Installing the Beam

Introduction

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 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 (see Hanging Joists, Related Projects). 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.

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 the top two examples at right. 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 one for your deck. With these connectors, always use the type and number of nails or screws recommended by the manufacturer.

2. Calculate beam 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 1/2-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 at right. Use short pieces of 4x4 as spacers and use 1/2-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.

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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 the top two examples at right. 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 one for your deck. With these connectors, always use the type and number of nails or screws recommended by the manufacturer.
2. Calculate beam 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 1/2-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 at right. Use short pieces of 4x4 as spacers and use 1/2-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.