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Fiber cement siding home:
This article discusses the selection and best-practices installation of fiber cement building siding products. In a series of companion pages we provide details about the properties of various fiber cement siding products from the major manufacturers, how to identify fiber cement siding, how to install fiber cement siding including guidelines for gaps, clearances, nailing schedules, end and cut sealing, siding joint or abutment caulking, painting or staining.
This article series provides descriptions of the field performance of fiber cement siding products and a detailed field investigation of several product failures: of fiber cement stain or coating peeling and cracking, and of gaps, shrinkage, and loose, buckled fiber cement siding.
We include fiber cement siding installation specifications, repair and maintenance and painting recommendations, and fiber cement siding manufacturer identification guides.
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This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Page top photo of fiber cement clapboard installation courtesy of Carson Dunlop Associates.
Does Fiber Cement Siding Contain Asbestos?
Modern fiber cement siding discussed here is not an asbestos product and does not contain asbestos. There may, however, be other health concerns related to silica or other dust produced by power saws or similar cutting operations in the modern material. Details about an older generation of fiber cement products, asbestos cement wall shingles and modern fiber cement wall shingles are discussed separately at SIDING, ASBESTOS CEMENT.
Also ASBESTOS & FIBER CEMENT ROOFING and also CORRUGATED ROOFING. CD Johnston also makes an interesting distinction between fiber-reinforced cements and concretes and fibercement products, noting that "The fiber content in these composites is much lower than in fibercement composites". 
Performance of Fiber Cement Building Siding
Modern fiber cement siding, while surely outperforming other materials like hardboard siding, is not impervious to mechanical damage, coating or paint failures, and a shrinkage and siding butt-joint cosmetic or leak issue, as our photo shows above and as discussed
"Modern" fiber cement building cladding has been around for more than 60 years, if we include its early form, cement-asbestos shingles such as those on the home shown below left (Dover Plains, NY).
Following the development of concern for asbestos safety, fiber cement shingles continue in production, but using reinforcing and filler materials other than asbestos.
Fiber-cement [in plank form], unlike it's shingle ancestors, is one of the newest entries into the siding field and holds promise in that the material can be fashioned to resemble almost any exterior cladding, holds paint well, and is essentially impervious to decay, insects, UV radiation, and fire.
Older fiber cement and asbestos-cement wall siding (photo at left) is vulnerable to impact damage. Repairs must be done with care to avoid breaking additional siding shingles when removing and replacing the bad ones.
Modern fiber-cement siding is made up primarily of Portland cement, sand, and wood fibers. It is chemically similar to older asbestos sidings but contains no asbestos, glass fibers, or formaldehyde.
Watch out: for silica dust hazards: while it does not contain asbestos, modern fiber cement siding products do, however, produce a very fine silica dust when cut with a saw or abrasive blade, which, if inhaled, can cause silicosis and other serious respiratory problems.
At left is the English language portion of the silica dust warning included with Hardieplank® lap siding. The company gives this advice in both English and in Spanish [click to see an enlarged image].
Fiber-cement boards are extremely straight and rigid when held edgewise, but they are much heavier than wood—about 20 pounds for a 12-foot length of 8-1/4 -inch siding.
They are flexible along the flat dimension, however, so any lumps in a wavy framing job will tend to telegraph through the siding.
Any fiber cement material is fairly brittle and, if not handled carefully, can crack or break. We found in particular that when picking up long lap siding boards it was critical to keep the board on edge - that is, its upper or lower horizontal edge is carried parallel to the ground.
Carrying a long fiber cement board "on the flat" risks breakage.
Shrinkage & Gaps in Fiber Cement Siding - watch out for "wet" fiber cement siding right from the manufacturer and watch out for siding butt joint gaps as wet siding shrinks
Warranties for fiber cement siding run
from 30 to 50 years depending on the manufacturer and
specific configuration. Fiber cement siding is cost-competitive with vinyl
and hardboard siding and significantly less expensive than
premium wood sidings. [Best Practices Guide to Residential Construction, by Steven Bliss,]
Watch out: based on our own field experience we do not quite agree with Steve Bliss's note above. We have seen serious shrinking 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.
The manufacturer told us that the material must be kept dry and "not installed" if the contractor thinks it's "wet". But from the service rep with whom we spoke we could not get the slightest definition of "wet" or "too much moisture" or "dry" fiber cement siding. 
