Wood Rot Repair
Updated: Sep 4
If you own a home, you have to maintain it, and fighting exterior wood rot is an ongoing battle for most homeowners. The rot is caused by a type of wood-destroying fungus, which is as damaging as termite infestation and can wreak havoc on everything from siding and trim to windows and doors. If the wood isn’t maintained with paint and caulk or some type of exterior sealant, then rot can take hold and completely decay it.
In some cases, you can catch the problem in its early stages and make repairs before a complete replacement is necessary. In other situations, you’ll need to remove the old wood components and replace them using methods to ensure long-term performance. Here’s a look at some repair options.
Scrape, Prime and Paint
The easiest way to combat wood rot is to prevent it from happening. The key to preventing rot is to protect the wood from moisture. Lingering moisture in the wood creates an environment conducive to the fungi growth which deteriorates the fibers.
When you see paint flaking or peeling on the house, make it a priority to scrape away the old coating and sand the surface if necessary to prep for recoating. The paint works as a layer of armor to protect against water and rot. Seal any open joints with a flexible caulk/sealant, recess any visible fastener heads, and spackle any holes, checks or cracks with a non-shrinking exterior wood filler. Seal any bare wood or repairs with a quality primer/sealer, then update the surface with one or two coats of exterior paint.
The same principle applies to stained and sealed wood on a deck or fence. You can determine its condition by simply splashing water on your deck. If the wood quickly absorbs the water, and the surface darkens, then the wood isn’t waterproof and it’s time for a maintenance coat. If the water beads, sits on the surface, or runs off, then the wood is adequately sealed.
Preventive maintenance is the best approach because deterioration of neglected outdoor wood is inevitable. If you end up having to make a repair, you’ll still have to follow up with all the sealing, priming, and painting that you skipped earlier.
Minor surface repairs to painted wood can be made using a variety of paste-like filler products, provided that the rotted wood is first removed from the problem area and the surface is prepped correctly. A screwdriver or chisel can make a good scraping/gouging tool to remove damaged material and wood rot. Clean away any residual dust and debris with compressed air or a shop vac.
Before adding a repair filler or epoxy product, consider applying a “Wood Hardener” to the damaged area. Also known as “Wood Restorer” (depending on brand), these quick-drying liquid products have a special resin that binds and reinforces damaged wood fibers to form a solid base for filling.
Water Putty—For minor rot repairs, Durham’s Water Putty is a powder product that is mixed with water on site. The putty is easy to use and clean up, and you get a lot of product for your money. Water putty should be used on vertical applications where water does not pool.
In a mixing container, add a little water at a time to the Water Putty powder and stir with a paint stick until you achieve a consistency similar to thick pancake batter. Apply the product in layers up 1/4-in. thick at a time. Allow each application to dry before adding more product. Once the damaged area has been completely filled and dried, sand the area smooth. Make sure to seal the repair with a waterproofing product such as exterior primer and paint.
Two-Part Fillers and Epoxies—For larger rot repairs, two-part wood fillers or two-part epoxies, which are available from a variety of manufacturers, require mixing together on-site to initiate the hardening process. They can create a very durable, permanent repair, but they’re also messier and more difficult to use due to the short working time before the product begins to stiffen. However, once dried, the repair can be sanded, shaped, planed, drilled, routed or sawed just like wood. This means the material can be molded into a three-dimensional wood replacement such as the broken corner of a window sill.
Surgical Wood Replacement
The best solution may be to cut out the damaged section of wood, carefully cut a replacement piece of pressure-treated wood to the exact same shape and dimensions, and glue the new piece into the construction with a high-quality, exterior-grade adhesive.
A buzz-cutter (a.k.a. oscillating multi-tool) equipped with a plunge blade is an excellent device for cutting out the problem. Sharp wood chisels can also be useful in skilled hands. Be sure to remove all loose chips, dust and debris from the damaged area.
After fabricating a replacement piece from treated wood, you should thoroughly coat all sides of the new piece—plus the damaged area—with exterior primer/sealer before fastening in place. The sealer will add an extra measure of moisture protection.
Fasten the replacement into the void with countersunk nails or screws combined with
an exterior-grade wood glue or construction adhesive. Fill the seams with a paintable,
exterior caulk/sealant, and tool the joints smooth. Apply primer and a matching coat of paint to complete the repair.
