Q. Does the Code Allow Ground-Supported Decks?
I want to build a freestanding deck where the joists are supported by horizontal pressure-treated 8x8 timbers lying on a bed of compacted crushed stone, with joists toenailed to the timbers — a normal residential deck, in other words, except the support beams would rest on the ground rather than being bolted to posts. The deck surface will be about 18 inches above the ground, with two steps up to the house’s entry door. Is this feasible, and are there any code issues I should be aware of?
A.Glenn Mathewson, technical adviser to the North American Deck and Railing Association, responds: Although the approach you describe doesn’t follow any industry standard practices for deck construction, there are some model code provisions that seem to argue for it. For example, code provisions that deal with wood foundations also involve wood members bearing on gravel or crushed stone. The usual minimum requirement in that application is for a 2x8 bottom member on an aggregate bed at least 6 inches deep. The 8x8 you propose should work, but it would be a good idea to cross-check the actual loads on the beams against the bearing capacity of your soil.
You also need to consider guards (as the code designates railings at the edge of a drop). They’re required where there’s a drop of 30 inches or more within a horizontal distance of 36 inches from an edge. Assuming that the ground in the area of your deck is relatively flat, guards probably wouldn’t be needed at the edge of the deck itself.
But the two risers leading to the house are subject to the same requirement. Since their combined height could come to 12 inches or more, adding that figure to the 18-inch height of the deck would equal or exceed the 30-inch threshold. In that case, a guard would be required at the edge of stairs within 36 inches of the edge of the deck. Stairs further than 36 inches from the edge would not require either a guard or a handrail, since graspable handrails are required only for stairs with four or more risers.