The term “deck pad” actually is an inappropriate term for a construction product that most often is used inappropriately, largely because of its name.
Many of the cement blocks used as deck support blocks actually are nothing more than 15 by 15, 18 by 18 or 24 by 24 inch patio slabs. These slabs are generally 1.5 to 2” thick, which is insufficient for most deck applications. Rather, a thickness of 3 or more inches on a pad is needed to bear the weight of most deck posts.
In addition, because deck pads seem so simple to use, they are often installed incorrectly. A deck pad for any sized deck needs to be installed only after a level area of the surface soil has been scraped away and a layer of one inch of sand per square foot laid down and packed in place, to allow for proper drainage.
By installing the pads in a manner that is not solid or level, excess weight bears down on a smaller area of the pad, and can contribute to the pad breaking. If the pad and deck are installed on a sloped surface, the posts (generally 4 by 4 inch) rest unevenly on the surface, and may actually slip downward over time, resulting in partial or complete collapse of the deck.
One of the better deck pad designs “traps” the four by four post in a raised square of concrete, preventing it from slipping. However, where the height of the deck exceeds two feet or more, those four by four supports become nothing more than free-standing stilts, relying on whatever anchor is used to hold them to the deck framework. With repeated vibration, or even strong winds, those pillars may shift.
The most appropriate use for deck pads is where the deck is not elevated at too high a distance from the ground, and where the deck is not excessively large. In some situations, deck pads may be used to support small extensions, or in between piles that are secured in the ground. In this manner, they act as supplementary supports.
Deck pads also can be used where the deck is securely anchored to the building, and where there is no angled pressure or weight on the pad. Ideally, deck pads work best where the cupped square opening allows the deck beams to rest fully in the cradle, rather than on a short or long post.
In softer soils, deck pads will rise and fall with varying moisture content, resulting in uneven deck plates. For this reason, they must be installed on packed porous surfaces.
Many cities specify, in their building codes, where pads may or may not be employed. If you are in doubt, and if your region does not have its own building code, review that of a nearby city, to determine what they see as the minimum standard for the use of deck pads versus footings, piles or foundations.
