Why the Substructure is the Most Critical Part of Any Deck

When people think of a deck, they often picture the exterior design and visible elements — railings, floor boards, and aesthetics of natural wood. But, the most important part of any deck is the part that isn’t seen: its substructure. A deck’s substructure provides the framework that holds up the deck. It includes everything beneath the floorboards and railings that keeps a deck strong and safe. To help you understand just how critical the substructure is, some common questions are addressed below.

A wooden deck on a sunny day

1) What are the components of a deck substructure? 

A deck’s substructure is made up of footingspostsledger boards and joists.

Footings provide the foundation for the deck. To build the footing, you start by pouring concrete deep enough into the ground that it goes below the frost line (the maximum depth of ground below which soil does not freeze in winter). Posts are then anchored into the concrete to provide a solid, strong structure on which you can build your deck.

Ledger boards are sturdy pieces of lumber that are used to attach the deck to the frame of the house. These boards must be attached to a solid structure. Some structures to avoid are brick veneers, overhangs, chimneys or bay windows.

Joists are pieces of wood that are used to support the framework of the deck as well. A general rule of thumb: the larger the deck, the larger the joists.

2) What makes for a sound substructure? 

There are a few critical items to keep in mind when building a deck substructure. Firstly, always make sure you are following your local building codes.  That’ll ensure you do things like make sure the footings are deep enough into the ground to provide sufficient support and that the posts on top of the footings are spaced out correctly. Typically, posts are about 6 feet apart, but it’s also worth checking with the American Wood Council (AWC) DCA6 guidelines, because everyone’s deck is different.

Depending on where you live, you can take extra precautions against water damage by applying an ice and water shield product that will protect the timbers from moisture, water or ice damage. Flashing, the metal guard that directs water away from sensitive areas, will help protect the deck from water damage as well.

3) What type of wood do I need to build a substructure? 

Pressure treated lumber is best for the substructure. Make sure to use the correct category  of treated lumber — it’s got to be rated for ground contact. Learn about the proper use categories here.

According to the North American Retail Hardware Association (NRHA), pressure-treated lumber is the go-to choice for your deck substructure, and what contractors overwhelmingly use. This is due to its load-bearing strength and affordability.

A few final tips

Keep these things in mind as you build:

  • When it comes to joists, the bigger, the better. So buy a size up if it is in your budget. If a span table or calculator  shows you need 2×8 joists, perhaps buy 2×10.
  • Install ledger boards with structural screws rather than nailing them to the house. This is a common mistake among DIY builders. Nails will not offer the support your deck needs, so you must invest in screws with high torque and strength.
  • Get a building permit. In doing so, you’ll be forced to go through the same process as a professional builder and will therefore have to get all the details right before building your deck’s framework.
  • When in doubt, ask a professional. While you may be a DIY guru, do not hesitate to ask a professional contractor’s advice, as it will help in the end.

Since it’s difficult to rebuild the substructure once it is constructed, you want to  build it correctly the first time around. While this may sound like a daunting task, so long as you take into account the AWC DCA6 guidelines, your local building codes and ask a pro when necessary, you’ll have all the right tools in your toolbox so to speak!|

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