Do I Use a Deck Block to Build My Deck?

About Deck Blocks

• Although frequently confused with the proprietary product "Dek-block," the general term "deck block" refers to several brands of pre-cast concrete piers that hold posts or joists in slots. Deck blocks resemble trapezoidal pyramids; they have broad, four-sided bases that slope to smaller, flat tops. Deck blocks sometimes have deep slots that hold conventional 4-by-4 deck posts. Alternatively, they have shallow slots that accommodate deck joists or girders placed on edge. In either case, the lumber simply sits in or on the block without anchorage from brackets, fasteners or adhesives.

Suitable Conditions for Deck Blocks

• Local climate and the conditions of the building site affect the performance of deck blocks. In standard installations, deck blocks sit directly on the ground's surface. However, areas that experience freezing temperatures generally require deck foundations or footings to extend below the frost line. Soil's freezing and thawing cycle causes the ground to shift, which often thrusts deck blocks out of alignment and permanently damages the deck structure. Check with your local building authority to determine if deck blocks are allowed in your area. Deck blocks require firm, well-drained soil to remain in position. If you're unsure about the condition of your soil, contact a professional builder or engineer for assistance.

Deck Blocks with Footings

• If your local building authority prohibits on-grade deck blocks, ask about placing blocks on poured footings. Footings are basically holes filled with concrete and finished with a flat top surface. Building codes sometimes allow deck blocks and other types of piers on top of footings that extend below the frost line. The idea is that if the footing remains stable, so will the deck block on top.

Alternatives to Deck Blocks

• The use of poured footings is the most common alternative to pre-cast deck blocks. Rather than top poured footings with deck blocks, you may insert anchor bolts into the wet concrete during construction or install post-base brackets on top of the footing after the concrete cures. One advantage of deck blocks is that they inherently raise wooden framing above ground level. To pour a footing that rises above grade, you must construct conventional concrete forms or use a prefabricated tube form.