Replace Damaged Lap Siding Boards So the Patch Disappears
One soft clapboard under a window is a $40 fix this spring and a $4,000 sheathing job in five years. The board comes out cleaner than you think.
Lap siding fails one board at a time, almost always in the same places: under windowsills, behind downspouts, at the bottom course where splash-back hits, and wherever a sprinkler soaks the wall. A soft board is not just cosmetic. Water is getting behind it, and the sheathing and framing behind that are next. The good news is that lap siding was designed to be repaired board by board. The technique is old, and it works as well on modern fiber cement as it does on 80-year-old cedar.
Understand how the boards are held
Each course of lap siding overlaps the one below by 1 to 1-1/2 inches, and the nails for each board are driven through its lower edge, passing just above the top of the board beneath. That means the board you want to remove is pinned by its own nails and by the nails of the course above, which pass through its top edge. Free both sets and it slides out. The face nails on the course above must stay, so you cut the hidden ones instead.
Find the true extent of the rot
Press an awl into the board every 6 inches, and 2 inches above and below the obvious damage. Soft wood takes the point with little resistance. Mark the good wood on each side of the rot, then plan your cuts at the nearest stud on each side, at least 16 inches from any joint in the courses above and below so the seams stagger. Check the boards above and below the same way; rot travels along the overlap.
Cut the board free without wounding its neighbors
Drive wedges or shims under the bottom edge of the course above to lift it 1/4 inch. Slide the oscillating tool’s metal blade up under that lifted edge and cut the nails passing through the top of the damaged board; you will feel each one let go. Then pull the face nails from the damaged board itself with a cat’s paw, or cut them the same way. Score the vertical cut lines with a speed square as a guide and finish them with a handsaw or a circular saw set to exactly the board thickness so you never nick the housewrap.
Fix what is behind it
With the board out, inspect the housewrap or felt and the sheathing. Probe the sheathing; if it is soft, cut it back to the studs and patch with matching thickness. Repair torn wrap with a patch tucked under the wrap above and taped at the sides and bottom only, so water still drains down and out. Slide a strip of flashing tape or a piece of felt behind each vertical joint you are about to create, 4 inches wide, centered on the seam.
Prime all six sides, then fit
Cut the new board 1/16 inch short of the opening so it slips in without splitting. Then prime it everywhere: face, back, both edges, and both cut ends. End grain drinks water ten times faster than the face, and an unprimed end is where the next rot starts. Let the primer dry overnight if you can. Slide the board up under the lifted course, tap the shims out, and check that its bottom edge lines up with the neighbors using a straightedge across three boards.
Nail on the original lines
Match the exposure of the existing nails: find the nail line on the neighboring boards and continue it across the new one at every stud. Drive stainless or hot-dipped galvanized siding nails 1 inch up from the bottom edge, snug but not dimpled; a crushed face invites water. For the course above, whose lower nails you cut, drive new nails 1/2 inch off the old holes and fill the old ones. Caulk the vertical joints lightly, prime the caulk, and topcoat two coats, feathering onto the neighbors.
Houses built before 1978 may have lead paint on the siding; wet-scrape and bag debris, do not sand dry, and wear a P100 respirator. Fiber-cement boards cut with a circular saw throw silica dust, so use a fiber-cement blade with a dust shroud or score-and-snap shears and keep the blade wet. And never nail into the top edge of the course below; that pins two boards together and they will split when they move.
Prime the ends you will never see again. That is the coat that decides whether you do this twice.
Keep the wall dry
Fix the cause while you are there. Redirect the downspout, move the sprinkler head, add a drip cap over the window, or extend the splash block. Walk the house every spring with the awl and probe the usual suspects. A siding wall that gets one board replaced every couple of years will last a century; one that waits for a “someday” full replacement will cost as much as a roof.
Keep the momentum going
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