Installing the Beam
On our basic deck design, the beam sits on top of the posts, and joists sit on top of the beam. Joists also can be set on the same plane as the beam by installing them with joist hangers. The beam should be pressure-treated unless you are using a naturally rot-resistant wood. Sometimes it is difficult to find a solid 4x beam in the width you need, and a 4x10 or 4x12 beam can be heavy. For these reasons, a built-up beam made of two 2xs can be more convenient.
To ensure that the outside edges of the beam align squarely with the ledger, reattach the mason’s lines, marking the sides of the deck before installing the beam. This is far more important than having the ends of the beam overhang the outside posts by the same distance. Check for square and measure the diagonals from beam to ledger before fastening the beam to the posts. Generally, beams should be installed with the crown side up. To find the crown, sight down the narrow edges of a board. If one edge seems to have a high spot (crown) on it, place a mark on this edge to remind you to set this edge up. In constructing a built-up beam, make sure the crowns of both boards are on the same side of the beam.
Tools: Hammer, circular saw, drill and bits, wrench, level.
1. Join beam to posts. The beam-to-post connection is critical to the integrity of your deck. The strongest connections are formed when the beam rests fully on top of the posts, as with two of the examples above. Metal brackets, straps, or ties add additional stability. You’ll be able to select from a wide variety of these fasteners at your home center or lumberyard. Your building supplier should be able to help you choose the right ones for your deck. With these connectors, always use the type and number of nails or screws recommended by the manufacturer.
2. Calculate joist overhang. On our basic deck design, the posts are centered 11 feet from the house, while the deck surface extends to 12 feet. This overhang, or cantilever, creates a more attractive deck because the beam is set back out of view. In general, joists can overhang the beam one-third, or about 33 percent, of the distance between the ledger and the beam. However, your local building code may dictate different requirements: Be sure to check on cantilever limits.
3. Construct a built-up beam. A beam composed of two 2x boards separated by W-inch spacers will match the width of 4x4 posts. The spacers also allow water to drain through the boards, ensuring a longer life than if they were nailed together. Use pressure-treated plywood to make the spacers, and point the tips to encourage water runoff. Use a spacer every 24 inches on center. Attach the spacers with 6d galvanized nails and drive 16d galvanized nails through the beam and spacers from both sides of the beam. Stagger joints and place them over posts.
4. Size up a beam. If you make a built-up beam using two 2x boards, keep in mind that the structural strength of the beam is not equal to a 4x beam. It is only equal to the width of the two boards (3 inches), not its finished width (3 1/2 inches). Spacers don’t add strength.
5. Place joints over posts. For a built-up beam, it is smart to locate even staggered joints over a post. If you are using a solid beam, however, you must center all joints over posts. Cut the beam squarely so as much of the beam as possible contacts the post.
6. Bolt beams to continuous posts. If your deck has posts running through the framing for railings, you should bolt a double beam to the posts as shown above. Use short pieces of 4x4 as spacers and use W-inch machine bolts with washers on both sides. To reduce the chance of splitting a post, offset the bolts horizontally. Note that this technique relies on the fasteners for much of its strength and integrity. It is not as strong as when the beam sits on top of the posts. It is a good idea to consult with a building professional before using this approach.
If properly installed, joist hangers are a more secure method of attaching joists to the ledger than the old technique of toenailing joists. In fact, most building codes require hangers for deck joists. Take care to buy joist hangers that match the size of joist you are installing. A 2x6 joist requires a different hanger than a 2x8 joist. Make sure the hangers are intended for exterior use. For built-up or 4x joists, use 3 1/2-inch-wide hangers. For end joists, use a heavy-duty right-angle bracket that attaches on the inside corner so no hardware is visible.
Tools: Fiammer, tape measure, pencil, framing square, speed square, circular saw.
1. Lay out joists on the ledger. Temporarily tack a scrap piece of 2x joist stock to the end of the ledger to represent the end joist you will install later. Hook your tape over the edge of this scrap and make a mark every 16 inches on the front edge of the ledger. Then go back with a square and extend each mark down the face of the ledger. Use a scrap of joist stock set on the joist side of each line to mark the width of each joist. Mark an X between the two lines to make it clear where each joist should go.
2. Attach joist hangers. Attaching a joist hanger can be trickier than it looks. Use the fasteners recommended by the joist manufacturer. Hold the hanger with one side aligned with the layout mark on the ledger. Use a scrap piece of joist stock to make sure the hanger is positioned so the joist top and ledger top are flush. Nail one side of the hanger to the ledger, leaving the other side loose.
3. Use special hangers for angles. Some deck designs have joists that meet the ledger at an angle. Forty-five degree hangers usually can handle a 40- to 50-degree angle. Use bendable seismic anchors for other angles. Cut joist ends at the appropriate angle for full bearing.
4. Install the joists. Measure and cut the joists to length. Both ends should be square and free of splits. The end joists will be 1 1/2 inches longer than the others to overlap the ledger. Apply preservative to the cut ends. Set each joist in place with the crown up. Make sure joists are straight and parallel. Nail in the other side of the hanger, then nail joists to the hangers. Overlap the end joists on the ledger ends, then attach angle brackets to the inside corner.
Keep It Square - The more care you take to install the joists square with the ledger, the easier the rest of the construction will be. Don’t assume you can square up things later. Use a framing square to align each joist at the ledger. Along the beam, use a spacer to test the position of each joist. To make the spacer, cut a perfectly square piece of joist stock exactly 14 1/2 inches long. Have a helper use the scrap to space and align the joists along the beam.
5. Attach joists to beam. With the joists straight and square with the ledger, fasten each with a 16d nail driven at an angle into the beam just deep enough to hold it in place. (After installing the header joist, you can remove these nails to allow the frame to be squared up.)
6. Install header joist. The header, or rim, joist is fastened to the ends of the joists. It’s helpful to mark a layout on the header similar to that on the ledger. While a helper aligns the tops, drive three 16d nails through the header into the joist. With the header installed, check the frame for square by measuring the diagonals. If necessary, remove the toenails at the beam to make minor adjustments. Finally, fasten joists to the beam permanently with two 16d nails toenailed through each side of the joist.