The durability of a home built with concrete block depends on close attention to these key details.
By Matthew Power
Who knew? At the recent meeting of the National Concrete Masonry Association (NCMA) in Baltimore, the guys (and they're pretty much all guys) who make concrete blocks voiced their frustration about selling their time-honored system to home builders. The issue: Builders go for the bottom dollar system every time.
"It's not fair to compare a stick-framed building to concrete block on a simple cost-per-square-foot basis says Rolland Johns, vice president of CEMEX, a Houston-based cement company. "That ignores the value that block offers."
That value, he says, includes termite resistance, durability, energy efficiency, fire resistance, and relatively stable costs.
So why don't more builders use structural concrete block? One big reason: a lack of trained masons. Another reason is that some builders fear that block walls will crack, leading to costly callbacks or litigation. The devil is in the details. Like all structural systems, block homes must be built correctly if they're to live up to their potential.
So whether you're an experienced block builder looking to improve durability or a stick framer looking for a time-tested alternative, here's a bare bones checklist to save you hassle down the road--and ensure a better quality finished product.
As with any type of structural system, many block problems can be avoided by careful attention to plumbing and electrical penetrations. Below-grade water supply lines should be insulated to prevent moisture from seeping into blocks (as well as noise control), and electrical penetrations should be foamed so they don't offer radon pathways.
Deflect and protect
Water must be kept away from below-grade blocks--particularly in freeze-thaw climates. That means proper grading, drainage systems, and a waterproofing membrane. Consider commercial-grade membranes or two-part systems. Exterior finish systems should incorporate weep systems and flashing to keep water from running directly down below-grade walls.
For below-grade applications, Building Science Corp. recommends closed-cell rigid foam applied to interior walls. Above grade, NCMA best practices specify foam insulation attached with non-conducting mechanical fasteners, such as plastic screws.
Place rebar properly
Vertical rebar should be installed within blocks at 5 1/8 inches from the earth side of the wall (with an 8-inch block) to compensate for soil pressures. NCMA recommends that the bar extend in a single section from the footer, through the below-grade blocks, into the above-grade walls. In addition, above-grade blocks near window or door penetrations should have horizontal reinforcement.
Apply a waterproofing seal at any point where wood or steel are supported by concrete block. Also, seal off the top of the block wall to prevent moist air from moving up into roof cavities and causing water damage.
Gap the slab
For below-grade blocks, the NAHB Research Center suggests adding an isolation joint between the concrete block wall and the slab on grade. This detail may reduce cracking. Also, the footer must extend a minimum of 2 inches from either side of the block wall, not to exceed the thickness of the footing.
Concrete Brick: Special Rules Concrete brick veneer tends to contract over time, while clay brick tends to expand. When using concrete brick over block walls:
For more complete technical advice, contact the NCMA at 703-713-1900, and ask for document TEK 10-4