Fiber cement siding installation defects: buckling, curling, gaps, leaks, loose boards (C) E.D. Sources & Effects of Moisture Absorption in Fiber Cement Siding
Field report & discussion of moisture levels in fiber cement siding

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Wet fiber cement siding; fiber cement shingle, shake or clapboards formed moisture sources & effects:

This article describes the probable causes & effects of moisture-uptake problems that occur in fiber cement siding shingle & shake installations in North America, including fiber cement shingle shrinkage gaps at butt joints, cracks, breaks, and loose or buckling shakes.

The article includes research on the sources, causes & effects of water absorption into fiber cement building products.

Possible Sources & Extents of Water Penetration into Fiber Cement Siding

Why Fibre Cement Siding Products Should be Dry When Leaving the Factory

"The Hatsheck process was initially developed for the production of asbestos composites, but it is now used for the manufacture of non-asbestos, cellulose fibre reinforced cement composites.

In the Hatscheck Process, unbleached cellulose fibers are re-pulped in warm water at an alkaline pH of 11 to 12.5; the re-pulped fibers are refined and then mixed with cement, silica sand, and other additives to form a mixture. The fibre cement mixture, is deposited on a felt band substrate, vacuum de-watered,and cured to form a fibre reinforced cement matrix in sheet form.
In autoclave technology, polymer based composites are manufactured by applying intense heat and pressure to eliminate moisture from fibre cement materials. This leads to composite consolidation and the result is an extremely strong and robust fibre composite material." - Shera™ Fibre Cement Building Materials, 2426/3 Mahaphant Building, Chareonkrung Road, Bangkorlhaem, Bangkok 10120, Thailand, Tel: Europe: +44 (29) 212 86505, Thailand: +66 (2) 291 2888 retrieved 12/20/2014, original source -

Protection of Fibre Cement Siding from Weather Before Installation

OPINION: Consider this simple argument: if fiber cement boards as shipped from the factory are coated with a waterproof coating on all six surfaces, why would they be vulnerable to weather or water damage prior to installation? It's only at installation that the boards are cut.

Now the surfaces of FC siding in storage, transit, handling, may be abraded, possibly compromising the water resistance of the material in less obvious or even subtle ways. See Dargontina (2007).

But I agree that manufacturers may vary in the quality and thickness of coating on the various surfaces of the fiber cement siding and that such might cause variation in moisture uptake during storage.

We'd need to see some independent lab tests but from the products I've seen water entry into the product seems to occur from the following sources:

Watch out: If fiber cement siding was installed wet or if the siding's interior surface becomes wet after installation shrinkage, buckling, and related problems may be encountered. Here is what the NichiBoard specification warns:

Do not install boards or components that appear to be damaged or defective. Do not install wet boards.
. - NichiBoard_3-part_Spec.docx retrieved 12/16/2014 original source

Reader follow-up: installed fiber cement siding absorbing water?

The reader-comments appearing at points in this article were provided by a reader whose siding is further illustrated and discussed

at SIDING, FIBER CEMENT SHINGLE-BOARD DEFECTS - Troubleshooting gaps, breaks, peeling, bowing, loose fiber cement siding: causes, remedies. Nichiha Fiber Cement products, performance & warranty field report.

Take a look at these photos taken this morning of the worse area.

Fiber cement siding after exposed to rain (C) InspectApedia E.D> Fiber cement siding after exposed to rain (C) InspectApedia E.D>

There's been a noticeable change -- to me at least -- after the heavy rains here these last few days!

Defects are still obvious but the boards look like they are lying much flatter! And gaps closing!

Sponge effect?

The photos I took when the apparent warping looked the worse were taken at the end of the summer (a long drying season here -- although we often do get wet fog). I really don't think it looks anywhere near as bad right now as it did back then.

(* that's my own home made duct tape flashing that I put up over the worse spots where house wrap had split)


If the installed fiber cement siding is absorbing water and expanding or swelling this may be worrisome since closing up board butt joint gaps in rain suggests that the material is absorbing water from normal weather conditions and that in this case siding movement in response to moisture changes may be a factor in buckling or fastener failures you reported earlier.

Now siding movement not itself weird: vinyl siding expands contracts in response to temperature swings and therefore is both overlapped and "hung" rather than nailed tightly to the building to avoid trouble. Wood clapboards and other materials also absorb moisture and then dry out as weather cycles occur.

But if the absorption and dryout cause product complaints or failures: buckling, bulging, fastener failure, and then shrinkage, gaps, more leaks, that does not sound promising.