Styles and Sizes of Fiber Cement Siding Products
Fiber-cement is available in a wide array of styles and finishes modeled after other materials ranging from horizontal wood siding to vertical sidings, wood shakes, bricks, and stones. The wood patterns are generally available either smooth or wood-grained and most are available factory-primed or finished as well as unfinished.
Our photo (left) is interesting because it shows two nearly-identical fiber cement wall shingles. The shingle on the right is a new replacement product that does not contain asbestos, while the shingle on the left is an older cousin that contains asbestos.
A clue to the presence of new fiber cement shingles on this home might be the observation that the shingle on the right is coated only with the factory primer while that on the left has been painted a few times. See ASBESTOS CEMENT SIDING for details.
Fiber-cement horizontal siding planks are typically 5-1/4 to 12-1/4 inches wide by 12 feet long and are designed for a 1-1/2 inch overlap. Vertical siding panels measure 4x8, 4x9, or 4x10 feet, and shake and shingle panels are typically 16x48 inches. The thickness of most siding materials is 1-5/inch. Smooth and textured soffit and trim boards are also available.
Fiber-cement soffit material is typically 1/4-inch and most trim stock is 7/16-inch thick, but manufacturers have recently introduced thicker profiles (see section on fiber-cement trim, page 34 in Best Construction).
Lap-Siding Fiber Cement Board Installation
Fiber-cement siding products install similarly to the wood products they imitate. They can go over wood-based sheathings or rigid foam, but they must be nailed or screwed directly to studs or 2x blocking. Fasteners should penetrate solid wood by 1 to 1-1/4 inches, depending on the manufacturer’s specifications.
Our fiber cement siding photo (left) courtesy of Carson Dunlop Associates, shows the product in end view (trade show booth installation detail) and includes the first-course bottom spacer behind the fiber cement clapboard.
The 12 foot long fiber-cement planks can be held edgewise by a single person, but the boards may break in two or deform if picked up flat. One person can install a plank by driving a single nail near its center to hold it in place against guide nails driven into the sheathing to mark the upper edge.
Manufacturers recommend leaving 1/8-inch between board ends and window casings and trim and caulking with a paintable 100% acrylic latex caulk. Butt joints between two planks can be either lightly butted and painted over or gapped 1/8-inch and caulked. Manufacturers recommend priming cut ends on site if the joints are not being caulked. As with other siding products, leave at least 1/2-inch clear at step and other flashings so the bottom edge does not soak up water.
Nailing & Butt-Joint Flashing Details for Nailing Fiber Cement Siding
Our photo (left) shows a pre-fabricated plastic flashing device intended to be inserted at the butt-joints of fiber-cement wall siding. (Photo courtesy Carson Dunlop Associates.)
Fiber-cement siding should be nailed directly to studs with nail penetration into solid wood of 1 to 1-1/4-inches, depending on the manufacturer’s specifications.
Pre drilling is required within 1/4inch of an edge or near sharp angles or other fragile shapes to avoid cracking. Pre drilling may also be required when nailing through foam sheathing to avoid cracking the siding.
Manufacturers require a hot-dipped galvanized or stainless-steel siding nail (or roofing nail for blind nailing) that should be driven flush with the surface.
Overdriving of nails can cause the material to shatter around the nail, weakening its holding power and, with some products, voiding the warranty. Staples and clip-head nails tend to penetrate too far, but coil nailers with adjustable depth-of drive work well. Some contractors hand-nail the siding to avoid problems.
Given the longevity of the siding, a long lasting corrosion-resistant nail is recommended. If fastening to metal studs, use corrosion-resistant pneumatic pins or self-tapping bugle-head screws.
Standard Nailing in Fiber Cement Siding
In most installations, horizontal fiber-cement siding is nailed top and bottom into each stud, with the lower exposed nail going through both layers of siding (see Figure 1-20).
[Click to enlarge any image]
Butt joints should lie over studs. This is the most durable installation. Color-matched galvanized nails are available for the exposed nails on prefinished sidings.
Blind Nailing Procedure for Fiber Cement Siding
Recommendations for blind nailing fiber cement siding products vary among manufacturers, but most permit “blind nailing” with siding planks less than 8-1/4-inches wide installed over 16-inch on center framing.
In this siding nailing technique, the fasteners are hidden just above the lap line of the overlapping plank and put a slight curve in the siding, pulling it tight to the wall. Roofing nails work well because of their large heads (see Figure 1-21 at left).
