How Do I Know How Much Weight My Deck Can Hold?

One of the prominent facilities at your backyard is the deck. Apart from the beautiful garden, awesome canopies that provide shade, you equally need a deck.

So, what makes up the deck?

Frame is the first and most important part of a deck that shows you how much weight your deck can carry.

So, how do you determine how much weight a deck can hold?

You can determine how much weight a deck can hold by calculating the total area in square footage, which is 100sqft x 50psf. Therefore, at this square footage value, your deck can hold an average of 4,000lbs – 5,000lbs load of weight.

For you to have a strong deck, you must ensure your frame is balanced. Otherwise, the legs and the stairs will be difficult to hold the deck.

To construct a strong frame for your deck in backyard, study the diagram below where the various parts of deck construction is listed and labeled.

•  a. concrete anchor bolts
•  b. ledger board
•  c. joist hangers
•  d. joist (typical)
•  e. 4×4 rail post
•  f. 6×6 structural post
•  g. stair stringer
•  h. hand rail cap
•  i. stair risers
•  j. 2×6 treads
•  k. concrete footing
•  l. footing form
•  m. outer joist (2×8)
•  n. rim beam (double 2×8)
•  o. galvanized post base
•  p. 2×2 rail pickets
•  q. 2×4 lower rail framing
•  r. 2×4 upper rail framing
•  s. galvanized stair stringer hanger
•  t. 2×6 decking
•  u. 2×6 stair rail cap
•  v. 2×8 stair stringer framing
•  w. carriage bolts
•  x. lag screws

Also for you to know the highest weight you deck can hold is by simply analyzing the load structure, the support post beam,and the soil capacity. 

Do a simple engineer calculation to know the weight of the load the deck will carry. 

These will give you an idea of the highest capacity of load your deck can carry for over years. 

Let us quickly have a study on the different types of load associated with decks. There are three major types of loads which are: Design Load, Live Load, and Dead Load.

Design Load

The design load is the total weight of load that your deck carries. For example, a design load is the sum total of dead load and live load.

Remember the load that is placed on the deck is measured by pounds per square foot (psf).

During the construction of a deck, the engineers take into account the dead load and the live load. This gives them an overall idea of how the  design load will be and builds the deck based on this knowledge. 

After this calculation, let’s  take a step further to study the structural frame and the soil capacity. But  before that let’s study the remaining two types of load.

Dead Load

This is the load exerted by the weight of the deck itself. often this load is about 10psf (per square foot).

Live Load

This is the load created by all external factors placed on the deck, (e.g., Furniture, planters, people) etc. This is usually about 40psf (per square foot).

with this knowledge of dead load and the live load you can then calculate the overall load your deck is going to carry which is called design load.

If dead load is 10psf and the live load is 40psf then design load is 10psf + 40psf = 50psf, this means that the total load is 50psf.

The table below shows the different types of load and the force exerted on a deck.

Name	Load Exerted	Load Math
Dead Load	Exerted by the weight of the deck itself	10psf
Live Load	created by all external factors placed on the deck	40psf
Design Load	Dead Load plus Live Load	50psf

The Thought of Math And Engineer That Get People Intimidated

Imagine after spending a lot of money on building a deck and the deck begins to sink a few years down-the-line. How will you feel you are going to feel bad right? To avoid this sink, learn the simple way to know how much weight your deck carries now.

Just because you are intimidated by the simple math and the engineering calculation. You couldn’t do a proper analysis of your deck before building. That is a waste of money.

All that you need is some basic math and simple engineering calculation to guide you on how to build a strong deck. 

The engineering calculations are very simple to understand. Just follow my explanation here you will know it. 

Likewise, to know the footing size before you begin your deck work. The Engineering calculation is explained below.

The engineering calculation is to multiply total area by per square foot. 

If the total area in square footage is 100sqft (square footage) x 50psf (per square foot), that is 100sqft x 50psf = 5,000lbs. This means your deck can support 5,000lbs load.

Now that you understand the different types of loads, the math, how engineering calculations work, and the footage size is time you learn how frame structure works.

Let’s begin by showing you how the framed structure works both at the top and the down.

Let us begin from the  frame top. The top of the frame is made up of rim joists — perimeter joists or band joists, while the middle structure is made up of infield joists or inner joists. 

After the top joists, and the middle joists we have the carriers beams. They are called the carrier beam because they carry the entire structure. Let us take a look at the snow load.

Snow Load

Snow load is the weight snow adds to your deck during winter. This means when building your deck you have to factor in the snow load. 

This increases the load weight upon deck during winter. To factor this in, a new calculation must be done. How to do it’s shown below.

If your initial purpose of load is over 75,000lb or 80,000lb hot tub on the deck, then you have to consider increasing the required load capacity of your deck to 100 psf. 

This increase will have an effect on the entire structure.

The structural strength of the deck needs to be increased by using 2×10 joists at 12″ On-Center spacing (O.C Spacing). With this increase your frame structure will carry the load.

The next thing to be considered is the beam spacing, support post size and how many footings required and how much weight will they exert on the soil.

Now let me take you through the entire deck on the wall.

The distance of the beam from the unsupported section of the ledger board is 8′. This means, the first midpoint is 4 from the house and take note of the separation between supported load areas as you move outwards from the house.

50 percent support force exerted over the deck between the beam and the house by the ledger board

Outward Supported Load Areas

Know the support load area of the deck. There is an extension of loan from the midpoint between any two support members such as the ledger board and the beams to the end of the deck.

Read on this it will make sense if you think about it.

For example, the distance from  the beam to the unsupported section of the ledger board is This means the first midpoint is 4’ from the house and marks the separation between supported load areas as you move outwards from the house.

This goes to tell us 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. 

Keep that in mind, let me jump ahead in a bit to show you that there are 6 supported load areas for this deck.

Just keep following the example, the next supported load area extends from the 4′ point outward to the beam and beyond to the end of the deck. 

Since it has no support member past the beam, 6’ is the length of this load area (from the 4′ mark to the 10′ mark).

Between the end post and the center post the width of these two supported load areas is extended.  

 From the end of the beam to end post is 1.5’. The distance between the center post and end post is 3.5′. This means the midpoint between the two posts is 1.75′. So 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 load area dimensions 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. 

Morealso, 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.

Inward Supported Load Areas

Let me refresh your mind that 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′.

The midpoint is between the end post to center post as we just calculated above.

From the end of the beam to the end post is 1.5’ the distance between the end post and the centre post is 3.5’. That is 1.75’ is the midpoint between the two posts.

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 Post (psf)

The below shows the load area calculation post (psf)

Area 1	Area 2	Area 3	Area 4	Area 5	Area 6
13×50 650	19.5×50 975	14×50 700	21×50 1050	13×50 650	19.5×50 975

The results in the table above show to us the loads that will be exerted on three posts. The two end posts contribute to load areas 2 and 6 will receive 975 lbs. The center post contribute to load area 4 will receive 1050 lbs

Load For Each Footing

Since we know the loads that we expect to be exerted on each post we can now design the size of our footing combined with any knowledge we may have about the soil type. In this instance 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 mainly the last point of concern. The type of soil determines how heavy the load can be before the footing is susceptible to settling. 

Note that organic soils are the worst. Check the soil before you commence work. If for example you have organic soils with rotting material you have to remove it and replace it 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 reason behind this 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.

For instant, the maximum load any of the footings can withstand is just over 1000 psf. It’s obvious that soil conditions would be a major concern in this deck building project. 

If you have any questionable soil, for your best interest contact soils engineers to run a quick test to know the best course of action.

You should by 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.