How to Remove and Replace a Section of Fiber Cement Lap Siding

Why this matters

Fiber cement lap siding is the opposite of vinyl. It does not bend, it does not slide, and it does not come off without cutting something. A plumber who cracks a board replacing a hose bib, or an HVAC tech who needs a new penetration through it, is going to cut fiber cement, and cutting it is a silica exposure that a power saw can make far worse than it needs to be. Then the board has to go back with its joints flashed, because a butt joint without flashing behind it sends water straight into the wall, and the stain on the sheathing shows up years later with nobody's name attached. Patching technique in general is in Siding and Trim Patch Repair; this card is the fiber cement section replacement.

Before you cut: is it really fiber cement, and how old

Modern fiber cement lap siding is wood-grain or smooth boards, usually 5/16 in thick, installed from the late 1980s onward. Cement-based siding on older houses, most often as shingles with a wavy or dimpled face, may be asbestos-cement, which was in wide use through the 1970s. They look alike to someone who has not seen both. On a house old enough to carry either, stop and run The Legacy Material Check Before You Disturb a Finish before any cut, drill or break. Asbestos-cement that is intact is not a hazard; breaking it is.

The silica control, stated once and applied at every cut

Fiber cement contains crystalline silica. Cutting, drilling or grinding it releases the respirable fraction, which causes silicosis and lung cancer. On construction work, 29 CFR 1926.1153 governs, and its Table 1 lists, in substance, handheld power saws with blades of 8 in or less cutting fiber-cement board outdoors, with a saw equipped with a commercially available dust collection system run as its maker specifies; done that way, Table 1 requires no respirator for that task. Check the current text of Table 1 for the full conditions. Three ways to cut, from least dust to most:

• Score and snap. A carbide scoring knife run along a straightedge several times, then the board snapped over an edge. Almost no airborne dust. Works for straight crosscuts in thin lap boards.
• Electric or pneumatic fiber cement shears. Make a strip of chips rather than dust.
• A dust-collecting circular saw, outdoors, with a fiber cement blade, per the Table 1 entry.

Cutting an installed board in place with an oscillating multi-tool is not that Table 1 entry. Capture the dust at the blade with a HEPA vacuum, keep the cut short, and wear a respirator under a written 29 CFR 1910.134 program. Never cut fiber cement indoors with a saw, and never dry-sweep or blow the dust; vacuum it with a HEPA vacuum.

The courses, drawn

 LAP COURSES, FACE VIEW (studs at 16 in on center)

   stud      stud      stud      stud      stud
     |         |         |         |         |
 ----o---------o---------+---------o---------o---
     course 3: butt joint (+) on a stud center
 ----o---------+---------o---------o---------o---
     course 2: joint one bay over, never stacked
               above the joint in the course below
 ----o---------o---------o---------+---------o---
     course 1: joint staggered again

 o = blind nail at the top edge of each board,
     about 3/4 to 1 in down, hidden by the lap of
     the course above
 + = butt joint on a stud center, with a strip of
     flashing behind it lapping both boards

Two things in the drawing decide the repair. The nails holding any board are hidden under the course above, so to free a board you reach under that lap, not through the face. And a new section creates two new butt joints, each of which needs to land where the instructions allow and needs flashing behind it.

The steps

1. Plan the section on the studs. Find the studs with a magnet on the old nail heads or a stud finder, and mark the section to remove so each end lands on the center of a stud, at least one stud bay away from the joints in the courses above and below. Reason: the new butt joints need solid fastening on each side, and joints stacked above one another give water a straight path. If you skip it: you cut between studs and have nothing to fasten the new ends to.

2. Release the course above by cutting its hold on this board. Slip thin shims under the bottom edge of the course above to open the lap a little, with no prying against the board, since fiber cement cracks rather than flexes. Slide a hacksaw blade, or a metal-cutting blade on an oscillating tool, up under the lap and cut through the blind nails holding the board you are removing. If the siding was face-nailed instead, two sets of nails hold the board: its own, visible near its bottom edge, and the course above's, which pass down through this board's top edge under the lap. Cut both. If you skip it: the board will not come out, and forcing it cracks the course above.

3. Cut the ends of the section on the stud lines. Cut vertically through the damaged board only, at each stud center, with the dust-controlled method above, and stop at the lap of the course below. Hazard: this is the dustiest act in the job; vacuum at the blade and wear the respirator under your program.

