Fiber Cement Problems: Production Causes of Cracks, Warping and Efflorescence

Fiber cement problems such as cracking, warping, delamination and efflorescence usually start with water: how much the board holds, how evenly it dries and how much it moves afterward. Selection and installation cause many site failures, but formula, pressing, curing and drying set the baseline. For a producer, most defects can be traced to a controllable process step.

This article is written for people who make the board, or plan to. It does not cover siding repair. It takes each defect and asks what in the plant could have caused it, what to measure, and how to separate a production fault from an installation fault. How the board is made is explained in our fiber cement board manufacturing process guide, and long-term fiber ageing in our cellulose fiber cement board article.

Fiber Cement Problems: A Producer’s View

Published reviews list a consistent set of field complaints:

• A technical advisory from IIBEC on fiber cement siding sold in the US describes panel ends shrinking apart, with gaps “greater than 3/16 in. (4.8 mm)”, plus cracks at panel edges or across the field, flaking, discoloration, warping and, in severe cases, siding fracturing and coming loose. It says these conditions may be associated with manufacturing and are not necessarily related to improper installation, while acknowledging that not following manufacturer specifications is a common reason for failure.
• A Western Australian government guideline (TG012) reports damage, cracking, efflorescence and cladding failure on several government buildings. Its review found recurring issues of inappropriate product selection, incorrect documentation, incorrect installation and insufficient quality control. South and west elevations developed the most defects.

Both matter to a plant. The first shows that buyers may blame the board. The second shows that many failures are not the board’s fault, and that you will only be able to prove it with good records.

Cracks and Edge Cracking in Fiber Cement Board

Edge cracking is the classic stack-yard defect. When cellulose fibers replaced asbestos in the Hatschek process, stacked sheets developed edge cracks, which Mtsweni’s University of the Witwatersrand study (2014) attributes to the hydrophilic nature of cellulose driving drying shrinkage. Giordano’s University of São Paulo dissertation (2011) describes the same mechanism on corrugated products: fiber cement commonly cracks along the edges because of moisture gradients that develop while piles are stored in the factory yard.

The mechanism is simple. The edges of a pile dry first and try to shrink, while the wet center resists. If the resulting tensile stress exceeds the young matrix’s strength, the edge cracks. It can happen before the board has left the plant, even though it may only be noticed later.

Process factors worth checking:

• Storage exposure. Wind, sun and low humidity on open piles increase the gradient. Mtsweni reports that kaolin used in partial replacement of cement, together with controlled exposure conditions, eliminated edge cracking in that study.
• Formula. Wollastonite microfibers, kaolin and certain admixtures were investigated as mitigation options in the same study. Results will vary with your materials, so test them on your own mix.
• Moisture at pressing. A 1950 process patent stated the trade-off plainly: “if the moisture content is too high deformation of the product will result in the pressing operation”, and if it is too low “the product will crack” (Waechter, US 2,531,016). The specific moisture range in that patent applied to its own product, but the principle is general: set a moisture window for pressing and measure it.

Cracks at fixings are a different case. TG012 lists differential movement, expansion, incorrect fixings and over-tightening as causes. A crack that starts at a fastener points to the site; one that follows a stack-edge pattern on boards still in the warehouse points to the plant.

Edge and Corner Damage

Broken corners tend to be blamed on transport, but many start in the plant. The board is at its weakest, and is handled most, between forming and the end of curing, and again when it is separated from the steel templates. Boards stuck to templates are a common cause of breakage.

Practical controls:

• Record the break rate at each station (stacking, press, release, dryer, saw, packing), so that losses can be located rather than guessed at.
• Keep templates clean and flat, since a dirty or bent template adds both sticking and warp.
• Check that the sheet has enough strength before handling, and adjust pre-curing when you change cement or weather conditions change.
• Trim enough edge to remove the weaker material the forming process can leave at the sheet edges.

On our reference designs, an automatic board release unit saves labor and reduces broken boards, and online inspection checks boards automatically so that defects are caught before packing. James Hardie also reports that volcanic ash formulations reduce edge cracking or crumbling when boards are nailed close to the edge (US 6,572,697), so workability can also be improved through the formula.

Warping: Moisture Gradients and Uneven Drying

A board warps when one face shrinks or expands more than the other. Warping is, in effect, the sheet-scale version of the edge-crack mechanism.

The production steps most likely to create a gradient are:

1. Drying. If one face leaves the dryer drier than the other, the board can bow as moisture equalizes. Cured boards still hold significant water; one Etex patent says drying is typically stopped at about 8 wt% humidity or less (US 12,227,456). The right target for your plant is the equilibrium the board will reach in your market, which differs between a dry desert climate and a humid coast.
2. Pressing and stacking. Over-wet sheets can deform under pressure, as the Waechter patent notes. Stacking straight and evenly over flat, clean templates helps sheets keep their shape as they gain strength.
3. Packing too early. A warm board packed in a sealed stack carries its gradient to the customer. Boards that are packed too wet can warp or shrink after installation.
4. Coating one face. A sealer or paint on one face changes how fast that side takes up and loses water. Check flatness on coated product through a wet-dry cycle, not just on the bare board.

