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Beams and Joist Sizing

10:43
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Why Most DIY Deck Frames Fail (And How to Succeed)

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Beams are critical structural components that connect footings and support the joists. The size of the beams depends on several factors, including the available clearance (especially for low-to-the-ground decks), the number of footings, and the desired span length. To create stronger beams, 'laminating' multiple pieces of lumber is common; for example, three 2x10s can be laminated together to form a very sturdy beam. Joists are supported by these beams and determine the deck's surface. Similar to beams, joist sizing (e.g., 2x6, 2x8, 2x10, 2x12) and spacing (12, 16, or 24 inches on-center) are dictated by load requirements, lumber species, and span tables. Online calculators and span tables are readily available to help determine the correct sizes and spacing. The presenter prefers using 2x10s for both beams (laminated) and joists (12 inches on-center) for consistency and a stiffer structure, especially when using composite decking, even if not strictly mandatory by the manufacturer.

Goal: Properly size beams and joists to meet structural requirements and desired deck characteristics.

Tools and materials

  • Lumber — Constructing beams and joists

What they say

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keep everyone on the same page. Now, for our beams, we like to triple them up. We like to do a laminated 2x10 beam. So, we'll take three 2x10s, laminate them together, and that will construct our beam. That's going to allow us for further spans between our footings and just a lot more support. You can also do a doubled 2x10, double 2x8. You can check these charts and find out all the information that you need for what size and how far they can span. Now, when it comes to the joist, you can also reference those same charts and it's going to tell you your allowable span. We'll put a link to one of these charts down in the description so you can reference it, but they're very easy to use and very handy. It's also going to tell you different types of lumber that you're going to use. Depending on your area, you'll have different species and what they're rated for. So, your allowable span is going to be dictated off of a few things. It's going to be number one the size of the lumber and how far apart they're installed. So you can do 12 in on center, 16 in on center, 24 in on center. You're going to want to reference also the material that you're going to use on top for what that spacing should be. We always install on 12in centers because we're using composite decking. It's not mandatory by the manufacturer, but we find it just provides a stiffer structure and it's only a couple extra boards, so we find it to be super worth it. This leads

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