Gaps at vertical trim joints of HardiePlank siding (C) Daniel FriedmanFiber Cement Siding Repair Advice & Specifications
How to repair fiber cement lap siding butt joint & trim joint gaps, damage, peeling paint, loose boards


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Fiber cement board lap siding repair guide.

This article describes repair procedures for cosmetic or functional siding issues that may arise such as loose siding, wind-damaged siding, siding paint or coating failures, and siding gaps, especially gaps at fiber cement siding butt joints where lap siding is installed.

Our page top photo shows loose and improperly installed fiber cement siding on a New York home. We quote from the siding manufacturer's installation guides, contractor guides, and appropriate codes & standards and we debate the pros and cons of caulking vs. flashing for certain siding repairs. We include sources for building siding back flashing & H-flashing products.

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Manufacturers' Advice for Repairing, Maintaining & Protecting Fiber Cement Siding

Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel FriedmanHere we tackle steps to stop ongoing leaks, damage, or to un-do damage from either a sloppy siding installation job or from more normal wear and tear as a building's exterior siding is exposed to weather, kick balls, peeing dogs, squirrels and raccoons, or to my grandsons and granddaughters, all six of whom (Chase, Tanner, Quinn, Sophie, Zoe, Chloe) could be hired as a home wear and tear test gang.

Article Contents

Our close-up photo of a butt joint gap in this fiber cement lap siding (above left) shows the width of the shrinkage gap.

The installers failed to install the recommended backer flashing / weatherproofing at any of the butt joints that we examined and we think the siding was installed wet and possibly not properly end-butted.

Cures for Loose Fiber Cement Siding Boards

Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel Friedman

Here as well as at page top are photos of loose fiber cement siding.

Considering that in our photo at left the siding should have been lapped (by the board above, typically 1 1/4"), for this end butt joint to be completely out of alignment with its neighbor we figure that the board above was also not properly nailed, possibly bulged as well.

Also notice that the boards are misaligned vertically as well. All of these clues suggest a super high-speed, sloppy siding installation by un-trained and un-supervised installers.

For more diagnosis we ask about the fiber cement siding's original installation & nailing: was the siding installed according to the product specifications for type of nail, nail spacing, overlap, back priming, butt joint and trim joint end priming & caulking? See the product's caulking, nailing, flashing, installation details provided by James Hardie and excerpted in our notes below.

Except in high wind areas, the product is installed using blind nailing (nails at the top of each siding course are covered by the succeeding course). Nails (or screws for metal stud applications) must be corrosion resistant, galvanized or stainless steel. Hot-dipped galvanized fasteners or stainless steel (near salt or ocean areas) are recommended.

Pin-Back Nailing repairs for loose fiber cement siding

Loose siding at house gable end (C) Daniel Friedman E GalowIn James Hardie's technical bulletin No. 17, the company provides a description of PinBack nailing for use at loose planks that were "high nailed" at original installation) or where there are problems with gaps, loose planks, or rattling noises.

At left we show a different loose siding problem: where the diagonal end-cut siding was installed against the home's sloping gable-end trim, the installer perhaps couldn't catch a stud, or perhaps s/he simply didn't want to move the ladder to nail the end of the siding board.

Now just a few years later the siding ends are curled away from the building. How can we fix this other than by a complete removal and replacement of this wall covering?


  • Pin Backs Pinning the plank down at the bottom edge is a common practice called “pin back.” It is used to correct “high nailing”, loose planks, gaps or rattling. Pinning of the butt joints with is not intended to increase wind load values; and shall be installed 3/8” from end and between 3/4” and 1” from bottom edge.

    The finish nail shall be nailed flush to the surface (not countersunk), must be corrosion resistant (e.g. galvanized or stainless) and does not provide structural support. For best aesthetics nail heads should be touched up to color match. [17]

Cures for Buckled Fiber Cement Lap Siding

Buckled fiber cement siding is most likely to occur because the installer failed to give the recommended gap between the ends of the fiber cement boards and abutting vertical trim at corners, windows, doors, etc.

