Cut the nails at the bottom of the row above the boards you’re replacing. Use a small hacksaw with a sharp blade. Cutting the nails will let you remove rotted boards without damaging those your’e saving. To get at the nails, carefully wedge out the bottom of the siding 1/8 to 3/16 in. with wooden shims.
Cut out bad sections with a circular saw or even a sharp utility knife. Be sure to stagger the joints (see Photo 1). Most hardboard siding is 7/16 in. thick, so set your blade depth carefully to avoid cutting into other courses. Use shims to lift the boards so you can easily get at the one you're cutting.
When you replace the boards, use a homemade “reveal set” jig to match the reveal of the existing siding. The courses should overlap by at least 1 in. Replace siding by working from the bottom up (you might need a helper here). Attach the siding with 10d galvanized box nails driven into the overlap at least 1/2 in. above the bottom edge. If you have insulated sheathing, predrill the nail holes to avoid crushing the insulation.
When hardboard siding is installed and maintained correctly, it can hold up for 30 or 40 years. But without proper attention, isolated areas can begin rotting in only a few years, especially near the foundation. Water splashes up from the ground, frequently soaking the vulnerable bottom edges. The paper face then flakes off, exposing the dark brown inner layers, and each soaking accelerates the rotting.
Replacing these rotted areas takes only a few basic tools and a few materials, but it can make a huge improvement in your home's appearance. Hardboard siding doesn't cost that much, so it's often worth it to replace an entire course. Doing this helps you avoid unnecessary butt joints.
- Prime the back and edges of the new siding. Thoroughly paint all exposed edges and grooves.
- Do not drive the nails flush or countersink them. The heads will break the paper face, allowing water to soak in and deteriorate the siding. Caulk any nailheads that break the paper face.
- Leave a 1/8-in. gap at corner and butt joints. Seal these joints with a 35-year, paintable acrylic latex caulk.
- Prevent water from splashing on the siding by installing gutters or repairing leaky ones. Also, adjust lawn sprinklers so they don't hit the siding.
- Where siding meets a roof, it will rot if the siding touches the shingles. When you replace these boards, make sure you have good flashing along the joint. Install the new siding so there's a 1-in. gap between siding and shingles.
- Consider replacing rotted areas with fiber-cement siding. It's 1/8 in. thinner than most hardboard, but it works in many cases, has a comparable cost, is highly rot resistant, and carries a 50-year warranty. For help finding a dealer in your area, call James Hardie Products (888-542-7343 or jameshardie.com) or ABTco (800-265-9829 or abtco.com).