Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel FriedmanFiber Cement Siding Gaps
How to diagnose the cause & choose a cure for fiber cement lap siding butt joint & trim joint gaps

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Gaps in fiber cement siding: clearance gaps, how big should they be;

where should there be no gap (butt joints). What are the causes of un-wanted gaps at the butt joints in fiber cement lap siding? How are such gaps prevented and if they have already occurred, how are they best repaired?

This article series includes field reports of fiber cement board lap siding butt joint gap problems, fiber cement moisture problems, and other fiber cement lap siding installation or in-service defects, their diagnosis & repair.

We include diagnostic inspection photos and text explain how to recognize, diagnose, and cure this problem.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Fiber Cement Lap Siding End Clearance & Butt Joint Gaps

Gaps at butt joints of HardiePlank siding on an 8-year-old home  (C) Daniel FriedmanFiber Cement siding installation butt joint gap complaints, cause, effects, cures

By 2005 we had found a few quotes from building owners complaining of apparent shrinkage in James HardiePlank siding, with gaps of as much as 1/4" opening between the butt joints of boards or between siding boards and abutting vertical trim.

[Click to enlarge any image]

Complaints about gaps, loose and curling siding and even siding loss continued to be reported in 2018.

"Anybody had their HardiePlank shrink? Ours developed about 1/4 inches gaps between the boards, which needed to be caulked."

"It [James HardiePlank Siding] was installed in August of 97 and there is some shrinkage but only 1/8 inch gap or so in some seams." [3]

And HardiePlank is not the only fiber (wood) cement board siding product with shrinkage questions reported in 2008:

" We have CT fiber cement board [CertainTeed Weatherboard™fiber cement siding] on our home. We built our house in 2004 and the siding is shrinking and warping and in the upper Midwest, this is a big problem for CT.

I believe these problems may have put our builder out of business as he has been trying to make repairs with no or little assistance from CT. "
... and ...
"I've seen it shrink, bow, peel, and pop nails within a year."

We had winds knock some siding off and it is loose. House is almost 10 years old. We were additionally told it is our responsibility to recaulk the joints every year, but from what I read here you shouldn’t?

We were told by 4 different contractors that our siding wasn’t installed properly, but the builder just came to look at it and said it was installed per manufactures instructions. We were told it was Hardiboard when we bought the house, but the builder says CertainTeed now (even though I don’t think it looks like it from your photos). It was nailed into particle board, I believe, but seems to have been nailed willy, nilly. - Anonymous by private email 2018/03/07-08

Our fiber cement lap siding photo (above left) illustrates the butt-joint gap problem. On this building we measured gaps between siding boards of as much as 5/16" at butt joints.

Make Sure You Have Correctly Identified the Siding Material and Brand

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

Watch out: informal surveys we performed in 2012 and 2013 in which we reviewed the top 20 website discussing fiber cement siding failures or performance issues we found a lot of confusion: hardboard siding was mistaken for fiber cement siding, and people were also inclined to assign a manufacturer's name to a product before it was positively identified.

In the siding gap complaint we investigated and documented here the original property owner thought that they had contracted for the installation of HardiePlank® fiber cement lap siding.

We found that another "Brand X" product had been installed instead. The owners did not get what they paid for in both the product itself and in the quality of its installation.

To identify fiber cement siding products as CertainTeed, Hardieplank or Brand "X" Fiber Cement Siding please see SIDING, FIBER CEMENT IDENTIFICATION .

Opinions about the cause of fiber cement board siding gaps and proper cures vary but making sure that the fiber cement siding is dry at delivery & proper siding storage prior to installation are important.

A few sources assert that HardiePlank and similar products do not warp or shrink, [7] leaving us surprised when we first heard of shrinkage complaints with these products back in 2005.

But as we document below, currently the company warns that if the product is not stored properly (flat, keep dry) prior to installation, if the material becomes wet or saturated the result may be shrinkage at the butt joints.

