In deck building, make room for common sense


September 11, 1993|By Karol V. Menzie and Randy Johnson

We have a friend who keeps asking us to write about the weird experiences of rehabbing: The perfectly functional light switch discovered sealed behind a paneled wall, the wires in the ceiling that aren't connected to anything, the attic framing reinforced with tomato stakes, the dead squirrel under the floorboards . . .

While these things are amusing (at least in retrospect), there's not much of a lesson in them. Anybody with a little common sense can figure out they're dumb or dangerous or wasteful.

But the really funny thing is how often common sense doesn't get applied to larger issues in construction. In one way it's understandable: in even a small project there's a lot going on, and a lot of conflicting needs. The smallest project can tie up a substantial sum of money; it always involves some inconvenience to the homeowner; and a seemingly trivial change can mean the difference between making money and breaking even to the contractor. It's an emotional undertaking.

So sometimes a short cut can seem like a solution.

That's the moment to call for common sense, and maybe a simple risk-benefit test.

Randy recently ran into this issue in the construction of a deck. The contractor wanted to set the support posts for a new deck on an existing concrete patio, rather than break through the slab and pour a new concrete footing, which would take more time and labor.

The contractor pointed out -- correctly -- that the code established by the Building Officials and Code Administrators Inc. (a widely accepted construction model) doesn't require footings if the deck is less than 10 feet off the ground and less than 100 square feet in size.

The point of a footing is to support a building load below the frost level, that is, how far down the ground typically freezes in the coldest weather. A standard footing is 8 inches of concrete at a depth that varies according to the climate. In the Mid-Atlantic, for instance, the bottom of the concrete footing should be 30 inches below grade (the level of the ground). Farther north, it must be deeper, and in the south, it can be shallower.

Secure footings keep the support posts from "heaving," or moving up and down as the ground thaws and freezes. Construction materials typically are not flexible enough to withstand even a little movement. And repeated movement in the supports could weaken the deck's connection to the house.

The building code establishes a minimum standard: It says, basically, "This much you must absolutely do."

Where the code leaves off is where common sense should begin. There are all sorts of factors that come into play in building a deck. How heavily will it be used? Will there be a lot of furniture? A barbecue grill? Planters? A hot tub? Two dozen friends partying every Saturday night? All those things affect the load -- the weight of the deck plus the weight of whatever is on it -- supported by the posts.

How stable is the weather? Are there cold snaps? Freeze and thaw cycles?

In addition, while codes like the BOCA are widely accepted, they are also widely amended. Many local jurisdictions mandate stricter or more detailed standards.

For instance, resting deck posts on an existing slab may be OK in Baltimore City, but it was not allowed in a similar situation in Baltimore County, and in nearby Harford County, the chief building inspector, R. Madison Mitchell, said "no way" could such a deck be constructed without footings. Mr. Mitchell said freeze and thaw cycles could cause the deck supports to move up and down "like a pogo stick."

Tim Sibol, of Skarda & Associates Structural Consultants, agreed deck supports should be set on footings, and not just rest on a slab. "Typically," Mr. Sibol said, "we would specify a 30-inch, round footing for any deck connected to the house."

Common sense says any deck should be built to maximum, not minimum, standards. If you have any questions about how secure the deck support will be, you might hire a structural engineer to review the plan. It won't cost much, and it could prevent a dispute with the contractor. Also, in most places, a structural engineer's opinion will outweigh that of a local building inspector.

And the exercise of common sense isn't always expensive. Digging two footings could cost as little as two hours of labor and $20 worth of concrete. Where's the sense in skimping?

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