Reader comment: claim that fiber cement board shrinkage or curling is due to absence of end cut primer

My building scientist says:

"Finished products may be either pre-primed or unprimed. It is important to note that coverage of pre-primed surfaces may vary, resulting in uneven protection. Fibers and matrixes may be further exposed through minor cracking, abrasion, and field cuts.

Manufacturers’ requirements for caulking or priming damaged surfaces are poorly achieved under routine installation practices.

These exposed fibers create important pathways for moisture absorption, freeze-thaw damage, and fungal colonization.

While they may span relatively small areas, these unprotected surfaces can significantly reduce the material’s overall durability even in the absence of extreme moisture or other deleterious agents."

He goes on to describe the various stages of the life cycle of cement fiber at the molecular level -- initially strengthening for a few years after manufacture and then gradually degrading depending upon installation factors. I see mine entering the post-5 year softening phase. - E.D.

Reply: Moisture entry through larger fiber cement board areas is the dominant effect

Really? It's the total area of board that is both exposed to water or moisture absorpation that is key in explaining both board curling complaints and, if installed wet or moist, board shrinkage gap complaints.

If the back surface of a fiber cement board is moisture permeable, or more moisture permeable than the weather side of the board, that is an enormously larger total board area than the end cut exposure. There were other reasons for priming end cuts including avoiding deterioration. We agree with the hint given in your building scientist's quote, that the water resistance of fiber cement board or panel coatings may be compromised by abrasion or other events.

In at least some fiber cement board products, the back surface is visibly less coated or sealed than the weather side.

Differences in moisture resistance may be present even when the coatings appear the same to the naked eye (confirmation research needed). If the boards were really waterproof on all surfaces the manufacturers would be less worried about protecting the product from the weather prior to installation - a requirement we see in the installation specifications of every fiber cement board installation guide we've surveyed across several manufacturers.

But it's also important to follow the water. The missing flashing throughout the installation of your siding forms a huge number of water penetrations into the wall, sending far more water down the back side of siding than just what would be absorbed into board end-cuts.

On homes where we removed FC siding we could often see the water marks running down the housewrap, and we also often found significant leaks at window and door tops that had been improperly flashed and sealed.

For the end-prime argument to be determining in water absorption we'd have to agree that the back of a board were waterproof. I'm not convinced that's the case.

Boards, even coated, are exposed to abrasion in handling and storage; and the boards of various types and materials that I've examined at suppliers and installed or on jobsites (which do NOT include this product line) manufacturers, I have not found that the back-side of a board was ever coated to the same standard as the weather side.

Here is what Nichiha says about the product's coating:

Every product we make comes off the line with three protective coatings; 2 coats of primer and an anti-efflorescence coating that prevents chalky residue from appearing.

The result is a product that holds paint better and looks better longer. We’re confident enough to back that up with the strongest warranty in the business. - original source: retrieved 12/16/14

And repeating an earlier quote from Nichiha:

Sierra Premium and NichiFrontier products MUST be kept dry, and stored in a covered area or covered with a tarp before installation.

Moisture saturation before installation may result in shrinkage and board damage. Do not install saturated boards. All pre finished products are required to be stored in a covered area.

If the boards can become problematically wet before installation, and if water is leaking into the wall after installation, it seems to me that the same risks about which the company warned still pertain.

Reader follow-up:

Don't agree. OK when it rains ... but that's almost never! (why I live here).

On the other hand -- lots of overnight fog and humidity. The house is often "dripping" wet in the morning before the sun comes out. That water easily infiltrates the butt joint/open ends and likely not much further. E.D.


When you can see the back surface of a board it will be instructive to compare its coating to the front side.

If we ultimately agree that the back surface is as water-resistant as the exposed weather side that leaves principally but not only the un-primed ends to suspect since IF there is water in the wall it has more time to soak on the back than on the air and sun exposed front of the siding.

And if failure to prime end cuts is the prime problem source we ought to be able to confirm that by in-situ examination of a representative number of butt joints, identifying those that were factory primed, those that were site primed (if any) or otherwise sealed (the manufacturer permits but does not prefer caulk), and those that were left cut-bare.

Keep in mind those repeated warnings from the manufacturer about protecting the material from weather before installation. What are they worried about? Water absorption into the material from immersion, lying flat, not draining. Similar water absorption problems may occur from wet wall conditions.

If/when siding is removed from this home one would want to look closely at the housewrap and at the backs of siding boards for water marks to indicate the areas and amounts of water passage.

Reader follow-up:

Fiber cement siding with factory-primed ends curling (C) InspectAPedia E.D.

[Click to enlarge any image]

I'm not ready ready to give up on back of board entry or blame it all on unpainted ends yet.

Take a look at attached photos.

