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How to build & install a built-up structural beam for decks, porches, & similar structures. This article describes how to build a beam used to support a deck or porch floor or similar structure. We describe building the beam out of 2x lumber in four easy-to-follow steps.
The article concludes with a description of different methods used to make a secure structural connection between posts and beams in wood-framed structures. This deck design-build article series describes construction details for decks and porches, including avoiding deck or porch collapse and unsafe deck stairs and railings.
The beam, or girder, is the structural member that spans from post to post.
Modest-sized decks typically are built with a single beam, but larger decks may require two or more.
[Click to enlarge any image]
There are several ways to make and mount a beam.
Regardless of the style of beam you use, you can be sure that it will be heavy. Plan to have a helper or two available when you need to lift a beam into place.
How to Construct a Built-up Deck Beam
When the design calls for placing a beam made of two 2 x boards over 4x4 posts, a very common arrangement, many deck builders like to construct the beam with Vi-inch spacers between the boards. This produces a beam that fits perfectly on top of the post, since 2X lumber is 1 1/2inches thick.
Additionally, the spacers allow water to drain through the boards, which prolongs the life of the beam. Use pressure-treated plywood for the spacers.
Note, however, that if your plans call for a single 4X beam, which would be 3 ½ inches thick, this built-up beam will not be sufficient. Structurally, it amounts only to a 3-inch-thick beam.
You can also make a built-up beam without spacers. Attach boards to each other with l0 d nails driven through both sides at least every 16 inches.
Apply a bead of silicone caulk to the joint between the boards to keep any moisture from penetrating.
The structural connectors shown at left are used at inside corners where a joist hanger or post & beam connector doesn't work - for example at the inside corner formed by the side rim joists and the ledger board or the inside of side rim joists and the front rim joist.
When Constructing a Built-Up Supporting Beam, Center Joints over Posts
When buying stock to use as beams, plan to have each joint fall directly on the center of a post. The adjoining sections of the beam should be cut as squarely as possible to provide maximum bearing on the post. With built-up beams, stagger the joints between boards, still making sure that each joint falls on the center of a post.
When Building Up a Beam Set All Lumber Crown Side Up
Lumber that is used for beams and joists nearly always has a visible crown, or arch, as shown below. Install both beams and joists with the crown facing up. That way, once the deck load begins pressing down on the crown, it will level itself out. In constructing a built-up beam, make sure the crown on both boards faces the same way
4-Steps to Build & Install the Deck Support Beam
Step 1: Find the Framing Lumber Crown
Locate and mark the crown for the beam. You can usually spot the crown by sighting down the edges of the board. Alternatively, set the board on edge on a flat surface, such as a concrete driveway or garage floor. You might want to mark the board with an arrow clearly indicating the crown.
Framing Note: Always install the board crown side up. When building a built-up beam, sight down each board, mark for the crown, and attach the boards with both crowns up.
Step 2: Assemble the Deck Beam
It is smart to build a beam that is a bit longer than needed. That way you can shift the joists slightly to square up the frame if necessary.
Once that is done, the beam can be cut to finished length. Assemble a built-up beam on a pair of sawhorses. Attach spacers to one board every 12 inches, then set the other board on top and drive three 12d nails at each spacer location.
Flip the two boards over and repeat the nailing pattern. Stagger joints between boards, and secure each joint with a spacer and nails.
Step 3: Set the Deck Beam in Place
If possible, position or reattach string lines representing the sides of the deck.
Align the beam with the string lines at each end. If you made the beam a bit longer than needed, overlap the string an equal distance on each end.
Lifting a typical deck beam overhead may require three or more relatively strong workers.
The safest way to install a high beam is to use scaffolding, which will also make joist installation much easier.
If you are concerned about a heavy beam tipping over as you try to set it in place, temporarily attach 2 x 4s to a couple of posts so that the 2 x 4s stick up by 8 to 10 inches.
Set post caps on the other posts and fasten them to the posts with a single nail each. Remember to install the beam with the crown side up.
Deck girder flashing tip: In our photo the deck beam and some deck joist top surfaces have been protected by additional flashing. Keeping water out from between the segments of built-up deck beams can substantially increase the life of the structure.
Obviously the time to make this decision is before nailing the deck flooring in place.
Since the deck fasteners will puncture flashing you might ask what good it's doing. Water enters an un-flashed deck girder at the gaps between the deck boards.
Step 4: Attach the Deck Beam to the Posts
With the beam safely in place, add post caps to any additional posts that need them.
Check the beam’s alignment with the string lines.
You can also check the alignment by measuring the diagonals; provided that the beam is equally spaced from the ledger on both sides, the beam will be properly positioned when the diagonals are identical.
Once you are confident that the beam is squared up with the ledger, attach the post caps to the posts and the beam with nails as recommended by the post cap manufacturer.
Our photo (left) illustrates a typical steel post-to-beam connector that ties a 4x4 post to a built-up deck girder. Other sizes of post and beam connectors are available for virtually every combination of post dimension and beam dimensions.
Post-To-Beam Connection Methods for Decks & Similar Structures
The strongest and most secure way to install a beam is directly on top of the posts. In most cases, the easiest way to connect the beam to the posts is with galvanized post caps.
Post caps are available to fit most typical post-and-beam configurations, even where the beam is narrower than the post. The top three illustrations at right show some of the more common connections.
Beams and posts can also be fastened together with wood cleats made from framing lumber, which must be bolted to both members.
Although the cleat method requires a bit more work, some people prefer the look of wood to less attractive metal caps, especially when the connection will be fairly visible.
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