ONE OF the nicest things about adding a deck to your house is that you probably can, if you want to, build it yourself.
You need to be comfortable working with a hammer, a power saw and large pieces of lumber, and you need a good grasp of the concepts of square, plumb and level. But beyond that, it's a straightforward process that a diligent home handy person can accomplish.
The first step is to design the deck. There are computer programs that will help you do this, and when you're finished print out a list of materials needed. Or you can get help from your local home-improvement center. Novices should stick to something simple, like a rectangle; keeping in mind that you can always add another level, or a gazebo, or built-in planters, later.
The next step is to lay out the deck outline on the ground on which it is being built. This is a sort of adult version of cat's cradle, where you get to play with string and stakes.
To make sure the corners are square to the house, stretch a string from the house to a couple of feet beyond the farthest extension of the deck. (If the deck will be 10 feet deep, measure out 12 feet.) Measure along the line 8 feet and place a mark there. Then measure 6 feet from the string along the house, and put a mark there. Stretch a tape to 10 feet and adjust the string so the two marks are at either end of the 10 feet. Stake the string at 12 feet, where it's out of your way.
Repeat the process at the other side to get two square corners. Check the diagonal measurements from one corner of the deck outline to the opposite; they should be within half an inch of each other. For the outside corners, measure out along the perpendicular strings 10 feet from the house and drive stakes along that line. The squaring process needs to be done twice, once before you dig the footings and again when you start putting up the support posts.
The next step is to locate the posts. They are never located at the edges of the deck; instead they are set in about two feet from the outside edge. The posts at either end should be set in from the side edges about a foot.
When the posts are located, dig the footings, remembering that the bottom of every footing should rest below the frost line (which varies by area; in Maryland it's 30 inches). Pour the concrete for the footing using a form you've built or a standard round form from a home-improvement center. The concrete has to cure about a week before you can build on it.
While the concrete is curing, you can install the ledger board on the house that will support the inside edge of the deck. The interior floor of your house will usually support decks that are built to the same height; that makes it easy to bolt the ledger to the rim joist or the blocking of the house floor.
If you have a single rim joist (1 1/2 -inch stock), use machine bolts or carriage bolts with the nuts on the inside. If you add blocking behind the rim joist, or if you have a double rim joist, you can use lag screws.
Clearly the deck floor should not be higher than the bottom of the door, so rain and snow can run off. And it's a good idea to clear snow from the bottom of the door to keep it from damaging the wood.
To find the interior floor height, use a nearby window or door; in fact, it's a good idea if you're installing a new door to cut the door opening before you install the ledger. Balance a level on the window or door sill (leveled, of course), and use a tape measure to find the distance from the level to the interior floor. Then measure the same distance from the outside down, and that will mark where the floor is on the inside.
When you mount the ledger, mark drilling holes at the top and bottom, 1 inch in from the edges, on 24-inch centers. Adjust the bolt locations so they will be out of the way when you start installing hangers for the deck joists, which will be on 16-inch centers. Prop the ledger against the wall and mark the joist locations on it. Drill the holes for the lag screws or bolts, using fasteners that are at least 1/2 inch in diameter. Then use the ledger as a template for marking the fastener holes in the house.
A simple way to prevent moisture from building up between the ledger and the house is to stack about 4 washers on the lag bolts or screws before you install the ledger. You can tighten the screws firmly and still leave a 3/8 -inch air gap. Before installing the ledger, lay a thick bead of silicone caulk around each hole in the house. The caulk will squeeze tight and seal the hole against moisture.
Once the concrete is cured, you can set the posts on the footings with post anchors. Use 6-by-6 posts, and try to choose ones that are straight and in good condition. Pressure-treated posts always crack, so start with the best ones you can find. The 6-by-6 posts can be notched at the top so a girder can be mounted on each side and still rest on wood -- something you can't do with a 4-by-4. Once the girders are installed, bolt the structure together through the outer girder, the tongue of the 6-by-6 and the interior girder. Space the posts no more than 6 feet apart. For instance, an 18-foot wide deck would require 4 posts.
If you're installing new siding along with the deck, take some extra precautions to protect your house. Use metal flashing at the top of the ledger under the siding to shed water. Depending on your design, you can probably find pre-bent stock flashing material at a building supply or roofing supply dealer. Make sure that a layer of building paper continues behind the siding and under the ledger -- or double the building paper behind the
ledger, to provide an extra layer of protection for the house.
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 homeworlark.net, or write to us c/o HOME WORK, The Sun, 501 N. Calvert St., Baltimore 21278.
Pub Date: 4/13/97