PUTTING TOGETHER the structure of a deck is a little like doing the proverbial jigsaw puzzle -- except, in the case of the deck, you have to cut out all the pieces yourself. This is the stage, however, when the deck begins to take shape, and all your hard work with the ledger (which fastens the deck to the house) and the main beam (which supports the far edge of the deck) will be rewarded.
When you're planning your deck, adhere to the standard lengths of lumber -- that is, the depth of the deck surface should be some regular measure such as 8, 10, 12, 14 or 16 feet. The point is a) not to waste lumber, because it is expensive, and b) to use as few of the longer, heavier and more expensive pieces as possible.
You can make the deck shallower than 8 feet, but unless it is some measure of 4, you will waste a lot of wood when you cut the boards down. You can also get boards longer than 16 feet, but they are very expensive and hard to find.
The depth of the deck will determine the span of the joists. The shorter the span, the smaller lumber you can use. However, you don't want to use anything smaller than 2-by-8, and anything larger than 2-by-12 would be a special order (read "expensive").
A span table Randy consults on a regular basis says that if the joists are set on the standard 16-inch centers, you can use a pressure-treated 2-by-8 to support a maximum span of 12 feet 10 inches. In practice, Randy uses 2-by-8s for spans up to 10 feet, 2-by-10s for spans up to 14 feet, and 2-by-12s for anything beyond 14 feet.
If you want a span of more than 18 feet, you should think about installing an additional beam at the midpoint; it could be less expensive to build the midpoint beam and use smaller lumber for the joists. On the other hand, the extra cost of the 18-foot joists may be the same or less as the extra beam and smaller lumber. You need to cost it out both ways.
(If you do want a larger deck, you might think about dividing it into sections, on different levels, perhaps, or on two sides of the house.)
Once you've decided what size of lumber you are going to use, the next step is to enclose the ends of the deck and the rim, to form a box. Cut the two side joists 1 1/2 inches longer than the length from the surface of the house to the far edge of the deck. Using 12-penny galvanized framing nails, nail the side joists to the end of the ledger and reinforce the inside corner with a right-angle (2-flange) framing anchor. Then nail the end joist to the ends of the side joists (which will protrude past the main beam) and reinforce on the inside corners with framing anchors. At this point, the outer edge of the box will simply be resting on the main beam.
Square the frame by pulling a tape measure from corner to corner to check diagonal measurements. (Always read the tape measure from the same side.) Adjust the frame so the diagonals are equal. Once the frame is square, nail the side joists down to the main beam with 2 12-penny nails, one on either side. Building codes require that where one joist bears the weight of another, a framing anchor must be used to reinforce the connection. At corners, where joist hangers won't fit, these framing anchors will help carry the load.
The next step is marking where the joists will go to support the floor of the deck. Using a tape measure that's marked at 16-inch intervals, mark the joist locations on the ledger. Then mark the 16-inch increments on the end joist, making sure you pull the tape from the same end on the end joist as you did on the ledger.
When you have all your marks, use a carpenter's square to draw a line down the face of the ledger and the end joist at each spot. (Unless you've been extremely careful, the last joist may be less than 16 inches from the side joist, which is fine.) Then draw an X on the side of every line where the joist will be installed. Make sure the Xs are on the same side of the lines on the ledger as they are on the rim. When you're finished, check everything again to make sure the marks are accurate.
Install joist hangers on each vertical line, lining up the bottom of the hanger with the bottom of the ledger. Nail just one side of the hanger, leaving the other side loose, so the joist will slip in easily. (Be sure to use the proper size hanger so the joist will be less likely to twist.)
Before you put the joists in the hangers, check each board for crowns, or bowed edges. If you find a bow, install that joist so the bow is up, so gravity and the deck loads will help flatten it. Mark the bows so you can keep track of them as you work.
Slip the joists into the hangers, and check to make sure they are flush across the top. Then finish nailing, using the nails that come with the hangers.
Another way to keep the joists from twisting is to install bridging -- short pieces of wood -- between them, especially in the middle of long spans. Check local building codes; bridging may be required in some installations.
Besides the nails, Randy uses a twist-type framing anchor at every joist to hold the joist down securely where it crosses over the main support beam. Installing the twist anchors and screwing down the decking should prevent a problem with twisting.
If you want to leave a hole in the decking for a spa, stairwell or tree, you should double the joists around it. Any time you interrupt more than one joist, the adjacent joists must be doubled to carry the load. When available, you can substitute a 4-by-10 or two 2-by-10s. A solid beam is stronger and more rot-resistant than a doubled joist.
Once you get the hang of the framing, it should go fairly quickly. It's nice to have a helper when you are moving lumber around, but for the price of a few cold drinks you should be able to persuade a teen-ager or neighbor to give you a hand.
Randy Johnson is a Baltimore home-improvement contractor. Karol Menzie is a feature writer for The Sun.
If you have questions, tips or experiences to share about working on houses, e-mail us at firstname.lastname@example.org, or write to us c/o HOME WORK, The Sun, 501 N. Calvert St., Baltimore 21278. Questions of general interest will be answered in the column; comments, tips and experiences will be reported in occasional columns.
Pub Date: 4/27/97