Cement board siding is typically very dimensionally stable. My experience has been that cement board siding normally would have less than 1/8 in. of shrinkage or expansion between two 12-foot sections of siding. This stability is proven by the fact that a thin profile cement board sheets have been used as a substrate or backer for natural stone and ceramic tile for years with very few issues with expansion and contraction. Even in wet environments such as showers, you seldom see cracked tiles or grout joints.

 

Cement board siding is made up of about 92% cement and 8% wood fiber. Cement board siding normally would have less than 1/16-in. thermal expansion in a 12-foot section. Manufacturers of cement board siding usually recommend leaving a small gap to allow for thermal expansion at siding butt joints. I always additionally recommend primed products.

 

WHY THE LARGE GAPS?

I have seen much larger separation gaps in the field. See picture (A) of the gap at the siding butt joint. The siding appears to have shrunk, but really hasn’t in this case.

 

Notice that the siding here (B) is actually very wavy. Often, as in this case, the wavy siding and butt joint gaps are caused by improper installation. The wavy siding and the separation gap at the butt joints are caused when the sheathing expanded.

 

Often cement board siding is installed over OSB sheathing. OSB swells when wet, much more than plywood does. In picture (C), the OSB has grown so much that I can almost stick my hand between the fire blocking and the OSB wall sheathing.

 

When the sheathing swells and bows, that causes the siding to bow as well. The result is separation at the butt joint, causing the siding to look as if it has shrunk. Most manufacturers of OSB panels recommend a small gap between OSB panels at the edges to allow for panel swell and expansion.

 

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