What to look for when calculating deck load.
As homeowners enter the summer months, many are looking into DIY projects that will allow them to upgrade their outdoor living space. As they do, a common question deck professionals receive is, “how much can my deck hold?”
The right answer is, of course, “that depends.” And it depends on quite a lot of variables. The answer you give a customer will never be as simple as just telling the customer that their deck can hold 2,000 pounds. That’s just not how it’s calculated.
The primary reasons that a customer may want to learn more about the weight load of their deck has to do with the current trends toward outdoor living. Many customers nowadays are looking at adding a hot tub to their deck, or an outdoor kitchen space complete with appliances.
Before you give them an answer, be aware that deck loads are a factor of the materials that you use, your local building codes, and the deflection of lumber. Even when we know what a deck’s weight load may be when building it, we don’t build decks to the load value of failure. We build decks to what is considered the “allowable amount of deflection.” That is a minimum standard that all reputable deck builders adhere to.
Of course, there are regional factors at play as well. Snow load will affect a deck’s weight load in a big way. Where I live and work, in the Minneapolis area, we need to build decks to support 50 lbs. per square foot. To support 50 lbs., first consider that 40 lbs. is what we consider “dead load.” Then the remaining 10 lbs. is considered “live load.” That’s the people and the “stuff” on the deck. Well, since none of us weigh 10 lbs., you have to consider that this load is the actual weight stretched out over the entire surface of the deck.
You also need to take into consideration the length of the deck’s joists. Also look up in your building code book, or ask your local inspectors for a chart that shows the species of wood that you want to use. For instance, if you’re going to build a deck with cedar joists, that load bearing capacity is far less than if you build it with Ponderosa pine, and that’s even less than if you build it with Southern yellow pine. At The Deck Store, we sell Southern yellow pine, grade No. 1, that is kiln dried.
How close together you place the joists also plays a role. Putting the joists 12″ apart, versus 16″ apart, changes the deflection value of those joists. So, you have to put this together as an entire package and find out why the customers are asking the question.
In short there’s no one right answer to “how much can my deck hold?” However, one rule of thumb to keep in mind while answering the question is that the answer commonly is that if a deck is properly designed, it’s built to hold 50 lbs. per square foot. It’s possible that it could hold much more weight than 50 lbs. per square foot. If you’re allowed to span 16′ with your joists 16″ on center, but you only span 8′ with your joists 12″ on center, then that deck can hold substantially more weight.
And, pardon the pun, but boards are only the surface of the issue. You also have to factor in joist hangers, fasteners, footings, and soil. You can’t build a deck that is going to have so much weight on it that the fasteners and hangers can’t support that load, or the beam that you will put in place or even the footings underneath it. You can make your joists able to hold thousands of pounds of weight and your footings could sink into the ground.
Most cities in their building codes are now using what is called a “wet service load.” Since, obviously, deck materials are generally outside, you have to calculate the load values based on the wet use of the treated lumber that is used.
As you can see, the answer to the question is far more than just how much weight can a deck hold. It’s how much weight can the soil hold, the footings, fasteners, joist hangars, etc., and how much can it hold while it’s wet.
The best thing you can do is to consult your local building codes and utilize resources available to you such as NADRA. As a NADRA member, I have found value in the access to peers and industry professionals.
Also be sure to check each product’s instructions to learn the recommended weight capacity for hangers, fasteners, etc. International Building Code section 507.3.1 says that composite decking must be installed per manufacturer’s published instructions. Even though you meet codes for joists and hangers, you still have to meet the manufacturer’s composite decking standards.
By knowing this, you can differentiate yourself from box stores or other competitors, and you can convert this knowledge into sales because you’ll establish yourself as the dealer in the market that consistently supplies the right materials and the right answers.