A single 8 ft length of uncut board with factory primed ends. E.D.

Something happened to the whole board? Both ends lifted and its pretty much making normal looking contact in the middle?

Fiber cement board end curl (C) InspectApedia E.D. Fiber cement board end curl (C) InspectApedia E.D.

Above, photos show the ends of a factory-primed board curling upwards at left (edge A) and right ends (edge B).


Even with a pound of field experience and a half-pound of forensic lab expertise I can never be as smart diagnosing by e-text nor by photos (I only see where someone else aimed the camera) as can be an on-site observer who has an ounce of sense.

But I'd like to know if you are seeing buckled FC siding ends only at ends that were field-cut and left un-painted, and I'd like to hear that you have inspected factory-sealed ends and found those boards behaving more nicely.

In your photos I see buckled board ends but I can't see the actual cut edge.

On your Edge A Gap photo at above left it looks to me as if the board is lifted for more than a foot and a half along its length.

The idea that moisture got that far into the length of the board only from an entry point at its cut-end is a challenging one.

But if I understand correctly you are showing end-curling at both ends of a board that was not cut in the field, whose ends remained factory sealed, and whose board ends are nonetheless curling upwards in response to moisture.

In my OPINION this would be consistent with water entering through the back side of the board and consistent with more water being present at butt joint openings both at this board and above it.

A related argument is simply that the board ends are innately less secured against curling than the board center.

Reader Follow-Up: Fiber Cement Siding Performance when Installed using Best Practices

Today I found another Nichiha [fiber cement-sided] house on the island (its a small place and I thought I'd already found them all)!

Every single house I'd seen (except one) had been installed by the same siding company that did mine -- and there were problems with every one! That exception is an interesting data point. Lots of details from the owner who was intimately involved in every aspect of the construction.

They used factory primed but unpainted Nichiha -- did "almost" everything right per install instructions with the exception of butt joints landing on studs and screwing down corners at butt joints.

They caulked vertical abutments, flashed every butt joint and then painted after the install.

The overlay was 1 1/2" rather than 1 1/4". Not one rattle or loose board. Very tight. The job was done by finish carpenters during the housing recession here about 4 years ago. Until today it was the best looking Nichiha I'd found.

All the boards are lying flat with no obvious defects.

But even here after about 4 years the boards were separating at butt joints -- no warping or curling BUT there are fairly uniform flat separations are occurring at many butt joints in the range of about 3/16" - 1/4" (that the owner is now attempting to caulk) (although at the time of this installation (and mine) the Nichiha recommendation was for a minimum 1/8 gap at joints (now they say moderate contact).

This house I found today was also done just about the same time as mine (according to hearsay) -- by the same installer who did mine! And from what little I was able to see from a cursory examination of one wall, the difference was night and day!

The areas I could examine were unbelievably tight! I couldn't easily identify a single butt joint! It took a while and I did locate a few -- but they were so tight they appeared to be hairline cracks! One big difference between this house and mine is that it doesn't have a large surface area exposed to the sun. I intend to go back again for a closer look at all sides of the house.

But one measurement I did take was overlay -- and this house also used 1 1/2" ! I don't know -- there may be something here! Not one rattle or even slightly loose board. Very tight.

I know that Nichiha came out with a new product line this year.

The width was changed from 9 1/4" to 9." The minimum overlay was left at 1 1/4".

This automatically gives you a slightly reduced exposure and a reveal the same as these houses. Maybe this is their attempt at trying to fix the issues I'm experiencing? Maybe I'll go even further, to 1 3/4", on my next install? - E.D. 12/17/2014

Reply: High pressure on siding contractors & job costs preclude perfectly dry fiber cement siding installations

I do not think the width reduction addresses shrinkage problems nor nailing issues as the head-lap was left unchanged. Manufacturers change siding width for current market - style reasons as well as to reduce the actual product production cost. Like shrinking chocolate bars sold at the same price as before.

Shrinkage gaps won't be affected by a reduced exposure. Buckling might be but not meaningfully.

The Best-Workmanship Fiber Cement Siding Installation Case You Observed

In my experience even in a "perfect" installation FC siding shrinkage shows up to at least some degree. No one measures moisture at the time of installation, the factory typically doesn't even acknowledge that moisture can be measured, and there is no explicit moisture standard (other than what I've published).

Rather the manufacturers give subjective guidance "don't install it if it's wet" - which is safe for them to say.

In my OPINION if FC cement later has opening butt joint gaps then it was installed at a moisture level that led to that trouble. How quickly the gaps appear, how wide they are, and where they are depend on original product moisture, sun exposure, and similar factors.

I do not (yet) think that we can explain any meaningful amount of FC shrinkage as occurring because dry-installed product got wet after installation.

