Properly flashed penetration blocking is a critical component in cement fiber siding systems. Although important, this exterior detail is often overlooked or executed incorrectly. One of the biggest violations of blocking requirements today, are furnace and tankless water heater exhaust vents. With appliances becoming more efficient and having significantly lower exhaust temperatures, capable of being exhausted through plastic piping, many installers just drill holes through the siding and apply a sealant around the pipe. High efficiency furnaces and tankless water heaters utilize 2 inch ABS or PVC pipes for intake and exhaust, which have an outside diameter of 2½ inches. Most cement fiber siding manufacturers require penetrations in the building envelope 1½ in diameter or larger to have blocking. This blocking should also be a minimum 3 inch radius greater than the radius of the penetration.
Penetration blocking not only protects the siding from potential mechanical damage at penetrations, it helps protect the materials from long term moisture exposure. A dripping hose bibb or condensate from exhaust systems can damage paint and deteriorate the siding materials. Although manufacturers typically recommend their manufactured blocks that match the siding materials, other types of blocking are available. The oldest form of blocking is wood, whether cedar, SPF or scrap framing materials. Newer products are constantly being introduced for all types of siding materials and installation methods vary. The most important component of any blocking method is proper flashing and compatibility with the weather resistant barrier (WRB) in use.
Not only does penetration blocking look aesthetically pleasing, it provides a flat, secure surface to mount electrical disconnects, receptacles, hose bibbs and light fixtures. Having a large flat surface to mount light fixtures is important not only to keep the fixtures vertical and parallel with the structure, but to avoid large gaps where a fixture may span several rows of siding that are difficult to seal and look unprofessional. Furthermore, blocking allows for minor modifications to be made without disturbing the building envelope during upgrades or remodels.