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.

The
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.

The
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.

**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 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.

To determine the maximum load capacity of your deck, start by calculating its total area and multiply by 50
psf. So a 100 sqft deck would be designed to support 5000 lbs.

Don't get confused with what weight you might think or want to load the deck with.

If
you drove a dump truck over it, yes this would throw all our
calculations out the window. But we are building a deck to support
known loads consistent with the purpose for which the structure is known
to be used for and the 50 psf number has a safety factor in it. That
is why engineers have settled on this as a safe value.

So with a
total deck load capacity of 5000 lbs we now move to the "slightly" more complicated
discussion about tributary areas and how this overall load is now
distributed around the entire deck and onto the soil below. Stick
around. This is the fun part.

Our
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
beam.

**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’
mark).

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**.

The ledger
is 10' long so every foot of ledger must be designed to carry **at least
200 lbs** of load.

Now
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
soil type.

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.

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.

Lets assume this soil is typical and can easily bear 1800 psf. We'll work with that.

If
each footing was 1 sqft then all of the weight of the given area would
be imposed over that 1 sqft area. Therefore, if the footing was twice
as large or 2 sqft only 525 lbs (half of 1050) would be imposed on each
sqft below Area C.

We can effectively reduce the force imposed
over a given area by using a larger footing size and distributing that
weight over the larger area.

We could build a square footing that
is 12"x12" or one sqft and we would be fine because all the tributary
ares carry total weights much less than the soil's bearing capacity of
1800 psf.

But let's use a round footing because we bought some
concrete tubular forms. If we use a 16" diameter form we can calculate
its surface area by using the formula for **area of a circle** as **π r²**.
This works out to approximately 1.4 sqft.

To
determine what weight per square foot is actually imposed on the soil
below each tributary area, we just divide its weight by the area of the
footing. Let's see what values we get for each footing.**F1**= 975 lbs/1.4 sqft= **696 lbs****F2**= 1050 lbs/1.4 sqft=**750 lbs****F3**= 975 lbs/1.4 sqft= **696 lbs**

In
every case the loads imposed are less than half and almost a third of
what the soil really could bear. So your deck will not sink into the
ground.

Go ahead and build the frame as strong as you wish. But
keep in mind the two concepts of the strength of the structure and the
bearing capacity of the soil. This is a really simple deck to make
these calculations with and is very helpful at illustrating the concept.

It
can get trickier as you change the complexity of the shape. But if you
know and understand these fundamental principles, deck load capacity
questions will be easy for you.

**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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