Buckling fiber cement siding may also occur if the product was not properly nailed.

It may be possible to cut an end gap clearance in place between the siding end and vertical trim, wearing proper protective gear to avoid silica dust hazards etc.

Also see Pin Back Nailing discussed above.

Cures for Coating Failures on Fiber Cement Siding

Peeling fiber cmeent siding (C) InspectApedia

At JAMES HARDIE HardiePlank Siding we discuss the failure of this factory-coated fiber cement siding installation.

  • Coating type originally used: what type of paint was used on the product? Was it back-primed (if recommended) or factory primed? What is the moisture resistance of the coating used?

    The siding should be painted (use an acrylic paint) within 180 days for primed products or within 90 days for unprimed products. Do not use stain; do not use oil/alkyd-based paints;

    Do not use stain on James Hardie products.

    The company also warns against painting when the surfaces are wet - a topic that we elaborate ad nauseam at PAINTING MISTAKES. The James Hardie company provides special touch-up coatings for ColorPlus products.
  • Painting fiber cement siding: Take care when re-painting, to avoid incompatible paints. The company recommends 100% acrylic paint. [14] See James Hardie Technical Bulletin 17 for painting and other fiber cement repair recommendations.
  • Sealing / caulking fiber cement siding: how were butt joints and end joints treated, flashed, caulked? Cut ends of fiber cement siding should have been sealed using 100% acrylic latex primer or paint (Certainteed) or in some cases using an approved caulk wiped on the cut surface (see our notes on Hardieplank installation). And caulk is required at certain joints such as vertical trim at windows, doors, corners.

    Caulk is not to be installed in J channel, F-channel, nor in fiber cement siding butt joints, though it may have been applied later by a homeowner or maintenance contractor.

    Check the caulking specifications provided in the installation guide by the manufacturer for your specific siding product. For example OEM specifications on caulking for fiber cement siding, see SIDING, FIBER CEMENT GAP & CAULK SPECS.

Frankly, my OPINION is there is no nice cure for fiber cement siding with a coating failure such as shown in our photo. As we discuss in excruciating detail in our article series beginning at PAINT & STAIN GUIDE, EXTERIOR, surface preparation is the key to a durable paint job.

Even the most sophisticated coating will fail if sprayed over dirt, for example. See PAINT SURFACE PREPARATION for more help.

How to Decide on a Remedy for Paint Failure on Siding

Dan Friedman Art Cady painting L.S. home, Parker Ave Pou7ghkeepsie (C) Daniel FriedmanChoosing the best remedy for failing paint on fiber cement siding or any other exterior building surface depend on at least these considerations:

  • What is the base product material, and what condition is it in. For example failed paint on hardboard siding, if the siding has delaminated, means it's time to replace the siding. SIDING HARDBOARD

    Watch out: some siding materials and surfaces simply cannot be effectively and economically "saved" by re-painting. Siding replacement may be a less costly option in a number of circumstances.
  • How do we remove peeling paint from a factory-painted fiber cement product, how much bad paint do we remove, how aggressive should we be? See MEDIA BLASTING for MOLD REMOVAL - a technique that can prepare a bad surface for painting.
  • How do we prepare the cleaned surface for re-painting? Is there chalking of remaining paint? Do we need to use a special primer? PAINT SURFACE PREPARATION
  • What paint or coating should be used? Talk with both the siding manufacturer as well as your paint supplier. For fiber cement siding the paint and primer need to be compatible with fiber cement products.
  • How should the coating be applied? Spray is fast and less costly but may not bond the paint as well to the surface. With Art Cady I had success working as a team, using spray to get the paint onto the surface and a roller to work it into the surface for better adhesion.

