Wood deck design and deck failure prevention

So if you are constructing a deck and you don’t want this to happen to you, you might be asking yourself “how strong should I build my deck?”  Of course, how strong you decide to build a deck depends on the anticipated loads you expect to have on your deck. And there can be quite a range, especially if you are anticipating a hot tub or large gatherings of people (like the alleged 200 people in the news story links above)…

Many people are intimidated with trying to figure out the load capacity for a deck and this is why it is always best to enlist a ParsCo engineering professional to assist you in doing the calculations. 

Many contractors aren’t sure where to begin so they just over build – which may be entirely unnecessary and cost you more money. Another problem that can arise from over building is a sinking deck. ParsCo professionals are thorough and if required will conduct a soils report to assist in determining the loading as it relates to the soils. 

But even if you build a strong deck it can gradually sink into the soil if you don’t take into account the size of footings in respect of the load for the deck. Once your deck starts sinking it can rip the ledger board away from the house or you will have to jack up the sunken area, excavate and pour a new larger footing. All of these situations will cost you even more money!

These issues cost money and when you consider the potential failure and replacement costs, the extra cost on the front end to do things right with ParsCo far outweighs the alternative. 

So for those do-it-yourself type of people that might be interested in building a simple deck for you and your family (not 200 people) you should always consider and utilize the following logic when designing and constructing a deck. 

The load that is placed on your deck is expressed in pounds per square foot (psf) and the total load or more appropriately, the design load, is comprised of the dead load and the live load.

Dead load is basically the load created by the weight of the deck itself. This is usually about 10 psf depending on the material you use. The live load is created by all the extras like furniture, planters, and people. This is usually about 40 psf (special consideration should be taken though). So using these figures as an example, the design load would be 50 psf.

Of course, if you expect a lot of snow to sit on your deck over the winter or envision an 8,000 lbs hot tub on the deck or 200 Spring Breakers even, this could increase the required load capacity of your deck up to 100 to 200 psf or more!

In order to be able to determine the amount of force that is exerted from the deck surface to the footings, it helps to conceptualize the path that the forces travel from their source to the ground.

The deck itself and anything on top of it exerts downward force which is transferred to all the beams and any ledger boards that are connected to the house. This is an axial load.  The force on the beams is then transferred down to the posts. The force on the posts is then distributed into the concrete pier and ultimately spread over the surface area of the footing which is then displaced over the soils. The larger the footing the more the force is spread out and the less chance of your deck sinking. This is all dependent on the soils though. 

If you can follow and understand this information then you may be interested in the AWC publication for deck construction. 

http://www.awc.org/publications/dca/dca6/dca6-09.pdf

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