An alternative repair method is to replace the rotted material with PVC, which is even more durable than treated wood. Commonly stocked in 3/4-in. thickness and a variety of standard widths, solid PVC boards won’t check, swell or rot, no matter how wet it gets. However, PVC trim is more expensive than treated wood, is limited to a few simple profiles, and can be difficult to find in some areas.
Note that stock PVC should not be used if the repair will be painted a dark color and exposed to the sun. When the dark color tone absorbs heat from the sunlight, the high temperature can warp and deform the PVC material, which should only be used for light-color applications or shielded from sunlight.
6 Tips for Replacing Rotted Exterior Trim
Window trim, door casing, handrails and fascia… These are a few common candidates for wood rot around most people’s homes. Here’s a little insight on how to replace rotted boards.
1. Before prying off the rotted boards, cut through the caulk joint with a sharp utility knife to separate the materials and avoid damage to paint or siding.
2. Scrape away old caulk and paint to provide a clean surface where new adhesives can bond.
3. When replacing the trim, use treated wood components and make sure to seal all sides and end grain of the replacement boards with exterior-grade primer. Alternatively, you can replace the rotted wood with PVC trim (must be painted a light color). Urethane moldings may be an option for more elaborate decor.
4. When cutting the boards to size, avoid cutting through any knots in the wood. If you do, the knot will most likely dislodge from the board and leave a big ugly hole right on the edge. Avoid this problem by moving the cut-line a few inches further up the board. You can trim those extra inches off the opposite end to get the board length you need.
5. Trim-head screws like the Fine Screws from U2 Fasteners have a small head that is easy conceal with filler, which is ideal when making repairs.
6. Caulk all gaps and seams with a quality elastomeric sealant. The HIR team uses the Duramaster sealant because it’s very flexible, paintable and cleans up with water. Finish up with a topcoat of exterior paint.
Working with PVC Trim
I planned to spruce up this roof overhang with a little caulk and paint. Plans changed when I found the extent of the rotted fascia (which had consisted of two separate boards with a gutter hiding the linear joint). This required complete removal of the fascia boards. To replace them, I ripped and edge-joined some 1x6 PVC stock to make a single 1x9 replacement board that wouldn’t rot.
I then installed the new PVC fascia along with a replacement gutter. Plus, I repaired the adjacent house siding with Durham’s Water Putty. Fascia damage is common on homes, so check its condition on your own home every once in a while.
Stock boards of PVC can also be cut to shape and used for surgical trim replacements in common rot-prone areas, such as exterior door casing. Polyurethane glue works well to adhere a PVC replacement piece into the cavity. Countersink the fastener heads and conceal them with non-shrinking exterior filler then sand smooth. Seal all seams with a paintable, flexible caulk, then prime and paint the repair.
Working with Repair Fillers
For minor vertical repairs, water putty is an inexpensive option that is easy to use and clean up. Once the putty dries, it is easy to sand smooth. However, the repair must be thoroughly sealed, primed and painted to protect it from water.
For more severe surface damage, Bondo filler is an option to repair a rotted wood cavity. Mix the two-part formula according to the instructions and work quickly to fill the void before the product hardens. Tool the repair as smooth as possible. Be prepared to sand the surface smooth with the surrounding construction.
Tips for Fascia Replacement
Replacing a rotted fascia board is a common headache for homeowners. Here are a few tips to make the job easier.
Unless you plan to reconstruct the soffit, it’s best to replace the fascia board with one of identical size and dimensions, otherwise the fit will be off. However, be sure to select very straight pressure-treated boards with no bend or warp, or else you’ll have trouble fastening it in place. Purchase long boards to reduce or eliminate any joints in the fascia.
Depending on the house design, some fascias have a groove in the rear to fit against the soffit. If that’s the case, you’ll need to reproduce the groove to the same measurements using a router.
Coat all sides, end grain, and also the router groove with a quality exterior-grade primer/sealer. Follow up with two coats of paint to encase the boards in protection.
Before hanging the fascia, mark the fastener locations with tape for quick identification.
Joints in the fascia boards should be cut at a 45-degree scarf joint located over solid framing. The faces of the miter should be coated with a quality exterior wood glue like Titebond III, and the mating boards should both be screwed together securely to the framing.
We recommend using three 3-in. Universal Screws at each fastener location. Countersink the heads and conceal with filler, then sand and paint.
Apply a drip edge to divert water away from the house, and if attaching a gutter, make sure the drip edge folds down inside the trough.