FEATURED ARTICLE BY TREX RAINESCAPE

In deck construction, one truth holds: if a deck’s substructure isn’t solid, nothing above it will be either. Beams and joists form the backbone of every deck. Get them right, and a project is set up for long-term success; get them wrong, and even the best decking boards can fail prematurely.

Here’s a guide for trade professionals on what beams and joists do, how they work together and how to install and protect them for long-lasting decks.

What Are Beams and Joists?

Deck beams are the main horizontal supports that span between posts or piers. They carry the weight of the joists, as well as everything above them, down into the footings.

Joists run perpendicular to the beams and support the decking boards. Together, these components form the essential framework that spreads weight evenly and helps keep the deck flat, level and strong.

Why Beams and Joists Matter

As the underlying structure of a deck, beams and joists perform several vital functions that directly impact stability and lifespan. Here’s why they’re so important:

  • Structural Support: A deck’s stability starts with its framework. Beams and joists form the primary load-bearing grid that supports the deck’s weight and keeps it secure. This includes:
    • Live Loads:Variable weight from people, furniture and movable objects.
    • Dead Loads:Fixed weight of the deck structure itself, including decking boards, railings and stationary features like hot tubs or planters.

If beams or joists are undersized, improperly spaced or made from inadequate materials, the deck may bounce, sag or fail completely. There are measures that can, and should, be taken to prevent this.

  • Ventilation and Drainage: Properly spaced beams and joists provide airflow that helps water dissipate from beneath the deck, reducing the risk of rot and decay. This is especially critical for decks built low to the ground or in high humid environments. Applying a quality joist tape like Trex® Protectseals the tops of beams and joists, locking out moisture and, in turn, helping the frame last longer.
  • Load Distribution: A vital role of beams and joists is weight distribution across the deck structure. Joists transfer the weight of the decking surface to the beams, which then deliver that load evenly to the footings. This balanced distribution prevents:
    • Warping: Deck boards twisting or bending.
    • Cupping: Boards forming a concave shape.
    • Creaking: Noisy and unstable movement in the decking.

When properly installed, this support helps the deck remain level, stable and free from stress-related damage.

Installation Tips

The installation of a deck plays a role in the structure’s longevity. Be mindful of both the products and the process.

  • Choose Quality Materials: Investing in high-quality wood helps to ensure the substructure will last as long as the decking. Use treated structural wood or approved composite framing for beams and joists. Avoid low-grade softwoods, which are prone to twisting and decay.
  • Set Proper Spacing: The ICC Digital Codes on spacingcite guidance on joist spans and deck spacing as provided by the International Residential Code (IRC). Beam spans depend on lumber size, species and anticipated loads.
  • Use the Right Fasteners: Outdoor decks need corrosion-resistant fixings. Hidden fasteners have no deck board penetration, which allows them to minimize water infiltration and reduces the risk of moisture damage. For additional protection, apply Trex Protect tape to joist and beam tops. It self-seals around any type of fastener, keeping water out and fasteners tight.

Common Issues and Solutions

  • Sagging Floors: If the top of the substructure feels bouncy or dips in places, those are indications of undersized or over-spanned joists. Check for cracks or soft spots and compare the spans to the proper tables. Reinforce weak areas or replace failing joists before laying down decking.
  • Shifting or Misaligned Beams: If posts aren’t properly anchored and adequately sized for the load, or the beams aren’t securely fastened to the posts, alignment can shift and throw off the entire frame. Check for properly sized footings or piers and always verify post spacing and beam levels. Use quality fasteners to ensure connections are square, strong and locked in place.
  • Moisture Damage and Rot: Water that sits on top of beams and joists can slowly soak into wood and cause hidden rot. This usually starts around screw holes and board edges. Sealing the tops of beams and joists with joist tape, such as Trex Protect, helps block out water and preserve the structural integrity of the foundation.

In deck building, a solid foundation isn’t optional – it’s everything. When it comes to joists and beams, take the time to get them right by choosing quality materials, using proper spacing and alignment, and protecting them from moisture to ensure decks remain strong and durable for years to come.

To learn more, visit https://trexprotect.com/.