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Fiber Cement Siding

Fiber cement building materials are being used more and more in new construction, primarily for siding. In fact, it is used by most large builders in about 20 percent of the new home market. The reasons are pretty simple – it looks like wood but is termite-resistant, water-resistant, non-combustible, can resist hurricane force winds (when face-nailed) and is warranted to last from 15 to 50 years.

Fiber cement siding is composed of cement, sand and cellulose fiber that has been autoclaved (cured with pressurized steam) to increase its strength and dimensional stability. While current manufacturing processes don’t allow for the reprocessing of fiber cement siding, the manufacture of the material itself utilizes wood chips reclaimed from wood processing (the cellulose). When combined with the long lifespan of the siding, the integrated wood fiber qualifies the material for “green” points under several different programs.

Painted finishes on fiber cement are far more durable than on wood, due to the uniformity and high density of the product and because fiber cement doesn’t absorb water as readily as wood.

There are a number of manufacturers of fiber cement siding, including WeatherBoardsTM manufactured by CertainTeed and WeatherSideTM manufactured by GAF. One of the better known is James Hardie Building Products, manufacturers of Hardiplank®, whose trade name has become synonymous with the product.


Fiber cement siding is generally molded to resemble wood products. Applications are designed to be installed in place of conventional clapboards, shingles and vertical siding. Although flat and embossed wood grain lap siding are the most common patterns, manufacturers are introducing a greater selection of alternate designs such as those that resemble stone or shingles.

Products are manufactured generally as planks or boards. Trim materials such as soffits are also available. Ensuring that trim is installed at the corners and around windows will help guard against leaks. When fiber cement materials are not available, vinyl should be used as aluminum trim will react adversely with the siding’s ingredients.

Products are generally available unfinished or pre-primed. Criterium Engineers recommends the use of pre-primed materials as that ensures coverage and improves installation efficiency. Ends should be primed in the field when cut.

These siding materials are offered pre-finished from the manufacturer in a variety of architectural matte colors. Although providing a good opaque painted surface, there are at least three drawbacks to specifying pre-finished siding. One is the potential problem of slightly varying color between dye lots. A second is matching the recommended touch-up at cut ends. The third is that recommended caulk joints become difficult to disguise. Because the manufacturers demand a premium for pre-finished siding, there is very little economic incentive compared with a field-painted building. Field painting also offers the advantages of providing the owner and architect with a limitless color palette.

Although greater lengths may be special ordered at premium cost, standard stock order length is 12 linear feet for both siding and trim. Joint-making will therefore generally constitute a greater labor factor than is usually the case with wood or other synthetic materials available at greater lengths.

It’s Always the Installation

While fiber cement siding is purported to be easier to install than other types of siding, paying attention to the manufacturer’s specifications is critical. That may not always be easy since these specifications can sometimes be vague. Language such as “leave appropriate gap between planks and trim” and “install planks in moderate contact” can leave installers scratching their heads.

Although fiber cement is relatively dimensionally stable, we have observed situations where the manufacturer’s specified spacing has not been provided. The result can be extremely problematic as the siding may buckle or wave and create opportunities for moisture to enter.

Moisture entry is a problem for all types of siding. For this reason, we recommend that joints be flashed and caulked. Flashing alone will allow water to enter the joint with the potential to travel inside the wall. Caulking alone can cause problems if water does get behind the siding (through imperfectly applied or aging caulk) because it will be more difficult for it to leave.

Furthermore, as suggested above and in the manufacturers’ literature, this type of siding will readily mirror defects in the sheathing or substrate if it is not properly supported. We have seen this in architectural features where framing was not the standard 16 or 24 inches on center.

Lastly, boards contain crystalline silica. Although installers tend to treat this material like wood, proper respirators should be worn when using a chop saw or other tools that create dust. The preferred method is to scribe and snap, but since installers are used to working with wood, they must be educated to the health hazards. Unfortunately, we rarely see proper respirators being used in the field by contractors employing this material.

Not Perfect in All Applications

Although fiber cement has proved to be a successful and reliable siding material, it is not perfect in all applications. A number of years ago, the James Hardie company made a roofing product meant to look like shakes or slate shingles. This product was the subject of a class action lawsuit. Failures, related presumably to freeze-thaw cycles, were observed on the upper West Coast. These failures involved delaminating (separating into layers), deconsolidating (crumbling or disintegrating) and cracking of the shingles.

Final approval of the settlement was reached on February 14, 2002. Owners of homes with these products have ten years from the date of installation, with a final deadline in 2012, during which to file a claim. Additional information is available at These products are no longer being manufactured.

The Engineer’s Opinion 

In the final analysis, fiber cement products represent a major product class with excellent performance results when used with proper attention to details at a cost that is slightly more than pine but significantly less than cedar and other long-lasting wood products.

Volume 18, Number 1

March 2007

Copyright © 2007

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