Therefore it is most likely that HardiePlank lap siding shrinkage gaps appearing at butt joints or possibly at vertical trim joints (those are usually caulked) are most likely due to the installation of siding that was at too high a moisture content at the time of installation.

Where did this "too wet" or "too moist" fiber cement problem originate? Before investigating the topic in depth we thought that the most common source of too-wet fiber cement siding was improper storage and handling prior to installation, exposing the siding to wet or saturating conditions.

Below: three more FC siding loose, gaps, mis-nailing photos from two different buildings at different locations.

Certainteed FC siding with nailing defects and siding blowing or falling off (C) InspectApedia.comJF Certainteed fiber cement siding gaps and caulk (C) JF

Above: siding gaps and siding falling-off of a home where FC siding appears to have been improperly-nailed. The closeup from the same buidling's fiber cement siding shows a caulked and separated FC butt joint and face-nailing of the boards. - Anonymous by private email 2018/03/08

[Click to enlarge any image]

Other factors in gap formation in fiber cement siding might include a combination of coating inadequacy, nailing defects and weather exposure.

Below: besides shrinkage gaps shown above, still another readers' photo below show more face-nailed and loose fiber cement siding boards, board butt joints not staggered up the wall, and curling up FC siding board ends. - Anonymous by private email 2018/03/05

Certainteed FC siding with nailing defects and siding blowing or falling off (C) InspectApedia.comJFBut we have also observed fiber cement siding that was delivered to the job wet from the supplier, even in cases where the product was still in the original, un-damaged factory-sealed plastic protective wrapping.

Where we observe an extensive problem with lap siding gaps in fiber-cement products it comes as no surprise that we are likely to find additional installation mistakes, errors, or omissions as well.

In several of our fiber cement siding photos above you can see that the siding was inadequately nailed and is quite loose.

On all of these buildings, some siding boards not only were lifting away from the structure at butt joints, we also found that some boards were misaligned by more than 1/4" vertically, giving a stair-step misalignment at the butt joints.

We trust the company's experience, opinions and advice about their product, but not everyone agrees, as this carpenter commented as shown just below:

"Both products [Hardieplank and CertainTeed fiber cement board lap siding] differ only slightly, but each does have there [their] molecular stamp.

People who are having problems with it are not installing it right. There is absolutely no way it should be warping, if it is installed right. The molecular make up won't allow it to shrink and swell as much as people are supposedly experiencing.

If you stick with the install specs, then problems will not occur. Spacing and gaps, nailing and nails, what kind and how you caulk, paint base and what you paint, vapor retarder vs barrier, sub-sheathing, everything that is included in the specs should be followed. ... They give us carpenters a bad name." [4]

Really? we disagree with the OPINION expressed just above at a DIY website - that installer's comments disagree with field reports, actual field experience, and consumer as well as installer observations as well as independent expert research on this topic.

Actual Causes of Shrinkage Gaps in Fiber Cement Siding Products

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

Our close-up photo of a butt joint gap in this fiber cement lap siding was made to document the width of the shrinkage gap.

But more information is at hand: the installers failed to install the recommended backer flashing / weatherproofing at any of the butt joints that we examined.

Observation of the location and size of gaps; normally shrinkage in a fiber cement siding product (or expansion) is uniform along the length of individual pieces.

Sources of Gaps in Fiber Cement Siding Products

Coefficients of Thermal & Moisture Expansion for Fiber Cement Products

There are two basic expansion/shrinkage forces in operation in fibre cement products: temperature and moisture. Of these, moisture is generally more significant.

Thermal expansion of fiber cement siding: Doggett (2012) places the coefficient of thermal expansion (COTE) for fiber cement products at 3.6-10 to 7.6-10 in/ in degree F, giving a linear expansion of less than 0.08" over ten feet of material.

Moisture expansion of fiber cement siding: Similarly the same author reports moisture-induced movement of 0.05% to 0.098% of the material length based on ASTM C1185 (which he criticizes as inadequatel because of the possibility of significantly more back-side absorbed moisture than anticipated by theh standard).

