I've had plenty of phone calls over the years from homeowners who are convinced they have to reside their entire house because some of their siding has been damaged by water. Yet when we take a look at these jobs, we often find that relatively little siding has to be replaced. Even better, the homeowners are surprised to learn that the fiber-cement siding we use as a replacement is often a perfect match for what they have. What's more, it resists damage from water and insects. What causes water damage? We get significant rainfall along the Gulf Coast where I live, and that leads to a lot of siding problems. But even in dryer parts of the country, lawn sprinklers that routinely wet siding or shrubs growing too close to the house to allow sufficient air circulation also lead to damaged siding. Most of the problems we see involve hardboard siding because it is so common here, and it's not always installed correctly. Carpenters might have overdriven the nails, improperly sealed joints or allowed cut edges to come too close to the ground. Over time, rain runoff soaks into the siding and causes mildew and rot. By the time the homeowners notice the siding is soft, they've got a real problem on their hands. The good news is that damage is usually limited to two areas: the first three courses of siding (2 feet or so up from the ground), and wood-framed chimneys, or chases, that aren't protected by a roof overhang. Where the siding has been installed properly, and where it's not soaked by rain, it's usually fine. We tear off the damaged hardboard and replace it with fiber-cement siding. The sheathing and framing sometimes need repairs too, but that is less common. In most cases, the original layer of building paper or housewrap at the bottom of the wall was enough to prevent water infiltration and damage beneath the siding. The repair is simple if you follow the details in the illustration above. The wood-grain pattern on the fiber-cement siding is often a perfect match with existing hardboard, although it's not as good a match with real rough-sawn wood. When the new siding has been painted — two coats of top-quality acrylic latex over a primer — the repair is hard to spot. Another plus is that fiber-cement can be brought closer to the ground than the 8-inch minimum I give hardboard. Several brands of fiber-cement siding are available; we're most familiar with Hardi-plank from James Hardie Building Products. The siding, made from portland cement, sand, cellulose and other additives, is available with smooth or simulated wood-grain finishes. Planks are 12 feet long x 5/16 inches thick., and they come in a half dozen widths, which means weather exposures can vary from 4 inches to 10 3/4 inches. Though it used to be a little more expensive than hardboard siding, fiber-cement siding now costs less in my area. Hardboard siding 8 inch wide costs 45 cents per linear foot, while fiber-cement runs 37 cents. There's one more thing homeowners like about fiber-cement siding: It holds paint better than wood. One house I worked on still looks great after eight years. Around here, most people start thinking about repainting after only three or four years.
Wood Replacement Patch
Depending on the extent and the size of the rot or damage to the wood, there are times when it's best to replace the damaged section with new wood. When it comes to siding, it is generally pretty easy to find and buy a clapboard section that will match the profile of the existing clapboards of your house.
Remove The Damaged Section
My favorite power tool for cutting out and removing the affected spots is an oscillating saw. These saws use a blade in an oscillating action to make plunge cuts into small areas, making them the perfect tool for this type of project. A simple homeowner model can be purchased for under $50.
Cut out a section of your siding where the patch needs to be made, being careful to only cut through the siding and not into the plywood sheeting underneath.
Gently slide a small prybar or 5-in1 tool behind the piece to be removed from the bottom side and pry out to loosen the nails holding it in place.
Once the siding and nails have been removed, take it into your local lumber store to match up the profile so you can purchase a section of clapboard long enough for your patch. While you're there you can also have a custom paint match done so you can touch-up your patch.
Prepare Your Patch
Measure and cut your patch to fit. If you don't have power tools you can purchase a plastic miter box and hand saw to do the job.
Before you nail-in the patch, apply a coat of a good bare wood primer to all six sides -- front, back, top, bottom, and each end. Also prime the ends of the siding you just cut on the house.
Once the primer has dried, you can even apply a couple of coats of paint on the siding before you install it.
When it's ready to be installed, take a high quality acrylic latex caulking and apply a bead to the back of the siding patch near the edge that will be butting-up to the existing siding. Also apply a bead to the back edge of the siding on the house where the patch will be butting-up to. This is called "back-caulking" and will help to prevent any water from making it's way behind the wood.
Install The Patch
Slide the clapboard piece into place and nail it into the plywood sheeting with some 7d or 8d ring-shank nails. Also nail down the loose edges of the existing siding that you cut. Use a nailset to set the heads of the nails just below the surface of the wood.
Fill your joints and nail holes with some wood filler, and apply your touch-up paint once the filler has dried. If your touch-up painting spot is too obvious, you can try painting the entire length of the clapboard where you did your replacement. As long as your color match is not too far off of the original you should be able to get by with not having to repaint a wall from corner to corner.
If you're replacing a section of trim the process is nearly identical to patching siding. Just make sure that when you nail-in your new patch that it is being fastened into some solid wood framing.
Epoxy Wood Repair Patch
Some areas of trim and window sills are either too difficult to remove, or too difficult to find a replacement piece that will match your existing profile. In this case your best option is an epoxy wood filler.
Epoxy wood repair products are generally a two-component (part A & part B) that must be mixed together in order to harden. Once they are mixed together you typically have a few minutes to mold and shape your compound before it dries and becomes unworkable. Once it has set-up, epoxy dries harder than the wood -- yet remains flexible enough to expand and contract with it. It can be nailed or screwed-into, can also be painted and sometimes even stained to match the surrounding color.
Prep The Area To Be Filled
Grind out the rotted wood with a trim router, die grinder, or rotary tool using a v-shaped grinding bit to get down to solid wood, because the epoxy will not stick to rotted wood.
Scrape off the paint down to the bare wood in the immediate surrounding area where your patch will be applied because the epoxy needs to bond to bare wood.
Tape off the surrounding areas (siding, trim, etc.) to keep from getting the epoxy on an area where you don't want it.
The wood must be dry before you can apply the epoxy.
Preserve & Prime
Drilling some holes in the wood surrounding the patch and injecting a borate wood preservative will help to prevent any further decay.
Brush on an epoxy primer to the exposed wood where the patch will be applied to help the epoxy adhere better.