Moisture Movement, Shrinkage and Gaps

Shrinkage gaps at butt joints are among the most visible complaints in the IIBEC advisory, and they trace back to wet-to-dry movement. EN 12467 includes moisture movement among its physical characteristics, so it is both a performance issue and a product-standard one. See our EN 12467 guide and ASTM C1186 guide for how each standard treats these properties.

Three published data points show how much the formula matters:

The third row is a reminder that fixes come with trade-offs. Density has an effect as well. Ni’s thesis tabulates water absorption of 32.1% for a 1.31 g/cm³ commercial sheet, 30.5% at 1.40 g/cm³ and 18.6% for a 1.62 g/cm³ compressed sheet (Ni, 1995). Pressing to higher density reduces the water the board can take up, so choose a press and product range with this in mind. On our reference designs, presses of 5,000, 7,000, 10,000 and 14,000 tonnes are matched to board density and output.

A lighter board is easier to install but will usually need more attention to moisture behavior. Make that decision per product, and test moisture movement every time the formula changes, not only at type approval. For a broader view of how materials affect this, see calcium silicate board raw materials.

Delamination Between Fiber Cement Layers

Sheet machines build board from many thin layers, so the bond between them is a built-in weak plane. A Hatschek monolayer may be only 0.1–0.6 mm thick (Redco, US 10,144,150). An Etex patent on co-cured layered boards states that a strong interfacial bond formed during co-curing “avoids delaminations during handling, cutting, installation and service” (US 12,227,456). In other words, the time to create that bond is while the layers are still green.

Points to investigate if a board splits or layers lift:

• Layer contact on the forming roll. Worn felts, surface drying of the film between wraps or an unstable slurry can weaken contact. Inspect the cross-section of broken samples; a clean split along a layer points here.
• Pressing. Pressing squeezes out water and improves the bond between layers. Check that pressure reaches the whole stack evenly and that water has a way out. Trapped water can force layers apart when pressure is released.
• Fines and fiber. Refining mainly improves drainage and solids retention on the machine; unrefined kraft fibers form open webs that drain fast and lose matrix (Ni, 1995). A change of pulp lot can therefore change layer quality.

Efflorescence: White Deposits and Lime Bloom

Efflorescence on fiber cement is usually lime bloom. Cedral’s roofing guidance explains it as a by-product of cement reacting with water, which migrates to the surface and forms a thin calcium carbonate deposit. It is more likely in winter and early in the slate’s life, typically disappears within 6 to 12 months and, the company says, does not affect strength or durability.

That is not the whole commercial story, though. A dark, through-colored facade panel with white patches looks like a defect. TG012 records that one government agency restricted a panel product by manufacturing date, after severe efflorescence on two projects. Appearance alone can decide whether a producer keeps a market.

Levers a plant can use:

• Reduce water transport. James Hardie describes efflorescence as a side effect of water permeation, and reports that sizing the fibers reduced water migration by more than nine times in an 8-hour test (US 6,676,745).
• Consider the curing route. Autoclave curing reacts ground silica with lime, which should leave less free calcium hydroxide. Air-cured boards rely on other controls. Both routes are in commercial use, and each needs its own efflorescence checks.
• Protect the board. Coatings and sealed edges limit water entry. TG012 lists sealing requirements for cut panel edges among the items to be specified, so your technical data should state them clearly.

Fibre Cement Slates: Problems Specific to Roof Products

Slates are thin, are exposed on one face to the weather and on the other to the roof void, and are often pressed to high density. They are covered by their own product standard, EN 492. That combination makes three problems more prominent:

1. Deformation under cyclic moisture. A BRE information paper (IP19/2001) tested slates under two months of cyclic condensation. Check the slate’s flatness after wet-dry cycling.
2. Lime bloom. Darker slates show it most, and customers will often complain in the first year.
3. Surface wear and growth. Cedral advises against high-pressure jet washing and steam cleaning, which can damage the surface. Say this clearly in your care instructions, because aggressive cleaning is hard to tell apart from a production defect later.

A Root-Cause Routine for the Plant

When a complaint comes in, work through it in this order:

1. Identify the batch. Trace the production date, the formula, the raw material lots, the press settings and the curing record. Without marks on each board or pack, this is impossible, so print them from day one.
2. Check retained samples. If a retained sample from the same batch shows the defect, the cause is likely in the plant. If it does not, look harder at site conditions.
3. Read the failure. Look at where the crack starts, whether the board split along a layer, and whether the white deposit is on the whole face or only near edges and joints.
4. Compare with the standard. Run density, bending strength wet and dry, moisture movement and water impermeability, then the durability tests that apply to the product’s exposure class.
5. Change one thing. Alter the likely cause, make a trial batch, and carry out the same tests against paired reference specimens.