Still this moisture-induced expansion can generate 0.12" of expansion over 10 feet.

Moisture expansion + thermal expansion if occurring simultaneously can produce movement exceeding the 1/8" movement standard. Doggett also notes that panel edge swelling (perhaps where edges are left un-sealed at cuts or where the edge seal has failed) can exceed 1.5% of the material thickness at saturation.

He also points out that a 1/8" gap left between FC siding products is too small to accomodate proper joint design for effective sealants that specifies a minimum sealant (caulk) width of 1/4" and a width to depth ratio of 2:1.

Really? For 1/2" thick Nichiha Fiber Cement siding boards the width would thus be calculated at one inch! Perhaps we've misundersdtood the author.

Doggett makes this important observation about the ASTM standard used for determining moisture movement in fiber cement siding products:

For the purpose of determining moisture movement, ASTM C1185 stipulates specimen exposures to practical equilibrium at a relative humidity range of 30-90% and a temperature of 73F (±4F).

Moisture movement testing is performed independently of water tightness

. In other words, the results assume that bulk water is not introduced and that bulk water is not transferred to the back side of the panel.

We argue that such assumptions are flawed and that joint performance should anticipate bulk water wetting to the back side. - Doggett (2012)

Comparison of the moisture-related expansion of wood with fiber cement

While expansion-compression is a significant factor in wood flooring that was installed dry then soaked-wet to cause swelling, the coefficients of expansion for wet wood are notably greater than for fiber cement siding, with wood moisture caused swelling varying by species and direction of wood grain.

While there is wide variation among wood species and among individual boards, Simpson & TenWolde put the shrinkage percent from green to oven-dry moisture content in Coastal Douglas Fir at 1.8% radial, 7.6% tangential, and 12.4% volumetric. - (Simpson 1999 US FPL)


References for Expansion Properties & Other Characteristics of Fiber Cement Siding

Appearance of Gaps in Fiber Cement Siding Products

The following conditions or details may affect how shrinkage gaps may appear in siding products:

Watch out: We have seen serious shrinkage in both counterfeit fiber cement siding and right from the factory James Hardieplank® fiber cement siding that we inspected and tested in 2012 and 2013 - DJF. Following the removal of "counterfeit siding" that had shrunk, disappointing the building owners, the contractor, Galow Homes, ordered new Hardieplank siding from a local building supplier.

As we document at SIDING, FIBER CEMENT MOISTURE LEVELS, new FC siding arrived visibly wet (inside its plastic wrapping) from the supplier and from the manufacturer. And as we also explain there, the definition of "wet or saturated" siding mentioned above is not given in the product literature nor easily obtained from at least some fiber cement manufacturers.

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

Our siding close-up photo at left illustrates what may be another clue in the history of this failed (unidentified brand) fiber cement installation. We can see a hard-dried layer of caulk that was a bit under 1/8" wide inside of the now wide-open butt joint between two siding boards.

We suspected that someone noticed these shrinkage gaps in the building siding earlier in the life of the installation, possibly very soon after installation or even during installation as the probably-wet siding boards dried and began to shrink. Someone attempted a cosmetic repair.

Unfortunately the repair was performed before all of the siding shrinkage had taken place. That this siding installation continued to shrink after the caulk application is evident by this image.

Also notice that the boards are misaligned vertically as well. All of these clues suggest a hasty installation by un-trained and un-supervised installers. More examples of siding installation errors are included below.

Open fiber cement siding butt joint shows no back flashing was installed (C) Daniel Friedman E Galow

Reader Question: 1/4-inch Gaps at Butt Joints in a CertainTeed siding installation? My contractor says it's factory Spec

I had CertainTeed factory painted fibre cement siding installed on my new home last year. The installer, contrary to the CertainTeed installation instructions, left a 1/8" gap at all butt joints. He now maintains that CertainTeed and other contractors state that a 1\4" gap at butt joints is acceptable.

