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Deck Building: Beams and Joists

How to Build a Sturdy Understructure for Your Outdoor Entertainment Center

A Deck Contractor

The next step in importance after deck design is the understructure: posts, beams, joists, and ledger boards. This article explains how to build a sturdy one.

And if you don’t want to tackle the job yourself, I highly recommend Angie’s List.. They can tell you how your local contractors performed.

Like any project that uses structural components, always be sure to check your local building code.

This series of articles begins with basic design issues and moves through to final finishing and refinishing. The links to the entire series are located at the bottom of this page and the lists on the prior articles are updated as they are posted.

Installing Your Deck Beams

Note: Always use screws, fasteners, nails, and metal components that are specially coated for the type of pressure-treated lumber you are using. In an attempt to get away from the arsenic-based formulations, the newer chemicals can corrode untreated metals prematurely. Forewarned is forearmed. The beams run across the tops of the posts and parallel to your ledger board with an attached deck (No ledger on a detached deck).

Nail or screw post caps to the tops of the posts in which to secure the beams.
Use a stake and string line to indicate where the beam should be located. Use the 3-4-5 squaring method to ensure that the string is perpendicular with the end of your ledger board. Remember that your joists butt into your ledger, and they ride on the top of your beams.
Cut Your beams to the proper length. A compound miter saw will give you a cleaner cut than a circular saw. If you're building a cantilevered structure, make sure you allow for an equal overhang past the ends of your outside posts. If you’re using 4” X 4” posts, it’ll take two 2” X ?” beams that are nailed or bolted together. (Where “?” is the height of your beams for your chosen deck configuration. See the lumber sizes and spacing article.)
Starting with the beam that’s farthest from your ledger board, set it in the post caps.
Now, just adjust your beam in the post caps until it’s about 1/16” from the string.
Screw or nail the beam into the post caps.
Install and secure your remaining beams as above.
Use 2” X 4” boards at a 45 degree angle to brace the beams to the posts.

Next, Install Your Joists

Find the length of your joists. Remember that the outside joists should butt to the end of your ledger (making use of a galvanized framing anchor), but the inside ones should be installed with joist hangers. And remember to take into consideration the length of your cantilever past the end beam and then add a couple of inches.
Now you’re ready to install your joist hangers. The distance between your joists will be mandated by your local construction code. Know it. Live it. Love it.
It’s time to install your joists. One end will go into the joist hanger. You can use galvanized rafter ties on the tops of your beams.
Mark the cantilever overhang on the outside joists and strike a chalk line across the tops of the joists.
Use your square to transfer these marks onto the sides of your joists.
Use your circular saw to cut off the ends of your joists.
Screw or nail a rim joist across the ends of the joists.

Check your local building code to find out if blocking is mandated between your joists to keep them from twisting. Even if not, I recommend taking the time to do it while you can get at it. “In for a dime, in for a dollar,” as they say.

Now is the time to put in blocking for posts for railing, if you plan to have them. Again, your building code might require them. The next deck article will take a look at planking (deck boards) and several options. Stay tuned!

Previous articles in the deck-building series:

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