What a great question.
The issues around deck load capacity is one I receive often from visitors asking how much weight they should design their deck for.
discussion quickly leads to the overall strength of the framed
structure. But that is only one part of it.
That's because any analysis of what load a structure may bear upon itself and its foundation must involve the support post network and soil bearing capacity.
It truly is a system - not unlike a chain - where the weakest link will lead to the failure of the deck.
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.
Yes, 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 with 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.
That's what we will discuss in the second half of this article.
The good news is that the concepts and the math used for determining loads on decks and other structures are really quite simple.
I’ll explain how you do it and you can go build that deck confident that it will be strong and stable and still standing years from now.
first area to think of is the actual framed deck. This structure is
comprised of perimeter joists - sometimes called rim joists or band
joists. Then there are the joists in the middle. These are sometimes
called infield joists or inner joists. You'll hear a number of terms.
And this framework is supported by a number of beams - sometimes called carrier beams because they "carry" the load of the structure.
The IRC and other similar codes in other countries, like Canada or the UK all work from a similar starting point for what a floor deck should be engineered to support.
These standards are borrowed
by deck builders and come from the actual code requirements used for
the floor deck of a residential home.
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.
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 lb hot tub on the deck this could increase the required load capacity of your deck up to 100 psf.
Not every place experiences seasonal changes like this dramatic series of photos shows. But you can see why you should consider all the forces that will be at work on your deck and build accordingly.
To avoid referring to complicated engineering tables and for the purpose of building a deck, let us start with the idea that using standard 2x8 softwood lumber at 16" o.c. joist spacing your deck will easily meet the 50 psf threshold.
Yes this is a concern but the
net effect or changes you might have to do to increase the strength of
the deck could be as simple as using 2x10 joists at 12" o.c. spacing.
The framed structure will typically handle the added weight quite easily.
The worry will be your beam spacing, support post size and most importantly how many footings and how much weight will they impose directly on the soil below.
This is critical because if you overload the soil more than it can bear, the deck will start to sink. A very bad thing.
deck is as simple as it gets in order to illustrate the concepts deck
load capacity and transfer of weight within each tributary area. This
deck is 10`x10` or 100 sqft.
There is a ledger board attached to the house. The joists run perpendicularly out from the house for 10 feet at 16 inches on center. The carrier beam runs perpendicular to the joists with its center at 8 feet from the house and the cantilever beyond its center point is 2 feet.
There are three support posts 3.5 feet from center to center. The beam cantilevers 1-6` past the outer posts to the perimeter of the outer most joist.
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. Area A equals 4x10 or 40 sqft.
There are four tributary areas on this deck: A, B, C and D.
Tributary Area A is confined between the midpoints of its two adjacent support members, the ledger board and the beam. The outside perimeter joists confine the width of the area.
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.
Area A equals 4x10 or 40 sqft.
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
Area B 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’
The width of Area B 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 Area B are 6x3.25 or 19.5 sqft.
Incidentally, tributary Area D is identical to B.
Tributary area C is slightly larger than B and D. It is 6`long but its width extends from the midpoint of footing F1 to F2 and F2 to F3. This distance is 3.5`. Area C is 6x3.5 or 21 sqft.
Load Calculation For Each Tributary Area
A= 40sqft x 50psf or 2000 lbs
B= 19.5sqft x 50 psf or 975 lbs
C= 21 sqft x 50 psf or 1050 lbs
D= 19.5sqft x 50 psf or 975 lbs
Notice that the middle tributary zone must carry more weight than the adjacent areas B and D. This is a common characteristic you will find in most decks and so sometime, if your bearing capacity of the soil is quite low, you may have to increase the size of the middle footings or add another support post in order to not overload the soil.
Area D is identical to Area B. Given that the shape and support configuration of the deck is symmetrical. The dimension are 6x3.25 or 19.5 sqft.
Lastly, Area A is supported by the ledger board across its entire length. We express this load value as lbs per lineal foot.
is 10' long so every foot of ledger must be designed to carry at least
200 lbs of load.
that we know the loads that we expect to be exerted on each post below
each tributary area and thereby onto the soil below, we can design the
size of our footings combined with any knowledge we may have about the
For example, I would design the footings for the other posts to also handle this 1050 lbs load - engineer up to the highest common denominator.
Richard Bergman is the editor of DecksGo.com and a builder of custom homes and too many decks and fences to mention. He is also an active product developer and patent holder. Richard holds a B.Comm and LLB degree and particularly enjoyed patent law.
Beyond theory, he loves taking ideas and turning them into physical realities and that is why he builds. He is always working on something interesting and loves to share his knowledge with those who may need some help.
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