I cannot find any documentation to support this, and I maintain that the requirement by CertainTeed to not leave a gap of any kind is to greatly minimize any shrinkage that might subsequently occur at the butt joints.

Could you help me with this? Thank you. - M.D 4/11/13

Reply: Certainteed emphatically advises against any gaps at their fiber cement siding butt joints; caulking is also not recommended. Current (2014) advice is "snug" but not "tight" butt joints.

Thank you for the interesting or I should say troublesome Certainteed fiber cement siding question - it helps us realize where we need to work on making our text more clear or more complete.

I've been working on the siding gap worry along with local contractors here in NY for some time and have investigated some cases in other states.

The three sources of siding butt joint gaps: improper installation, improper product storage, thermal movement

My OPINION from what I've learned is that there are three distinct sources of butt joint gaps in fiber cement siding installations

  1. Improper siding installation: if the fiber cement product butt joint gaps are due to installation error the installer did not follow the manufacturer's specifications. [This is apparently your case]
  2. Siding shrinkage: If siding butt joint gaps are due to shrinkage, the central problem is one of shrinkage that occurs principally if the siding was not stored properly prior to installation - particularly, if it got wet. High moisture at time of installation is going to give gaps. Ugly gaps.

    Watch out: we also have observed siding that was wet at the time of delivery from the supplier, even wet from the manufacturer.
  3. Thermal /moisture-caused movement of siding: some siding expansion & contraction due to temperature changes and due to moderate environmental moisture level changes (excluding board backside wetting from wall leaks) is normal.

    Manufacturers such as CertainTeed note that the total amount of movement in their fiber cement products is less than in similar-looking wood siding (that may also be more affected by moisture changes). While allowance should have been made for thermal movement at time of installation (end gaps at vertical trim) this phenomenon may make modest gaps appear at butt joints when the siding has been in service for a time.

    Watch out: in long continuous runs of fiber cement (or other types) of lap siding, failure to allow adequate clearances at the ends where the siding abuts vertical trim may result in buckling and gaps just such as the ones you describe in your observations.

And there is no very nice fix on pre-finished siding in particular - caulk + paint looks ... well, like caulk + paint. Caulking is NOT recommended by the manufacturer as I note in detail below - a point stated in several ways in the instructions.

On painted siding, and provided the shrinkage is finished, one can possibly make a suitable cosmetic repair depending on the ability to match the textures involved and depending on coating compatibility - CertainTeed advises checking with the coating manufacturer.

If there is a butt joint gap not due to erroneous installation but rather because of moisture due to improper material storage, how do we know shrinkage has finished? Measuring moisture in spot checks would help but not be a complete answer as moisture varies considerably over a building's exterior..

You haven't told me exactly which product you've installed - an important detail. I infer that it's CertainTeed's WeatherBoard siding. But from what your installer should have read in the installation manual there should be NO gap at butt joints. And that joint should also not have been caulked.

Certainly 1/4" butt joint gaps in WeatherBoard type siding (but not shake siding) would be absurd and it violates the company's installation instructions (and possibly your product warranty).

Manufacturer expressly advises against gaps in butt joints in fiber cement siding

Certainteed is emphatic on this point as it's the just about the only caveat that is repeated so many times throughout the instructions. No gaps at butt joints. No caulk at butt joints.

Other locations should have gaps and should be caulked, such as at vertical trim abutments and at horizontal zee flashing requires a gap but is not caulked.

So where did your contractor look to "make up" his explanation that his siding installation was correct?

The specification for a 1/4" gap IS provided but not for butt joints. That spacing is clearance at siding bottom or between siding bottom and horizontal trim flashing shown on pg. 29 [5]

If you don't have the installation manual you can download it from Certainteed at, or directly at , or you can contact the company's Consumer Help Line: 800-782-8777,


Continue reading at SIDING, FIBER CEMENT GAP CURES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see SIDING, FIBER CEMENT SHINGLE-BOARD DEFECTS for an extensive field report on a Chinese brand 1/2" thick fiber cement siding installation with gaps, loose boards, buckling and other complaints.

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