How to Open and Close Wood Lap Siding Around a Penetration

Why this matters

Wood clapboard is where a penetration is easiest to do badly. A hole saw goes straight through it in thirty seconds, the line set or conduit goes in, a bead of caulk goes around it, and for a year it looks fine. Then the caulk lets go, water runs down the face of the course above, follows the line into the wall, and the sheathing behind it rots in a circle nobody sees until the paint blisters. A penetration that lasts is built the other way: the siding comes off around the spot, the opening is flashed at the layer behind the siding, a mounting block gives the line a flat sealed face to pass through, and the clapboard goes back around the block with its cut ends primed.

This card follows one job, start to finish, with the reason for each move. The flashing details at the barrier are owned by How to Flash an Exterior Wall Penetration Into the WRB and are cited, not repeated.

The job

An HVAC tech is installing a ductless mini-split on a 1952 house. The line set, condensate drain and control wire have to pass through an exterior wall to the outdoor unit, through a 3 in hole. The wall is painted wood clapboard with about a 4 in exposure, over board sheathing covered with asphalt felt. The paint is several layers deep.

First: the paint, before any tool touches it

The house was built before 1978, so the paint is presumed to contain lead unless it has been tested and found not to. Cutting, prying and drilling painted clapboard makes lead dust and chips, and that is a hazard on two routes: the occupants, through dust tracked inside and paint chips in the soil a child plays in, and the worker, through dust breathed in and swallowed from unwashed hands.

• The occupant side is EPA's Renovation, Repair and Painting rule, 40 CFR 745 Subpart E, which applies to paid work that disturbs paint in housing built before 1978, with state programs administering it in some states. It has a minor repair and maintenance exclusion based on the area disturbed, 20 sq ft or less on an exterior, but the exclusion does not reach demolition of painted surfaces or the practices the rule prohibits. Whether cutting out a section of siding is covered is a question many shops settle by treating it as covered work done by a certified renovator. This is not legal advice; read the rule and your state's program.
• The worker side is OSHA's lead standard for construction, 29 CFR 1926.62, which governs your employees' exposure regardless of the EPA rule.

In practice on this job that means: plastic sheeting on the ground under the work out at least 10 ft, or as far as the shop's lead-safe practice sets; windows and doors near the work closed; no power sanding, planing or heat on the paint; cuts made with a hand saw or an oscillating tool with a HEPA vacuum at the blade; a respirator under a written 29 CFR 1910.134 program as the shop's 1926.62 assessment requires; and hands washed before eating, drinking or getting in the truck. See related: The Legacy Material Check Before You Disturb a Finish.

Locating the penetration on the courses

1. Pick the spot on the inside first, then find it outside. The indoor unit's bracket sets where the hole goes. The tech drills a 1/4 in pilot hole from inside at the bracket's line-set location, angled slightly downward toward the outside, and finds the bit outside. Reason: a pilot from inside guarantees the opening lines up with the bracket, and the small bit shows what is in the wall before the big bit goes through. The tech scanned the bay for wiring and pipe first and drilled only after the scan was clear, with a dust shroud and the vacuum at the interior wall, since the interior paint is the same age as the siding.

2. Read the courses around the pilot. Outside, the pilot came out 1 1/2 in below the butt of one course, in the exposed face of the next course down, which shows 4 in. The mounting block chosen for the job is about 7 in tall. A 7 in block on 4 in courses covers parts of three courses: the one the pilot is in, part of the one above it, and part of the one below. Reason: the block has to sit flat on the sheathing, not on tapered clapboard, so everything it covers comes off.

Opening

3. Wedge the courses and cut the nails. Old clapboard is usually face-nailed near the butt, often through the top of the course below. The tech drives thin wood shims under the butts above and below the work to open the laps a little, then slides a hacksaw blade, wrapped in tape at one end as a handle, up under each lap and cuts the nails holding the boards to be removed, at the studs on either side of the opening. Reason: pried clapboard splits along the grain, and a split board is a board to replace, in a profile and wood that may be hard to match. If you skip it: the course above splits at its butt as you lever.

4. Cut the opening in the clapboard, one size larger than the block. With the nails cut, the tech marks the block's outline plus about 1/8 in all round on the face, and cuts the clapboard out within that outline with an oscillating tool and the HEPA vacuum held at the blade, stopping at the felt. The cut pieces come out in short lengths. Reason: the 1/8 in leaves room for sealant and for the wood to move. Hazard: this is the cut that makes the lead dust; it is done inside the sheeting, with the respirator on, never with a circular saw on painted wood.