Cures for Open Butt Joint Gaps in Fiber Cement Lap Siding Installations

Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel Friedman

Cosmetic issues: Building owners should also be made aware of possible future cosmetic issues, depending on the repair method used and workmanship;

For cosmetic concerns, if building condition permits a slower approach, consider trying more than one product and method for gap sealing, examining the durability and appearance of the repair after 12 months or longer of weather exposure on the most-challenged building sides.

Details about repair approaches to open butt joints in fiber cement lap siding are give at SIDING, FIBER CEMENT GAP CURES.

There we also discuss adding back flashing to seal open lap siding butt joints against leaks into the wall system.

Cures for Algae, Fungus, Moss, Lichens on Building Siding

Algae, fungus, moss or lichens on siding are usually due to a combination of moisture and shade on the building wall. Plants too close to the building wall, repeated wetting by sprinkler systems, or unnecessarily dense close shade tree growth are conditions that can usually be remedied.


For more about algae, lichens, moss growth on buildings see ALGAE, FUNGUS, LICHENS, MOSS.

Adapted, expanded & excerpted from CertainTeed Corporation's "CertainTeed WeatherBoards™ Fiber Cement Siding Installation Manual" [5a], from "Fiber Cement Siding Best Practices for Effective Job Site Management"[5b] and from JamesHardie Corporation installation instructions and bulletins for JamesHardie HardiePlank lap siding. [12][12b] and finally also from applicable building codes & standards.

How to Remove Individual Fiber Cement Shingles or Boards

Reader Question: how do I remove individual or bottom fiber cement siding boards?

20 January 2015 Tony said:
I need to remove a bottom Hardie Lap siding board because it has deteriated from the furnace condensation outlet. I have replacement boards but how do I remove the bottom board by itself when there is a board above it and covering the nails, etc.


Great Question, Tony. I have had to deal with this problem on FC shingle siding, lap siding and also on slate roofs.

If the FC siding has been painted such that the lower edge of the upper board is sealed tightly to the one below, cut the paint seal with a utility knife.

Then use one of the following methods to cut the nails holding the lower board:

If the lower edge of the upper board above the one to be removed is face-nailed through the lower edge of the upper board, reaching between the two boards, cut the nails with a hacksaw blade or slating tool as I describe next:

Slate hook (C) Daniel FriedmanFor slate roofs there is a hammer-tool that can be used for some siding removal - the tool slips up under the upper slate, a sharp flat hook slides over the nail, an outside handle of the tool is hammered down to cut the nail - but you may have trouble finding this tool.

But you can see it - the bright blue tool illustrated near the top of the article - at SLATE ROOF SOURCES & TOOLS and shown at left.

Alternatively, working carefully so as not to break the Hardie Lap boards above the ones to be removed, you can use a hacksaw blade to slip up between the boards, slide it horizontally to find the next nail, then saw through the nail. Buy a few metal-cutting hacksaw blades and a simple handle that attaches to one end of the blade leaving about 6" of free hacksaw blade.

If the FC lap siding boards are nailed very tightly you may be able to reach up under the board you are going to remove, using a flat bar, to pry GENTLY at the top of the board, beneath thus the board to be removed and the lower edge of the board course above it, prying thus between the back of the lower board and the wall sheathing, just enough to open the boards enough to go back to the hacksaw. Don't pry too much or you'l break the board above.

If the lower board was blind-nailed, that is nailed to the wall through its upper edge such that the second course lapped over the nails holding the first course board in place, you'll need to ....

Gently pry up the lower board using a flat bar beneath its lower edge just enough to be able to slide the flat bar up to the upper edge of the board to find where it is nailed.

From that position you probably can't use the hacksaw trick I describe above, but you can use either the slating tool I describe to cut the nails OR you can use a thin flat bar to drive up, cutting each nail in position.

Take care not to pry too much or you'll break the boards above.

More details about removing fragile or brittle siding materials are at ASBESTOS CEMENT SHINGLE REMOVAL


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