4. Remove the section and inspect behind it. Lift the piece out. Look at the water-resistive barrier: torn wrap or a wet stain is fixed now, before anything goes back. Reason: this is the only time that layer is exposed.

5. Cut the new board to the opening. Measure the opening at the top and bottom, cut the replacement to fit with the butt ends in light, moderate contact with the old boards, the way most makers specify, not jammed tight and not gapped. Score and snap the crosscuts where the board allows it. Prime or seal the cut ends with what the instructions name.

6. Flash each butt joint. Slide a strip of flashing behind each new butt joint, over the barrier and overlapping both boards, resting on the lap of the course below. Reason: the joint itself is not watertight; the flashing behind it is the seal. If you skip it: water running down the face enters the joint and has a clear path to the sheathing.

7. Fasten the new section. The course above now covers the new board's top edge, so it cannot be blind-nailed. Face-nail it at each stud, typically about 3/4 to 1 in up from the bottom edge so the nail passes through the lap and catches the course below, with corrosion-resistant nails, and keep nails the minimum distance from the board ends the instructions state. Also face-nail the two old board ends at the shared studs. Drive heads snug to the face, not countersunk. Reason: an overdriven head crushes the board and loses its hold. Hazard: a nail gun kicks off a hard face; keep your free hand clear of the line of fire and wear eye protection.

8. Seal and paint. Caulk where the board meets trim and around any penetration, with the sealant the instructions name. Whether butt joints themselves get caulk varies by maker; most rely on the flashing and some say not to caulk them, so follow the sheet. Touch up the face nails and paint the board. See related: How to Get a Color Match From a Paint Chip.

Worked example: a cracked board at a hose bib

A plumber replaces a leaking frost-free sillcock on a 2009 house with fiber cement lap siding. The old sillcock had been screwed hard against the siding, and the board around it is cracked across its face. The boards are 8 1/4 in wide, laid with a 1 1/4 in lap, so each course shows 8 1/4 minus 1 1/4, which is 7 in. Studs are at 16 in on center. The house was built in 2009, outside the asbestos-cement window, and the boards are wood-grain lap.

Plan. The crack runs about 10 in either side of the pipe. The nearest stud lines either side of the crack are 32 in apart, two bays, with one stud between. The joints in the course above are at least one bay away from both lines. So the section is 32 in long, ending on stud centers, with three studs behind it.

Removal. Shims opened the lap above about 1/8 in. Three blind nails under the lap, one at each stud, were cut with a hacksaw blade. The two vertical cuts were made with an oscillating tool and a fiber cement blade, with a HEPA vacuum hose taped at the blade and the plumber in a half-face respirator with P100 filters under the shop's program. The section lifted out. The house wrap behind it had a 2 in tear where the old sillcock flange had been forced; it was taped with the wrap maker's tape.

New section. The opening measured 32 in at top and bottom. The replacement was cut to 32 in by score and snap on the tailgate, outdoors, with no saw, its ends primed, and bored for the new sillcock with clearance for sealant. Flashing went behind both butt joints. The board was face-nailed at the three studs, and the two old board ends were face-nailed at the end studs: five face nails in all. Sealant around the sillcock, the board painted and the nail heads touched up.

Result. A 32 in section, two flashed butt joints in moderate contact, nailed into all three studs, the barrier repaired before closing. The ticket records the board size and exposure, the section length, the cutting method and dust controls used, the barrier repair, and the paint used.

What changes the method

• Pre-finished boards. Factory-painted boards are touched up with the maker's touch-up kit, and a cut edge on a pre-finished board shows if left raw.
• The damage reaches the course above or below. Each course is a separate section with its own two joints, and the joints are staggered between courses.
• A shingle or panel profile instead of lap. The fastening and the flashing are different; work from that product's instructions.

References

• OSHA 29 CFR 1926.1153, respirable crystalline silica in construction, Table 1; 29 CFR 1910.134, respiratory protection
• See related: Siding and Trim Patch Repair; How to Flash an Exterior Wall Penetration Into the WRB
• See related: The Legacy Material Check Before You Disturb a Finish
• Fiber cement manufacturer installation instructions (lap, nailing, joint flashing, clearances)