An occasional extra face nail may still be required to hold the lower edge tight to the wall where there is a bump or bulge in the framing. Since the lower edges remain unsecured, blind nailing is not suitable for high-wind areas.
Reader Question: 1/4-inch gaps between siding boards and next course?
How tight should the 5.25" (4" reveal) lap boards rest one on top of the other? For a new install, I would expect the bottom of the board to lay flat on the top of the one under it, however, on a job, I am seeing boards with 1/4"+ gaps, but I can't find any literature saying what the max should be. - Mike 3/13/2013
Normally the lower edges of lap siding are touching to 1/16" on the top edge of the course below.
Given this loose installation, I'd take a look also at the nail lines to be sure that the courses were lapped according to the manufacturer's specifications.
Cutting Methods for Fiber Cement Siding
Ordinary carbide-tipped blades produce less dust but wear out within a few hours compared to a few months for abrasive blades. In the last few years, manufacturers have responded with specialized diamond tipped blades and tools, making the work easier and safer.
The new fiber-cement blades cut smoother, create less dust, and outlast ordinary carbide blades. When used with the new dust-collecting saws designed for fiber-cement, cutting is safe and effective.
Many contractors also use electric shears such as the fiber cement shears produced by Malco Products. (Photo above courtesy Malco Products.)
Fiber cement cutting shears are similar to a sheet-metal nibbler but specially adapted for fiber-cement. These make a clean cut with little dust, but are not as fast as a circular saw and cannot cut through multiple boards at once. Scoring and snapping, as for drywall, is also an option for quick cuts where a crisp edge is not needed.
Finishing & Painting Details for Fiber Cement Siding
After installation, small dents or chips can be filled with any cementious patching compound. Before priming or applying the top coat to pre primed material, wipe away any dust from cutting with a damp cloth or sponge or lightly hose down the siding and allow it to dry thoroughly.
If the siding has been hosed down or power washed (unprimed siding only), allow at least two sunny days before priming. Painting should be completed within 90 days of installation to avoid deterioration of the surface from prolonged exposure to water.
For unprimed siding, manufacturers recommend an alkali-resistant, 100% acrylic primer specifically approved by the paint supplier for fiber-cement. Back-priming is not necessary; in fact, some manufacturers recommend against back-priming so any trapped moisture can dry from the back of the siding.
For the top coat, use a 100% acrylic latex paint. Because fiber-cement is dimensionally stable and largely inert, it holds paint well. Estimates range from 7 to 15 years for a quality paint job. Some of the prefinished products carry 15-year warranties on the finish.
Our photo (above left) of paint failure on a fiber-cement siding installation was provided by BC home inspector Hugh Cairns. Mr. Cairns is a Canadian home inspector located in B.C. and is an occasional contributor to InspectAPedia.com.
The illustration below is of a Chinese-made fiber cement siding product produced by Ningbo Yihe Greenboard Co., Ltd. While we have no specific complaint about this product, it and similar fiber-cement siding board products may add to difficulties in determining the origin and manufacture of various modern fiber-cement siding products.
The siding above is described by the Zhejiang manufacturer as:
Watch out: That fiber cement siding is available from a variety of sources is evident from the list below. In addition, some our investigations of fiber cement siding failures (paint or coating failures, shrinkage, gaps discussed at SIDING, FIBER CEMENT GAPS and SIDING, FIBER CEMENT DEFECTS) led to an assertion by U.S. manufacturers that even though the homeowner had been told that a U.S. product was being installed (Hardieplank for example) in fact the manufacturer has told the homeowner that it was not their product.
In a conversation with James Hardie about Hardieplank and in a second one with Certainteed as well as field inspection of Hardieplank confirmed to be the case: building owners may have been paying a premium for counterfeit products and more, when there is a product failure they may have no recourse. [Personal communications, DJF with both companies, April - June 2013].
Identification of Chinese-Made Fiber Cement siding may be aided by this additional detail offered by the manufacturer: beginning in 2012, fiber cement siding from the supplier listed above is provided only with a smooth finish. The embossed wood grain patterns that were used before 2012 have, according to the manufacturer "been mostly destroyed".
However in the spring of 2013 the company contacted us with this update
-- Adapted with permission from Best Practices Guide to Residential Construction.
Continue reading at SIDING, FIBER CEMENT GAP & CAULK SPECS or select a topic from the More Reading links shown below.
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