Q&A / 

Cement Siding

DEAR TIM: The cement siding on parts of my house is disintegrating. When my builder sold me on this fiber-cement siding, he claimed it would last forever. Is my cement board siding defective, did the builder make a mistake or is it a combination of these two? What are the best practices that should be followed when cement-siding installation is part of a new construction or remodeling project? Brent W., Meredith, NH

DEAR BRENT: Based on your photos, it's my professional opinion that your fiber cement siding installation was doomed from the start. Because the crumbling cement siding is in just a few areas, and the other siding on the rest of the house in is perfect condition, the problem is a workmanship error. Cement house siding is a fantastic product when installed correctly.

Cement siding looks just like wood lap siding. While it doesn't rot like wood, it isn't immune from damage. PHOTO CREDIT: Brent W.

Cement siding looks just like wood lap siding. While it doesn't rot like wood, it isn't immune from damage. PHOTO CREDIT: Brent W.

Installing fiber-cement siding is not really that hard to do, but there are published best practices that the major cement-siding manufacturers make available. Follow these instructions and your fiber-cement siding can last for well over 100 years. I can show you houses in the Midwest and other cities east of the Mississippi River that are covered with cement siding that was installed before World War I, and it looks as good as the day it was installed, and some of it has never been painted. Cement siding's pedigree reminds me of a Triple-Crown winner.

Your cement siding failed at three locations: your front porch, rear deck and along one roof edge. The reason for the disintegration of your fiber-cement board siding is simple. The siding was installed too close to these surfaces. The best-practices installation guide shows that a 2-inch gap should exist between the bottom of the siding and each of these horizontal, sloped or vertical surfaces.

The crumbling is most pronounced at your porch and deck because the cement board siding is in direct contact with the porch and decking. Water that accumulates on these flat surfaces readily soaks into the siding. In cold weather, this water-soaked siding freezes. Since water expands approximately 9 percent when it turns into ice, this expansion blasts the siding apart. Creating the 2-inch gap prevents the siding from sitting in water, and it promotes rapid drying after the storm has passed.

Cut or sheared edges of cement siding, while inherently durable, can have extra protection added if the carpenters have finish paint on hand while installing the product. If they apply a heavy coat of urethane acrylic exterior paint to the cut edges, it prevents any splashed water from soaking into the siding. It's extremely hard to paint these edges after the cement lap siding is installed, especially the angled cuts where the siding abuts a sloping roof. It takes just a minute to paint each cut edge, but this practice can add decades of life to the tender edges.

Before you, or a contractor, attempt to install fiber-cement siding, you must read the written instructions. Some manufacturers have different instructions for different regions of the USA. Be sure you have the correct set of written guidelines for your fiber-cement house siding. These can be obtained for free online in a matter of seconds. You can't assume the carpenter, builder or subcontractor is going to do the job right. It's your responsibility to know how the job should be done, and it should be in your contract that the written instructions are going to be followed to the letter.

The best-practices guidelines cover many aspects and details you'll encounter when you or your contractor starts to work with your fiber-cement exterior siding. Cutting methods, flashing details, nailing suggestions, use of water barriers, etc. are all discussed. These practices are not difficult to implement, nor do they require expensive tools or techniques.

Note that the building code can be in conflict with the best practices for installing cement siding. The code may actually have more stringent requirements. For example, instead of the 2-inch gap called for by the siding manufacturer at a horizontal grade application, the building code may call for a gap of 4 or 6 inches. Be sure you have a full grasp of the building code as well as the best practices so you don't have to correct a mistake.

Cement home siding used to be made with asbestos as the aggregate that was mixed with the cement paste. For all intents and purposes, this process created thin sheets of solid rock. This is why many older cities and towns have cement siding houses that are still in service. All many of them need is a good washing to make them look like new.

Because asbestos has been linked to lung cancer, its use in building materials has been halted. Wood fibers were substituted in fiber-cement siding. If it's made at the time you're ready to build, try to locate a fiber-cement siding whose wood fibers have been treated to prevent wood rot. Untreated wood fibers are susceptible to rot if water somehow gets into the siding.

One of the best characteristics of cement board siding is its ability to hold paint. Cement siding is not hygroscopic, so it doesn't expand and contract like regular wood siding should it get wet. This means paint sticks to cement siding like it sticks to metal. If you apply a urethane-acrylic exterior paint to clean cement siding, you may not paint again for 15 or 20 years.

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