Main Structural Deck Components: GIRDERS Part III of VI

Last week we continued looking at a six-part series on the topic of the structural elements and load path of a typical deck system. Today we will continue in the second part of this series looking at main girder or beam which supports a deck system.

The outline of the series follows:

  1. Footings
  2. Columns
  3. Girders
  • Connecting a Post to the Girder 
  • Girders Transfer and Aggregate the Joist Load
  • Girder Sizing and Calculations
  • Built-up Girder Beams
  • Girder Hardware Connections

4. Joists

5. Ledgers

5. Rim Joists

We have created several different graphics and diagrams for explanation and analysis in this article, but we will look at some of these graphics again in future articles to examine them more closely.

Connecting a Post to the Horizontal Girder Above

Connecting the vertical deck posts securely to the horizontal beams or girders they support is a critical structural detail. Proper fastening at this junction transfers loads evenly and prevents separation or racking over time. Several different hardware components and techniques are commonly used to achieve a robust post-to-beam connection.

One of the simplest methods involves cutting a shoulder into the top of the post that matches the width and depth of the beam. The beam then rests directly on this flat, horizontal shoulder cut into the post top. While providing some bearing surface, this approach alone does not laterally restrain the beam.

girder main structural deck components

To lock the beam into the shoulder cut, approved metal post caps or post anchors are installed. These connectors have a bent section that wraps over the top of the beam while the straight section lags into the post using angled nails. This secures the beam in both the vertical and horizontal directions.

In cases where the posts pass straight through and intersect the middle of the beam, sturdy mounted post bases get anchored atop the post before installing the beam. Approved hardware like framing anchors, hurricane ties, or column caps then link the beam directly to the top of the secured post base below.

For even greater load transfer, some deck designs call for specialty beam hangers or joist hangers at each post top. These “U”-shaped galvanized brackets are secured at the sides of the beam and attached to the post with multiple structural screws. Beam hangers connect the beam while resistibg uplift and lateral force.

Proper structural connections can be arranged in a multiple of different configurations.   Effectively a redundant metal-reinforced series of connections can be used to support the girder down into the vertical supports without relying solely on ineffective toe nailed fasteners. This ensures the entire load path transfers and integrally connects.

Girders Transfer and Aggregate the Joist Load

The girder or main beam is the primary horizontal framing member that supports the deck’s floor joists above. This critical component transfers the accumulated weight from the joists and decking surface down into the vertical posts and footings. Proper sizing and construction of the girder is essential for ensuring the deck’s structural integrity.

imortance of girder in main structural deck components

(This diagram shows how a deck girder is essentially the bridge and supporting element like the backbone between the post of the deck and the deck joist.)

The girder typically bears directly on the support posts below, with the post tops cut to create a shouldered bearing point which inherently stabilizes the girder by nestling and connecting the girder directly to the post on two planes. This shoulder helps prevent the girder from sliding or shifting laterally.  A connection by shouldering the girder is a much stronger positioning because by being in contact with the post below on two sides inherently resist movement. Specialized structural ties, or other hardware is used to securely attach the girder to the posts for a rigid connection capable of resisting lateral and uplift forces.

Girder Sizing and Calculations

To determine the appropriate girder size, deck builders refer to span tables provided in the building code or by the lumber grading agencies. These tables specify the maximum allowable span between post supports based on factors like:

  • Girder/beam dimensions (width and height)
  • Lumber species and grade
  • Joist span and spacing supported by the girder
  • Any concentrated load conditions like hot tubs
  • Environmental adjustments for ground snow/rain loads

Common girder sizes like triple 2x10s or double 2x12s are sturdy enough for most residential decks under 14-16 feet in length. Longer spans or very heavy loads may require built-up or engineered girders (or even steel beams).

In cases where the required girder depth exceeds what is readily available in single solid timbers, multiple individual boards can be fastened together into a built-up configuration. Properly staggering and nailing the laminations per structural standards allows smaller lumber to act as a larger unified beam section.

Built-up Girder Beams

When using multiple individual boards or lumber pieces to build up a larger girder or beam, properly fastening and adhesively bonding them together can create a unified element that is stronger than the sum of its parts. There are a few key reasons this built-up configuration increases strength.

By securely fastening the individual boards together with structural screws, lag bolts, or toothed metal plates, you essentially create a “composite” member. The shear connectors between the boards allow them to act as a unified cross-section rather than separate elements. This composite action enhances the overall stiffness and load distribution ability.

Built-up beams allow optimizing the cross-sectional geometry for structural efficiency that may not be possible with off-the-shelf lumber dimensions. For example, using multiple 2×6 boards adhered into a rectangular shape can create a deeper beam section more capable of resisting bending stresses than a solid 4×6.

pattern layout of girder in main structural deck components

Any single piece of sawn lumber will likely contain some defects like knots, slope of grain, etc. that create weak points. But when combining multiple elements tied together, these defects get dispersed and their impact is minimized across the built-up section.

In addition to mechanical fasteners, using structural adhesives like construction adhesives or structural wood glues further solidifies the built-up beam into a monolithic unit. The adhesive transfer shears between laminations, enhancing overall stiffness.

When properly assembled per code requirements on fastener spacing, staggering lamination ends, and application of the structural adhesive, testing has shown built-up beams can actually exceed the strength of comparable solid-sawn members of the same size. The harmonized, defect-dispersed cross-section optimized by the composite behavior creates an amplification effect.

Girder Hardware Connections

Throughbolts, structural screws, or specialized wood adhesives may further enhance the beam’s composite action when using built-up members. Some manufacturers offer a variety of heavy-duty barrel hanger ties explicitly engineered for sandwiching and splicing multiple plies together.

With a properly sized and fastened main beam or girder in place, the perpendicular floor joists can span across and attach to the girder’s top edge using hurricane clips or modern alternative structural ties. This allows the load to transfer in a unified path down through the deck’s substructure to the footings below with ample safety factors against deflection or failure. Designing, constructing, connecting this primary girder properly is important for deck bearing and uplift resistance strength.

why girder is main structural deck components

(The diagram above shows an example of a relatively simple deck layout with joists supported by a girder shouldered and set into three posts. Those posts are then each transferred to a concrete footing and are connected by a footing post structural connector tie.)

The picture above shows the girder supporting the joists. The joist run perpendicular to the rear facade of the building. In this case the joists are connected to a ledger board and those joist bear, farther away from the building on top of the girder. The girder is supported by the post below and those posts are supported by the concrete footings. All of these elements are connected together and particularly the girder is connected to the joists above through hurricane clips or their modern alternative which are galvanized steel structural supports nailed to both the joist themselves and to the girder. Like many of the structural connectors, the clips used at each joist connection both help maintain rigidity which allows for the load to be transferred, but they also help with uplift resistance. Uplift resistance is required to resist the force which can cause decks and other building elements to be lifted up into the air from high winds. The pressure exerted on building systems during the strongest wind gusts is significant and although it doesn’t happen every day.  Normally the most potentially devastating wins happen at the worst seasonal wind storms.  In those extreme weather events, the force can be significant and even tear buildings apart.

Use a contractor who understands and cares about doing things right.  Always, feel free to reach out to us here at Dupont Decks and Patios.  You can call us at (202) 774-9128.  You can find us online at and you can email us there as well at

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