Deck Building Beam Options


The Deck Building Beam sits on the footings, it carries the joists and gives the main support to the surface decking.

Depending on which type of deck you want to build - there are different structural options to take into consideration.

It's important to understand the difference between a "Corner Post Deck" or a "Cantilevered Deck" so that you can decide which method of deck building beam you want to use.


The Corner Post Deck

By far the easiest deck to measure out and construct is the "Corner Post Deck" - mainly because the simplicity in constructing the joints and the ease of making smooth wood joins.

This type of deck building beam is usually made up of two pieces of 2 x 8 or 2 x 10 pressure treated or Redwood lumber. These two pieces will both be attached to the decking posts - one on top of the other.

Deck Building Beam Option


The Inner Beam will be 2" shorter that the outer beam one each end (4" shorter in total). As you can see in our illustration - this will mean that the 2" of the 2 x 8 "Outer Joist" will fit neatly against the post and Inner Beam and butted up against the inside of the outer End Beam.

The footings and posts are located in the extreme corners of the deck with additional footings every 6' - depending on the size of deck you plan to build.

Because the footings and posts are in the corners and the end deck building beam and outer joists are attached to them, this creates the limit of your decking frame and means that your decking boards will not go beyond this size.

The main disadvantage is this method is that the footings will be visible when looking at your deck from your garden - which will give you an elevated or side view.

The Cantilevered Deck

This form of deck construction creates a much more attractive appearance and more skilled finish.

The deck building beam for a cantilevered deck is easy to build and does give more strength by spreading out the load it supports - onto beam lumber placed either side of the footing posts.

To explain this method fully we have given you a plan view and a side view.

deck building image


Depending on the size of the structure you want to build you will probably use 4 x 4 posts with 2 x 8 beams for a ground level deck design with or without a couple of steps or 6 x 6 posts and 2 x 10 beams to support the weight of a bigger and higher deck design structure.

Unlike the corner post method where the joists hang from the side of the outer beam to the ledger - in this case the excess post above the timber beam would be trimmed back and the joists will actually rest on the top of the beams and will be attached from the ledger across to a "header joist".

The other main difference and advantage of a cantilevered deck is that the boards can over-hang beyond the footing and beam support structure. There are different rules and building regulations for this in different states and countries, but usually you are allowed to extend out to about one third of the total length of the joists.

Obviously a overhang beyond this will be dangerous in that it will create a weaker structure - however careful planning will mean that when looking at your deck from the garden the footings, posts and deck building beam suport structure will not be visible - and this creates a much more attractive deck.

The Third Option

New Rules Regarding Deck Building Beam

In some states the local building office requires that a different method is used - this is known as the "Post-Beam Cap Installation"

This method requires that the deck building beam actually rests on top of the posts and the two are attached by post beam caps as you can see in this illustration.

post beam caps deck construction


If you are using 4 x 4's and a double deck building beam of TWO 2 x 8's making a total depth of 4, matching the size of the post then a standard "single piece cap" can be used. However if the post and the total beam are not the same size you should be able to find the "two-piece cap" version of the same decking hardware.

A Post beam Cap is a handy piece of decking hardware rather like the post anchor. The post beam cap is screwed down onto the top of the post and goes up to screw into the side of the beam.

Related Deck Design Information

We have more information articles on deck design and deck design available to help you with:

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