What a great question.
In fact, we receive regularly emails from our visitors asking how to strong they should build their decks. 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.
People Get Intimidated By The Thought of Math and Engineering
Many people are intimidated with trying to figure out the load capacity for a deck. Even some 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.
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.
But Some Basic Math Is All That’s Needed
The good news is that the concepts and the math used for determining loads on decks and other structures are really quite simple. In this article I?ll explain how you do it and you can go build that deck confident that it will be well built and still standing years from now.
Where Do You Start?
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. The live load is created by all the extras like furniture, planters, and people. This is usually about 40 psf. Together 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 this could increase the required load capacity of your deck up to 100 psf.
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. 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. The larger the footing the more the force is spread out and the less chance of your deck sinking.
Our Example – A 10′ x 10’Deck
Since there is no diagram I’ll keep this as simple as possible to illustrate the concepts of load transfer mentioned above. The deck I will refer to will be 10′ x 10′.
There will be a ledger board attached to the house. The joists will run perpendicularly out from the house. There will be a beam running parallel to and 8′ away from the house. There will be 3 posts supporting the beam. Two posts will be located 1.5′ from either end of the post and the other post will be located in the center of the beam at the 5′ mark.
If you have a sheet of paper and a pencil you should be able to draw this out quite easily.
Outer Supported Load Areas
First, determine the supported load areas of the deck. A supported load area extends from the midpoint between any two support members such as the ledger board and the beams in this case, to the next midpoint between support members, or until the end of the deck.
This should make sense if you think about it.
For example the unsupported section from the ledger board to the beam is a distance of 8′. Therefore the first midpoint is 4′ from the house and marks the separation between supported load areas as you move outwards from the house.
This means that the force exerted over the deck between the beam and the house is supported 50% by the ledger board and house and 50% by the beam. I’ll jump ahead a bit just to tell you that there are 6 supported load areas for this deck.
Second, following this logic the next supported load area extends from the 4′ point outward to the beam and beyond to the end of the deck. Since there is no support member past the beam the length of this load area is 6′ (from the 4′ mark to the 10′ mark).
Third, the width of these two supported load areas extends to the midpoint between the end post and the center post.
The end post is 1.5′ from the end of the beam. The distance between the end post and the center post is 3.5′. Therefore the midpoint between the two posts is 1.75′. That means the total width of the first supported load area extends from the end of the beam to the 3.25′ mark along the beam (1.5′ + 1.75′).
The dimensions of load area 1 are 4’x3.25′ and equals 13 square feet. The dimensions of load area 2 are 6’x3.25′ and equals 19.5 square feet. And since the deck is symmetrical, the load areas 5 and 6 on the opposite side of the deck are identical. This leaves only the load areas 3 and 4 in the center area of the deck to be determined.
Inner Supported Load Areas
The supported load areas in the center of the deck (areas 3 and 4) are already set out for us. The length of load area 4 extends from the ledger board outwards 4′.
Its width starts at the midpoint between our end post and the center post as we just calculated above. Its endpoint is the midpoint on the other side of the center post between the final end post.
So if the beam is 10′ long and our starting point is at the 3.25′ mark and extends another 1.75′ past the center post that translates into 1.75′ to the center post plus another 1.75′ past the center post ? a total width of 3.5′.
Therefore, the load area 3 is 4’x3.5′ and equals 14 square feet. Load area 4 is 6’x3.5′ and equals 21 square feet. Now all we have to do to figure out how much weight each supported load area will be subject to is to multiply the square foot numbers by our design load weight of 50 psf.
Load Area Calculation Load/Post (psf)
1 13×50 650 2 19.5×50 975 3 14×50 700 4 21×50 1050 5 13×50 650 6 19.5×50 975
Loads Per Post
The results in the table identify the loads that will be exerted on our three posts. The two end posts attributed to load areas 2 and 6 will receive 975 lbs. The center post attributed to load area 4 will receive 1050 lbs.
Load For Each Footing
Now that we know the loads that we expect to be exerted on each post we can design the size of our footing combined with any knowledge we may have about the soil type. In this case our center post has to be able to handle at least 1050 lbs.
I would design the footings for the other posts to also handle this load – engineer up to the highest common denominator.
Bearing Capacity of Soil
This is the last area of concern. The type of soil determines how heavy the load can be before the footing is susceptible to settling. Organic soils are the worst. If you have organic soils with rotting material it must be removed and replaced with granular stone and compacted before a footing can be installed.
The other types of soils most commonly encountered are clays which have varying degrees of moisture.
The concept is that the more moisture retained in the soil, the lower its bearing capacity. The typical range of bearing capacity for clays, starting with the softest with higher moisture content to the hardest with lowest moisture content is between 2000 psf and 8000 psf or more, respectively.
In our example the maximum load any of the footings will encounter is just over 1000 psf. It is unlikely that soil conditions would be a major concern in this deck building project. If you do find the soil is questionable, the best solution is to get a soils engineer to run a quick test to determine the best course of action.
You should now be ready to go off and start building your brand new deck confident that it will handle the load you are going to throw at it.
What Do You Do With This Information?
So now you at least know how much force the components of your deck will subjected to based upon your design load that you selected. Great! But to put this valuable deck building info to use you now have to know how these numbers translate into determining the actual size of posts, joists, beams, footings and even the species of lumber to use when it comes time to start building your deck.
For that you will need some engineering books or construction calculation software which are available. But for now at least you understand the importance of load capacity.
For other interesting articles, information and ideas on designing decks visit Ideas-For-Deck-Designs.com
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