palm sander and 60-grit paper. Next, the entire patch is coated with alkyd primer. To ensure maximum longevity, the patch should be covered with two full coats of high-quality paint within two weeks.


4 1⁄2-in. trim saw (model #314): Porter-Cable Corp.; (800) 321-9443.

Slot-cutting bit for router (model #146-0832): Eagle-America Corp.; (888) 872-7637.

Epoxy primer (Primkote #8006-1): Abatron Inc.; (800) 445-1754.

Plastic biscuits (Lamello K20): Colonial Saw Inc.; (781) 585-4364.

West System epoxy: Gougeon Brothers Inc.; (989) 684-7286.

John Michael Davis is a restoration carpenter in New Orleans, Louisiana. Photos by Tom O’Brien. Drawings: Rick Daskam.

From Fine Homebuilding No. 246 — Download a PDF of this article.


Image Credits:

  1. Tom O’Brien

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