What protects a plant is a written moisture window at every stage, records it can trust, and a lab that runs the same tests week after week.

FAQ

What are the problems with fibre cement?

The main problems with fibre cement are edge and field cracking, warping, delamination, efflorescence and shrinkage gaps at joints, along with broken corners from handling. Most of them come from how water moves through the board. Selection and installation errors cause many failures too, so producers need batch records and retained samples to separate plant faults from site faults.

What are common fibre cement slates problems?

Common fibre cement slates problems are lime bloom, deformation under cyclic moisture, moss growth and surface damage from aggressive cleaning. Lime bloom usually fades and, according to manufacturer guidance, does not affect strength. Slates fall under EN 492, which sets their product requirements, so plants should add flatness and coating checks after wet-dry cycling to the tests it requires.

Does efflorescence mean the fiber cement board is defective?

Not usually in the structural sense. Lime bloom is a thin calcium carbonate deposit and, according to manufacturer guidance, typically fades within 6 to 12 months without affecting strength or durability. Severe or recurring efflorescence is different: buyers may reject the product on appearance. A plant should treat it as a signal to check water absorption, free lime, curing records and coatings.

Why do fiber cement sheets crack at the edges in the stack?

Edges dry faster than the center of a pile. As the edges try to shrink, the wet center restrains them, and the tensile stress can crack the young matrix. Cellulose makes the effect stronger because it readily exchanges water with the air. Sheltering piles from wind and sun, setting moisture limits and adjusting the formula are the usual controls.

Do autoclaved boards have fewer problems than air-cured boards?

Neither route eliminates these defects, and they fail differently. Autoclaving reacts ground silica with lime, which should reduce the free lime behind efflorescence, and published research reports differences in how fibers age under the two routes. Air-cured boards are commercially established with their own synthetic fibers and formulas. Compare the routes by testing a specific formula, not by general reputation.

Planning a Fiber Cement Board Plant?

See how forming, pressing, curing, drying and online inspection fit together on our fiber cement board production line page, which covers both flow-on and Hatschek forming machines. Our calcium silicate insulation plant setup guide covers a different product, but its planning sequence is a useful reference. For the forming step itself, see the Hatschek process.

Sources

• IIBEC, Technical Advisory No. 017-2019, Fiber-Cement Siding: Inherent Issues Affecting Performance: https://iibec.org/publication-post/iibec-technical-advisory-no-017-2019-fiber-cement-siding-inherent-issues-affecting-performance/
• Government of Western Australia, TG012 Fibre cement and compressed fibre cement cladding: https://www.wa.gov.au/system/files/2025-11/tg012-fibre-cement-and-compressed-fibre-cement-cladding.pdf
• N. B. Mtsweni, Impact of different materials on cracking of corrugated fibrecement sheets, University of the Witwatersrand, 2014: https://wiredspace.wits.ac.za/items/fb829034-57fd-4b16-ba1d-23cfce84340a
• B. L. Giordano, Control of cracking in fiber cement apply concepts of functionally graded materials, University of São Paulo, 2011: https://teses.usp.br/teses/disponiveis/3/3146/tde-20092012-182239/en.php
• O. H. Waechter, US 2,531,016 A, Treatment of fibro-cement products, 1950: https://patents.google.com/patent/US2531016A/en
• James Hardie Research, US 6,572,697 B2, Fiber cement building materials with low density additives: https://patents.google.com/patent/US6572697B2/en
• James Hardie Research, US 6,676,745 B2, Fiber cement composite materials using sized cellulose fibers: https://patents.google.com/patent/US6676745
• Etex, US 12,227,456, Fiber cement products and methods for the production thereof: https://patents.google.com/patent/US12227456
• Redco NV, US 10,144,150 B2, Hatschek process for the production of fiber cement plates: https://patents.google.com/patent/US10144150
• BRE, IP19/2001, The performance of fibre cement slates: effects of condensation: https://bregroup.com/store/bookshop/the-performance-of-fibre-cement-slates-effects-of-condensation-downloadable-version-
• Y. Ni, Natural Fibre Reinforced Cement Composites, PhD thesis, Victoria University of Technology / CSIRO, 1995: https://vu-9.eprints-hosting.org/15654/1/Ni_1995compressed.pdf
• Cedral, Roofs: care and maintenance: https://media.cedral.world/pd17712/original/401374203/cedral_roofs_-_care_and_maintenance.pdf
• C. A. Fioroni, Strategies for improving physical-mechanical performance and dimensional stabilisation of fibre cement, University of São Paulo, 2024: https://bdtd.ibict.br/vufind/Record/USP_da0915789fd9775dd26fd01c15e330b3
• SS-EN 492:2012+A2:2018, Fibre-cement slates and fittings: Product specification and test methods: https://online.sis.se/en/produkter/construction-materials-and-building/construction-materials/products-in-fibrereinforced-cement/ss-en-4922012a22018