What a great question. 

The question around deck load capacity is one I receive often from visitors asking how much weight should they 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 is 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. 

The Entire Deck, Footings And Soil Are A System

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 are not 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 do not consider 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 is what we will cover in the second half of this article.

But Some Basic Math Is All That Is Needed

The good news is the concepts and the math used for determining loads on decks and other structures are quite simple.

I will explain how to do it and then you can go build that deck with confidence knowing it will be strong and stable and standing years from now.

Let's Start At The Top And Work Down

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 will hear a number of terms.

This framework is supported by a number of beams - sometimes called carrier beams because they "carry" the load of the structure. 

Design Load

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 all locations experience seasonal changes like the one depicted in the series of photos.  But given the amount of snow piled up on this deck you can see why it is important to consider all the forces that will be at work on your deck and then calculate and build accordingly.

Framing For The Design Load

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.

What About Snow Load Or a Spa Tub

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.

Our deck is as simple as it gets 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. 

Defining Tributary Areas Of The Deck

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.