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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.
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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.
And HardiePlank is not the only fiber (wood) cement board siding product with shrinkage questions reported in 2008:
Our fiber cement lap siding photo (above left) illustrates this 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
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
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,  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. Other factors might include a combination of coating inadequacy, nailing defects and weather exposure.
But 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. At above left, the siding was inadequately nailed and is quite loose. On this building 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:
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.
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
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:
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)
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.
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.
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
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 
If you don't have the installation manual you can download it from Certainteed at certainteed.com, or directly at www.certainteed.com/resources/fc017.pdf , 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 the More Reading links shown below.
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