How Much Weight Can a Deck Hold?
A new deck built in compliance with the IRC building code can handle, at minimum, a load of 50 pounds per square foot (psf). As time passes, however, the load capacity of your outdoor space can diminish. In this article, learn what factors influence deck load capacity and how to maintain and even strengthen it.
Factors Influencing Deck Weight Capacity
The weight-bearing capacity of a wood deck frame depends on many factors.
1. Wood grade
Wood is graded based on its appearance and defects. “Select structural” lumber is the best, followed by No. 1 & better (BTR), No. 2 & BTR, etc. For redwood, look for the term “Construction heart.” In my experience, no money is saved buying low-grade lumber. It takes longer to install and doesn’t give the same result.
2. Deck Design
Your design specifications will affect the spacing and size of the deck footings, beams, and joists. These supporting elements must distribute the deck’s load evenly to ensure structural integrity. To protect the frame structure, aim to create a waterproof deck. Our company offers several products that make the job easier both for DIYers and professionals.
3. Post, Beam, and Joist Spacing
The closer the spacing between the posts, beams, and joists, the more weight the deck can bear. Span tables referenced in deck building codes and deck substructure building guides enable deck builders to specify the joist spacing for the different sizes of treated lumber to obtain the desired load rating.
4. Load Distribution
Distributing the deck load evenly promotes structural integrity. This is especially important today because outdoor living spaces have more square footage than before and can include heavy items like hot tubs or kitchens. Such acute load points will likely require beefier timber and/or footings under them.
5. Fasteners and Hardware
Don’t skimp on the screws, bolts, joist hangers, and post anchors that hold the deck together. Use only corrosion-resistant fasteners and hardware that are rated to handle the calculated loads at the ledger board and elsewhere. Install them per the manufacturer’s instructions to ensure deck safety.
6. Age and Upkeep
Wood decks naturally weaken as they age. To prolong a deck’s service life, the wood needs protection. Otherwise, the effects of age will appear sooner.
Calculating the Load Requirements
I don’t recommend you do these calculations yourself, but here are the basics. There are two elements to the total load, aka design load: dead load and live load. Consult a deck builder or engineer to get these figures right because they are critical to structural integrity. And the expense adds just a fraction to the overall deck installation cost.
This element encompasses the deck itself, including the decking materials, the joists and beams, the posts, and any other accessories, such as railings and built-in planters and benches.
To estimate the live load, add up the weight of the furniture, other accessory items, and the maximum number of people that will be on the deck at a given time.
Combine the dead load and live load to get the total load. For complex or large decks, or if you have any doubts about the accuracy of your calculations, hire a structural engineer to perform them in compliance with the local building code. It could require you to account for the snow load on the deck.
Importance of Beam and Joist Spacing
Beams and joists provide the primary deck support, and spacing them properly is fundamental to the structural integrity, safety, and longevity of the deck. The closer the spacing, the smaller the lumber can be. The greater the span, the larger the lumber must be.
The IRC and local building codes specify the maximum allowable spans for support beams and joists based on wood species, joist size, intended loads, etc. Adhere to these codes to ensure the deck is structurally sound.
Significance of Support Posts and Foundations
Properly placed and spaced, support posts transfer the load from the joists and beams to the foundations (footings) in the ground, ensuring that the weight is distributed safely. Proper spacing prevents excessive deflection of the deck structure.
The type of soil on which the support posts and foundations rest is also an important factor. If you’re building in an area where expansive or soft soils are known, test the soil-bearing capacity. It may require special footings. Check whether the size and depth of the footing holes must be inspected before you add the concrete. It’s not a job you want to do twice.
For DIYers who aren’t structural engineers, here are some clarifications on deck structure.
Does the Deck’s Weight Capacity Change Over Time?
Yes, a deck’s weight capacity changes as it ages, but it’s a matter of degrees, and well-maintained decks preserve their weight capacity better than neglected decks.
The weight capacity declines I see most often stem from moisture or insects that lead to rot and decay. Water can also cause the wood to swell or warp. Wet wood is also weaker than dry wood and more likely to grow mold. In addition, moisture can accelerate corrosion of the fasteners and hardware, possibly compromising structural integrity. The deck might also settle or shift due to erosion or the freeze-thaw cycle.
Can You Strengthen an Existing Deck to Increase Its Weight Capacity?
Yes, by reinforcing key structural elements, you can strengthen your deck to increase the amount of weight it can hold. This could include placing additional support posts and footings beneath the deck to reduce the spans of the beams and joists.
It’s also possible to “sister” new joists and/or beams to the sides of existing ones. This increases load-bearing capacity because there is more material to carry the load. Another approach is to bolt steel plates to the structural elements. If you’re unsure how to strengthen your deck, ask a structural engineer or contractor for help.
How Often Should You Check Your Deck’s Weight Capacity?
I recommend that homeowners check the overall condition and weight capacity of their deck at least once yearly while cleaning it. Look for signs of wear, rot, insect damage, and loose or corroded fasteners.
If you retrofitted the deck to include heavy structures or expanded it, make sure the modified deck can accommodate the changes. If you move into a home with a deck, have a professional examine it. Also, inspect the deck after severe storms or earthquakes. Watch for sagging, cracks, loose connections, or any unusual movements.
Do Different Decking Materials Have Varying Weight Limits?
Yes, the weight limits of decking materials, including various wood species, composite decking, and PVC decking, do vary.
Softwood species like pine generally have lower density and strength compared to hardwoods. That includes pressure-treated wood, which has moderate weight-bearing capacity. Hardwood decking species like ipe, teak, and mahogany are denser and stronger.
Composite decking is made from a combination of wood fibers, plastic, and binding agents. As a result, composite decking deflects more than most wood and requires more structural support. Hollow composite deck boards typically have a lower weight-bearing capacity than solid composite boards. Follow the manufacturer’s installation instructions.
The weight rating of PVC decking is similar to that of composite decking.