5. Inspect the sheathing and the felt. The felt was brittle but intact, and the sheathing boards dry. Reason: this is the one time you see behind the siding. A soft or stained board here is a repair before the block goes on, not after.

The barrier and the block

6. Open the felt and flash the opening. The tech cuts the felt for the block as the WRB card describes, with the upper flap preserved so it can lap over the head flashing, and drills the 3 in hole through the sheathing at the pilot, sloped down toward the outside so water in the sleeve runs out. See related: How to Flash an Exterior Wall Penetration Into the WRB. On a wall with house wrap instead of felt, the same layering applies with the wrap maker's tape.

7. Mount the block. The block is fastened through the sheathing with stainless or coated screws, set in a bead of sealant on its back edges except the bottom, which is left open so any water behind it drains. A head flashing goes over its top, tucked up under the felt flap and taped, so water coming down the wall is turned out over the block rather than behind it. Reason: the block gives the line set a flat, sealable face, and the head flashing is what makes the whole assembly shed water by lapping rather than by caulk. If you skip the head flashing: the top of the block becomes a shelf that sends water behind the siding.

8. Run the line set and seal it to the block. The line set, drain and wire go through the sleeve, sloped to the outside, and the block's own cap or sealing collar closes around them as its instructions describe. The drain line keeps its slope to the outside all the way out.

Closing

9. Cut the clapboard back around the block. The pieces removed in step 4 were short lengths, so the tech refits the courses with new clapboard of matching profile where the old pieces were too short or cracked, cut to leave about 1/8 in at the block's sides. The course above laps down over the head flashing. Reason: clapboard and block must not be tight, because wood swells across its width as it takes on moisture, and a board jammed against the block cracks its paint film and opens a path for water.

10. Prime every cut end before it goes back. All end grain gets a coat of exterior primer, including the ends of the old boards left on the wall at the edges of the opening. Reason: end grain takes up water many times faster than the face, and an unprimed end is where a clapboard starts to rot. If you skip it: the paint at the ends of the repair fails first, in a neat frame around the block.

11. Renail, at the original pattern. Face nails go back near the butt of each course into the studs, with corrosion-resistant siding nails, in the old pattern so no new nail splits a board at its end. Nail heads are set flush, filled and spot-primed.

12. Seal and paint. Sealant at the block's sides where it meets the clapboard, not along the top edge of the course below the block, which has to stay open to drain. The patch gets finish paint matched to the house. See related: How to Get a Color Match From a Paint Chip.

13. Clean up as lead work. The sheeting is misted, folded inward and bagged, the ground under it checked for chips, and the area cleaned with a HEPA vacuum. Reason: the job is not done while paint chips from it are in a flowerbed.

How it came out

One block, flat on the sheathing, sealed on three sides and open at the bottom, a head flashing tucked under the felt, the line set sealed at the block, three partial courses refitted with 1/8 in clearance at the block, every cut end primed. The ticket records the pre-1978 presumption and the lead practices used, the felt condition behind the siding, the block and flashing used, and the paint.

The penetration this replaced, on a neighbor's identical house the tech had seen the year before, had been a 3 in hole saw straight through the clapboard and a ring of caulk. The difference shows in the one place nobody looks, on the sheathing, three or four years on.

What changes the method

• No barrier at all. Some older houses have siding nailed straight to board sheathing with no felt. There is no layer to lap into, so the head flashing is the whole defense; extend it well up under the course above and seal it to the sheathing.
• Shingle or board-and-batten siding. The block still sits flat on the sheathing and the flashing logic is the same, but the cut-and-refit follows that siding's own lapping.
• A small single line, such as a cable or a 1/2 in conduit. A purpose-made siding mount for a single cable, set over a small flashed hole, may stand in for a full block; the same rule of lapping, not caulking, applies.

References

• EPA 40 CFR Part 745 Subpart E, Renovation, Repair and Painting; OSHA 29 CFR 1926.62, lead in construction; 29 CFR 1910.134, respiratory protection
• See related: How to Flash an Exterior Wall Penetration Into the WRB
• See related: The Legacy Material Check Before You Disturb a Finish; How to Seal a Roof Penetration That Will Not Leak
• Mounting block manufacturer installation instructions