Hello Deck Enthusiasts!

John Trava, PE
JSD Structural Engineer 

‘Tis the season for grilling, swimming, and lounging outside with friends and family.  What better place to do these activities than in our backyards and on our decks! Many families look to build and do maintenance before the 4th of July. Though JSD typically focuses on commercial projects, let us help you explore important considerations and safety tips for sturdy and stylish deck structures for your home!

1. A Structure is Only as Good as Its Foundations

The soil below your deck is extremely important and can cause huge problems in the end. Think about it: when walking through snow, we all know that the load from our feet is too much for the surface of the snow to handle and our feet sink into the snow.  To combat this, people attach snowshoes to their feet which spreads the load over a larger area and keeps them from sinking.  Suddenly the snow’s capacity is not overloaded because instead of being concentrated in a small area it is spread out over a large area.  So, picture the posts of your deck as your feet and the soil below as the snow.  We need to add snowshoes to your deck!

The Uniform Dwelling Code of Wisconsin requires a minimum 12-inch diameter concrete pier below each post. The piers should also stretch at least four feet below the top of the soil and will spread out the load over a larger area to protect against something called frost heaving.  Frost heaving is when the water in the soil freezes over the winter and expands, pushing everything on top of it up and down with the temperature.  By adding concrete piers below your posts, you are ensuring that the deck has a solid foundation and that it will not move around with the temperature.

2. Secure and Protect Your Ledgers

To understand the issues that might arise, we need to understand what a ledger is.  On a typical deck, one side is attached to the house and the other is supported by posts.  The side attached to the house consists of a board that is screwed or bolted into the outside wall, called a “ledger,” and a system of hangers and joists that make up the deck itself.

The failures of decks are frequently caused when the ledger fails and is ripped off the house.  Ledger failure can be prevented by using the proper fasteners and adding flashing over the top of the ledger.  Wood screws should never be used to attach a ledger to a house.  The Uniform Dwelling Code requires a minimum of 1/2-inch diameter lag screws or through bolts at various spacing depending on the span of your deck.  Local building inspectors are happy to help, and tables are available online in SPS 321 of the Wisconsin Administrative Code for those that like some light reading. 

If the fasteners are properly sized and installed, the next failure occurs when the ledger or wall itself rots from water infiltration.  A piece of metal flashing runs inside the wall of the house and over the ledger preventing any water getting trapped behind the ledger and rotting out the wood.  With the ledger being the weak link on most decks, proper installation will add years to the life of your deck!

3. Don’t Forget About the Wind!

When we are designing our deck, we are always very concerned about all the weight that will sit on top of it. Many people overlook what is going to push against it though. Wind is a real thing and if it can move an oak tree, it can move your deck. Without lateral bracing, decks tend to sway back and forth when loads are shifted or wind blows.  The solution to this problem is actually an easy fix. Any tall deck should be installed with knee-braces at the corners.  Knee-braces are diagonal members that lock in the corners of a deck to keep the frame rigid and stiff.  Knee-bracing can be installed using either toe-nailing or brackets, but if toe-nailed, use lag screws as much as possible.

We hope this short “JSD is Looking Out for You” piece is informative and inspiring as you embark on your deck projects.  Stay tuned for more insights and updates on structural engineering in future editions as our team stands ready for your commercial needs!