How To – Building A Beam For Your Deck

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    Decks and Porch additions require that the joists be connected to the home with a Ledger Board and joist hangers. On the opposite side (away from the home) you rest the joists on top of a beam that is supported by posts.

    In this HowTo we will cover the steps for installing the posts and building the beam for our deck project.

    When designing your deck you will know its dimensions in our project the deck is 10 feet by 20 feet. For best use of materials this means we will use 10 foot long joists running the short distance away from the house. Knowing the dimension of your beam, the joists, the location of the footings and the posts that will support the beam is necessary before any work begins because you need to know the size and location of the beam to know where to place your footings and posts.

    To support the joists we need a beam under them on the outside dimension away from the house.

    The size and location of the beam is decided by a beam span chart that lumber manufacturers provide to contractors. You can learn more about span charts in our other HowTo  Framing and Span Charts.

    We have found that a beam made of Two 2″x10″ running the 20foot length will be right for this job. We can set the Beam in 1 foot from the outside of the joists to hide it under the deck and provide good support.

    The 2×10 beam will need to sit on 6″x6″ posts which are connected to our footings.

    To begin the work we mark on our ledger board the bottom dimension of our joists. This is 7-1/2 inches down from the top of the ledger.

    Then we install a string line at the bottom mark of our joist dimension and run it directly parallel with the location of our beam.

    This will give us the height of our posts.

    Install a piece of 6×6 post material in the post base and use the string line to mark where to cut it. You will also need to notch half of the top of the post 9-1/2 inches down and 1-1/2 inches in to accept the beam.

    To notch the post simply make a number of cross cuts close together in the area to be removed and then break out the waste with a hammer and clean up the work with a chisel.

    You will notice we also need to notch the end of our beam to get it under our ledger board. The beam won’t support the ledger board but since it is right there why not give it a little extra support.

    Clamp the 2×10’s to the posts and then nail them together every 6 inches on center with hot dipped galvanized nails. Remember to nail from both sides of the beam.

    Also remember to check the level of the beam as you work. 

    To attach the beams to the posts we will use carriage bolts with flat and lock washers. As the beam shrinks after losing moisture from the pressure treating these bolts will need to be re-tightened. Expect to re-tighten them once with in about a month and then again in about 6 months.

    When you have the beam completed and have bolted it together you can double check your distances between the ledger on the wall and the beam. Slight adjustments can be made by loosening the nuts in the post mounts.

    Finally tighten everything down and you are ready to start adding joists.

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