For cutting fiber-cement siding, it's best to use a 4- to 6-tooth polycrystalline diamond-tipped (PCD) blade made specifically for fiber-cement.
Regular carbide blades will cut fiber cement, but they dull relatively quickly, and if you attempt a gang cut, the blades overheat and can warp.
A good alternative to a PCD blade is electric shears, which work like motorized tin snips. While they cut slower than a circular saw, they can make straight and curved cuts without much dust.
Silica Dust Warning
Cutting fiber-cement with a circular saw produces serious clouds of dust. This dust has a 15-30% crystalline silica content, making it a serious health hazard. The MSDS of one popular fiber cement product warns that repeated inhalation may lead to lung damage, and the National Institute of Occupational Safety and Health (NIOSH) links the inhalation of silica dust to a deadly lung disease called silicosis.
When cutting fiber cement, it's critical to wear a respirator and/or use a dust-collecting circular saw attached to a shop vac. NIOSH recommends:
A saw with a built-in dust collection container or shroud that partially encloses the saw blade. (Ed. note 11/20/15: As reported on sister site Tools of the Trade, Skilsaw has a new fiber-cement saw coming April 2016.)
- A shop vacuum with an air-flow rate of 30 CFM or higher.
- A hose that's at least 1.25-inch in diameter with few or now bends.
- Use of a high efficiency disposable filter bag a pre-filter in the shop vac.
- Use of PCD blades.