Patching Siding Where a Vent Came Out

Pulling an old dryer vent or exhaust duct out of an exterior wall leaves a round hole through every layer of the wall. A tube of caulk and a plastic cover will hide it, but a real patch that disappears and keeps water out means rebuilding each layer, in order, from the inside out.

Look before you patch

Take the vent hood off and pull the duct. Then look at what’s around the hole. Thump the sheathing: if it’s soft or water-stained, cut out and patch those areas before going further ¹. The paint on old siding very likely contains lead, so work lead-safe: plastic on the ground, mist before scraping, and clean up carefully ².

Patch the sheathing

Square up the round hole in the sheathing into a neat rectangle, fasten wooden cleats or blocking behind its edges so a patch has something to nail to, and cut a patch of the same thickness. In an old house the sheathing is often boards, so a piece of board or plywood of matching thickness works.

Patch the paper

Behind the siding is a water-resistive barrier: building paper or felt in an old house, housewrap in a newer one. Patch it shingle-style: the new piece tucks under the layer above it and laps over the layer below, so water running down the wall always flows onto the outside of the next piece down. Seal any cuts or tears with a sealant or tape approved for that barrier ³.

How clapboards work

Before cutting siding, understand how it’s fastened. Clapboards are nailed near their bottom edge into the studs, and each board above laps the one below by about 3/4 inch. Ideally the nails just miss the top of the board below, so each board is held along only one edge and can swell and shrink without splitting. In practice, most old siding is nailed through both boards ³, which matters when you go to remove a piece.

Clapboards split easily. This Old House’s Tom Silva advises patience and hand tools, since the vibration of power tools can crack the surrounding boards and turn a small patch into a big one ¹. Fine Homebuilding’s Andy Engel uses an oscillating multitool with care ³. Either way, go slowly.

Cut back the siding

Don’t patch with one short piece over the hole; that leaves a column of joints that shows. Instead, remove sections of each course the hole touches, cutting each one at a stud, and stagger the joints so they fall on different studs from course to course.

To remove a section ³:

1. Mark a square line at a stud, and score it with a knife to prevent splintering.
2. Cut along the line up to the bottom of the board above.
3. Punch the visible nails in the section through with a nail set.
4. Slip a metal-cutting blade between the boards and cut the nails holding the board above.
5. Pry the upper board out slightly, finish the cut with a knife, and slide the piece out.
6. Trim any nail stubs flush with the sheathing.

This Old House’s alternative is to score the damaged board lengthwise about halfway up and pry off the lower half first, leaving the upper half in place as a guide until last ¹. Pull every old nail before installing new boards; leftover nails split new clapboards ¹.

New boards

• Match the old siding: butt thickness, exposure (add an inch for the overlap), texture, and wood species ¹.
• Prime all six sides before installing, with oil-based primer if the siding is painted ^{3 4}.
• Flash each new end joint: slip a strip of flashing about 2 inches wide, a little longer than the board is tall, behind the joint, lapped onto the board below ³.
• Slip each board in at least an inch under the course above and align its butt with its neighbors ¹.
• Nail with stainless-steel ring-shank siding nails, which won’t rust or streak the paint; 7d or 8d into the studs. Keep nails about 3/4 inch up from the butt and at least 1-1/2 inches from an end joint ¹, just high enough to miss the board below ³.
• Caulk the ends with paintable caulk ⁴, renail the board above, and paint to match.

Try it

Before the real job, practice on the back of the house or a garage: remove and replace one short section of a clapboard at a stud, using the nail-cutting trick. Once you can do one without cracking the board above, the vent patch is just four of them, staggered.