If the product were installed really dry, butted tightly, then got wet, it'd expand and buckle.

What is also interesting is the observation that at least some FC siding comes visibly wet right from the factory in the original factory-sealed plastic wrap.

If at a particular job the FC siding that was delivered was in fact quite dry that job will have the least shrinkage showing up later.

"Send it Back if Wet" vs On-Site Drying of FC Siding

On the "everything right job" that I've detailed the contractor tried leaving boards in the sun, moving them indoors into a dry garage at night, measuring moisture, and keeping work otherwise protected.

We even took care to prevent boards from contacting a dry concrete floor.

IT is in fact completely impractical for a contractor to measure every board much less to try to dry them out before installation.

There is not enough dry, conditioned, sheltered space, not enough measurement time, and the job bid won't include the costs of such an approach.

When the manufacturer said that the contractor should just send back or refuse to accept doubtful FC siding she was being at best glib.

The contractor has not only bid on a job without assuming extra time and labor to try to dry materials, but w/he has a work schedule committed.

Bailing out on a wet FC siding job for re-scheduling, hoping for a delivery of more dry product (which is itself unpredictable) leads to very large financial losses for the contractor as well as angry clients whose work is delayed.

In sum the contractor is under terrible pressure to keep going and do the best s/he can to fix or minimize problems as the job proceeds. I suspect that it's not one in 1000 cases that a siding contractor rejects FC siding even if it is visibly wet in the package.

Market Impact of Wet FC Siding Shrinkage

Brand X Fiber Cement Siding being installed soaking wet in wet weather (C) E.D.

However from a few siding contractor conversations I've had on this topic, once the contractor has been burned in job cost, time, losses, or customer complaints, the contractor is simply going to either refuse to install FC siding on future jobs or will take great care to explicitly exclude cosmetic or shrinkage or warping-related issues from any warranty or job responsibility. That's what they tell me.

This problem is not unique to siding of course. There are plenty of other frustrating or costly examples in construction such as some roofing products, even nuisance tripping of AFCI protected electrical circuits.

Reader follow-up:

It's rained again since those first pictures I sent you of the job down the street from me. In between they've since been working their way down the stack installing the wet boards. - E.D. 12/17/2014

More Research Needed on Fiber Cement Siding Complaints

There has been considerable research on fiber cement materials and their properties, including fiber cement siding failures & complaints, cited in the companion articles listed below.

Yet in our OPINION the science is incomplete: we'd really want to make more controlled measurements of moisture levels and shrinkage as well as more detailed analysis of details or hypotheses such as:

Composition of fibre cement siding boards and panels - research citations

NichiBoard specifications indicate the product's composition:

Fiber cement panels are manufactured from a pressed, stamped, and autoclaved mix of Portland cement, fly ash, recycled rejects, and wood fiber bundles. - NichiBoard_3-part_Spec.docx retrieved 12/16/2014 original source

The product specification also lists first among its performance requirements the following:

A. Fiber Cement Cladding – Must comply with ASTM C-1186, Type A, Grade II requirements:

1. Linear Variation with Change in Moisture Content: 0.08% linear change.

2. Wet Flexural Strength, lower limit: 1015 psi.

3. Water Tightness: No water droplets observed on any specimen.

4. Freeze-thaw: No damage or defects observed.

5. Warm Water: No evidence of cracking, delamination, swelling, or other defects observed.

6. Heat-Rain: No crazing, cracking, or other deleterious effects, surface or joint changes observed in any specimen.
- NichiBoard_3-part_Spec.docx retrieved 12/16/2014 original source

That some fiber cement siding products contain wood or cellulose fibers including materials from cotton or paper is clear from some research on the topic. Fiberglass is also used in some fibre-reinforced cement products while asbestos was previously used.
See ASBESTOS CEMENT PRODUCTS for a history of the contents & manufacture of these materials.

Bottom line:

Fiber cement manufacturers advise not to install the siding if it is too moist, too wet, or in words of one, "saturated", but no one will tell us what is "too moist". Worse the customer service rep with whom I spoke told me (incorrectly) that "... it is impossible to measure the moisture level in fiber cement siding" at the same time that she told me "... if the siding is too wet the contractor should refuse the shipment, send it back, or not install it". [paraphrasing]

It is true that a particular moisture meter may not use a scale that has been calibrated specifically for fiber cement products, and that a pin type moisture meter may not adequately measure moisture deeper in the material.

But the choice of instruments is broad and includes equipment perfectly capable of making comparison measurements of moisture levels in materials, permitting calibration to fiber cement that is dry, moist, or wet as needed.


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