White Paper: Cement Fiber Board Installation
“Lap siding is a popular exterior finish for both residential and commercial ICF jobs. Cement-board siding is particularly popular, due to its durability, ease of installation and fire-resistant qualities. “
Clark Ricks, ICF Builder Magazine
Installing Cement Fiber Board siding over an ICF is becoming more prevalent. There is little instruction from the siding or backer board manufacturers, due to the variations among ICF blocks.
BuildBlock Building Systems offers three different block styles. BuildBlock and BuildLock are similar and will be treated together. GlobalBlock has a much different attachment system and will be described separately.
The starting point for any cement fiber board installation is to look at the manufacturer’s installation instructions. Fastener spacing and other aspects will be determined by the manufacturer and may vary from manufacturer to manufacturer, and even among a manufacturer’s products. Compare the spacing required for the fasteners to the attachment availability of the forms. BuildBlock and BuildLock both have attachment furring strips 6” horizontally, with hard points every 8” vertically on each furring strip. The hard points (denoted with BB on the forms) offer 450lb – 500lb pull-out strength.
Ring shank nails are typically specified by the manufacturers of cement fiber board siding. These are acceptable for hanging the siding onto the ICF, but they should be alternated with screws for increasing the holding power of the fasteners. It is also vital to take care when using a pneumatic nailer for attaching the siding that you maintain the integrity of the plastic webs. Over pressuring or over driving the nails can damage the webs by cracking them, reducing the holding capacity. Do not use nailer below 40°, and instead only use screws for attachments. BuildBlock recommends using screws for all attachment points as the properties of plastic vary greatly from wood.
Screws should be regularly interspersed with the ring shank nails, and should be long enough to fully penetrate the web attachment, including the BB hard point. Typically 1-5/8” – 2” screws are used. They should be coarse thread, and coated for outdoor use. There are several hardened screws made especially for cement fiber board, with small flutes on the underside of the countersink head to assist with driving them flush.
It is not necessary to cover the foam with a weather barrier, but many jurisdictions are pushing for this. The exterior of the ICF can be covered with standard house wrap or other weather barriers. It is important to mark the locations of the BB hard points when wrapping the ICF so that they may be located during siding attachment.
GlobalBlock is a special case for siding attachment as it offers optional web attachment points, which must be placed while stacking the forms. These attachments are located every 12” horizontally and 16” vertically. For some siding applications, the fastener spacing is tighter than this. In that application these forms will need to have sheathing or furring strips attached at those points, and the siding attached to those strips or sheathing. When adding wood to the exterior of the ICF in that manner, it may be advantageous to add the house wrap. We do not recommend ring shank nails to be used with GlobalBlock.