How do I replace rotted exterior trim?

Replacing rotted exterior trim is a carpentry job most handy homeowners can do on a single-storey wall: find out how far the rot goes, take out only what is soft, put in a new piece primed on every face, and stop the water that caused it. That last part matters most. Decayed wood does not hold paint,^[1] and a new board installed the same way as the old one will rot the same way. In the National Park Service's words: "If the cause of material failure is not addressed, the replacement will also fail."^[2]

Before you start

Lead paint. If the house was built before 1978, the trim may carry lead paint, and the EPA advises assuming the paint contains lead if you cannot determine the age of the house.^[4] Lead-safe work for a homeowner means plastic sheeting under the work, misting surfaces before you sand, scrape or cut, a HEPA vacuum for cleanup, and a disposable N-100 respirator.^[4] Avoid dry sanding old paint; where sanding is unavoidable, mist first and use wet-dry sandpaper.^[4]

Permits. Virginia's rules are simple here: the Uniform Statewide Building Code lists installation or replacement of trim, and application of paint, among the ordinary repairs that need no permit application.^[5] Two exceptions are worth knowing. A building official may require a permit for exempt work in a special flood hazard area,^[5] and historic districts have their own approvals. Loudoun County, for example, requires a Certificate of Appropriateness for changing materials on an existing structure in its historic districts, but not for replacement in kind or repainting.^[6] If you live in a historic district and want to swap wood for PVC, ask your locality first.

Ladders. Keep this a ground-floor job. For the trim you do reach from a ladder, set the base out one-fourth of the height to where the ladder rests, never stand on the top two rungs, keep three points of contact, and skip ladder work in wind over 20 miles per hour.^[7] Metal ladders must not be used near electrical wires,^[7] which matters at the corner where the service drop meets the house.

Find out how far the rot goes

Rot rarely shows its full size. Water gets in at a bottom end, a miter or an open joint, and the surface can look sound over decayed wood; the Forest Products Laboratory advises probing several areas with a small-tipped screwdriver.^[1] An awl works too: sound wood resists the tip, rotted wood lets it sink in or comes away in soft, stringy fibers. Check the bottom edges and ends of every piece near the damage, because liquid water enters wood 10 to 100 times faster through end grain than through the other faces.^[1]

Repair or replace

Small, shallow soft spots in otherwise sound trim can be repaired. Cellulose wood fillers must be rated for exterior use and need a protective finish.^[2] Two-part epoxies are tougher: two-part epoxies bond tenaciously with wood and can be sawn, nailed or sanded, but they are only for areas that will be painted, because they do not take stain and deteriorate in sunlight.^[2] Either way, dig out all the soft wood first; filler over rot only hides it.

A larger damaged section of a long or decorative piece can take a spliced patch, the carpenter's "dutchman": chisel out the decayed wood, glue in a piece of the same kind of seasoned wood with a waterproof adhesive such as an epoxy formulated for wood, then plane or sand it flush.^[2]

A board that is soft through, or soft along much of its length, should be replaced. Plain casing, corner boards and rake boards are usually cheap enough that replacing the whole piece is faster than patching it.

Choosing the replacement

Match the old board's thickness and profile so the paint lines and caulk joints work. Beyond that, the material decides how long the repair lasts.

• Wood. White pine, western redcedar and redwood hold paint well; southern yellow pine and Douglas-fir, with their wide dense latewood bands, are harder to keep painted.^[1] Finger-jointed boards are common for fascia and trim, and paint on them often fails in a patchwork pattern or at a joint where the pieces move differently.^[1]
• Cellular PVC. Versatex says its cellular PVC trim will not swell or shrink like wood experiencing a moisture cycle,^[8] which makes it a sensible choice at sills, near the ground and anywhere splash-back keeps wood wet. It moves with temperature instead, so follow the maker's instructions: Versatex calls for fasteners that penetrate a full 1 1/4 inches into the framing, placed no more than 2 inches from the end of a board, and paint colors with a light reflective value of 55 or greater.^[8]

Putting the new piece up

The steps above cover the whole job. Three details make the difference between a repair that lasts and one you redo in a few years.

Prime it before it goes up. "Priming before installation (even in your garage) is the best way to extend paint life."^[1] The back and the cut ends are the faces you can never reach again once the board is nailed on.

Keep end grain out of standing water. "Leave space below the bottom of any board exposed to weather so that the bottom edge does not stay wet after a rain."^[1] Vertical casing that sits tight on a sill or a porch floor wicks water up its end grain; a small gap, sealed with primer, drains instead.

Caulk with restraint. Caulk the vertical joints between trim and siding, but do not seal the horizontal overlap of lap siding,^[1] which is how water that gets behind the siding drains out.

Then paint. Bare primed wood is not a finished surface, and research cited by the Forest Products Laboratory found that surface degradation from as little as one week of sun exposure before painting significantly shortened the life of the paint.^[1] Two topcoats over primer is the usual system.

If the old trim keeps cracking or peeling elsewhere, read why paint on window trim cracks, and for the habits that stop the next repair, see how to keep exterior trim from rotting.

Stop and call a pro if

• The sheathing, a sill plate, a header or any framing behind the trim is soft or wet. That is structural repair, not trim.
• The rot is in a porch column or post. The National Park Service notes that column repairs are usually work for an experienced carpenter because they may involve supporting the roof above.^[2]
• You find mud tubes or tunnels in the wood. "A termite tunnel is a sure sign of infestation."^[1] Call a pest control company before any carpentry.
• The trim is above the first storey, or it is fascia or rake board along a roofline. Two-storey ladder work and anything at the roof edge are jobs for a crew with the right access equipment.
• The window or door frame itself is rotted, not just the casing around it.
• The house was built before 1978, the job will disturb a lot of paint, and you cannot keep the dust contained.

Getting it fixed on Loclis

On Loclis you describe the trim repair once, with photos of the damage, and local businesses quote it; a business that needs to see it first can offer an estimate visit. See what trim replacement costs in Virginia, browse shutters and exterior trim pros, find exterior pros around Blacksburg, or read more exterior answers.