How to Size a Deck Beam

A beam is a solid structural member that is supported by posts and footings that accepts and transfers the load from joists. Once you know the distance between support posts and the length of the joists that will be supported you can determine the required size by checking our beam-sizing chart. The size of a beam is a function of the amount of weight it must support and the distance it must span.

A common beam is composed of the same size and type of lumber used for the joists by nailing 2 or 3 together side by side. Even if you are allowed to use a smaller beam, you should maintain the same depth of material as the joists to present a uniform appearance. Never position your post “sandwiched” between two boards as is sometimes seen as a beam construction technique. The relationship between directly applied and fastened boards is what bonds the strength of a multiply beam.

Occasionally a beam application may require spanning a long distance that exceeds the design limits for conventional beams. In these cases you may have to use an engineered beam to meet the required load capacities. Engineered beams are fairly expensive, so you might want to add another post to reduce the span if possible. Your local building supplier should be capable to use your plans to determine which size and type of beam will work. Steel I beams are the strongest form of beam available and can be used to carry extremely long spans 20 ft + but are very expensive, heavy and difficult to work with.

Popular Framing Articles
Deck Support Columns

How to Decorate Deck Support Columns

Browse some examples of decorative deck support posts for tall decks.

Deck Framing

Building A Deck Over An Air Conditioner

Things to consider when building a deck near an air conditioning unit. You may want to relocate the AC unit if possible to avoid some problems.

Deck Framing

How to Build a Multi-Level Deck

Multi-level decks are very popular. Learn how to build a deck that steps up or down using shared posts and footing connections.

Next Step: Decking Articles
Decking Installation

How to Install Decking Over Existing Concrete Slab, Patio, or Porch

Discover the best techniques to build a deck over concrete, ensuring proper drainage and stability, while creating a beautiful outdoor space.

Decking Installation

Picture Frame Decking

Install a picture frame or perimeter board to finish the edge of your deck with style.

Decking Installation

How to Install Composite Decking

Learn about how to install composite decking. Topics include expansion and contraction issues and hidden fastener systems.

Get more helpful resources delivered right to your inbox

Explore Articles by Topic

Popular Framing Articles
Deck Framing

Why is Joist Protection so Important for Your Deck?

If you’ve ever seen a piece of wood left out in the weather for any period of time, you know what happens: decay. Whether through wet rot, insect damage, or mildew, the fibers begin to break down.

Deck Framing

How to Brace a Swaying Deck

Learn how to install a diagonal brace on your deck frame to prevent racking. Check out our detailed image.

Deck Support Beams

Engineered Deck Beam

Use treated engineered deck beams for long spans with fewer posts and footings. Engineered beams can be designed and specially ordered from your lumberyard.

Next Step: Decking Articles
Decking Installation

What to Consider When Building A Wrap-Around Deck

Looking to add a wrap-around deck to your home but unsure of what the process involves? Check out this guide to learn more about top considerations when building a wrap-around deck.

Decking Installation

Tile decking

Installing tile over a standard deck frame in an exterior environment is a recipe for trouble.

Decking Installation

Decking over decking

No; this is not a recommended practice.

Get more helpful resources delivered right to your inbox

Explore Articles by Topic