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BOB HEIDENREICH: Deck Math III

Part 2: The importance of calculating joist hanger loads.


BY: BOB HEIDENREICH

Frigid temperatures and fallen leaves remind us all too well that winter is upon us. And while deck building may be in hibernation, the slowdown means opportunity— opportunity to focus on next year. What can my business do that it’s not already doing? Where can it improve? How can it become more efficient? These are just a few questions that will help you grow your sales for next year. In the meantime, focusing on techniques to improve the quality of your business can go a long way.

If you recall, last month (link) I discussed how to calculate footing loads and the weight of a deck with a few formulas, making estimating easier. Now, I would like to discuss how you can estimate joist hanger loads when constructing a deck.

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As you may know, joist hangers can only carry a certain amount of weight and, just like footing loads, that weight can be calculated in a similar fashion.

Joists hangers connect to the ledger board, but before you select any hanger, you want to know how much weight your deck is going to carry and how much load each hanger should hold. First, if you refer to the manufacturer’s guide, you’ll be able to determine the structural properties of joist hangers, and how much weight each can hold.

Now, let’s say we’re constructing a deck that is 16 feet deep, but has a cantilevered beam, leaving a span of 14 feet. Joist span is the distance from the bearing point of the beam to the house, and if the span is 14 feet, that means half the weight goes to the beam and the other half goes towards the house.

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If there’s one thing you need to know about joist hangers, it’s that spacing matters. If each joist is spaced 12 inches on center, then you need to calculate the weight each end of the joist is going to carry. The 12 inches of separation means that each joist is carrying seven square feet of load—half the span and then half the distance out, which happens to be seven feet. Because you want a deck that can hold at least 55 pounds-per square foot, you multiply seven by 55, showing you that you’ll need a joist hanger that can support 385 pounds.

Upon inspecting various joist hangers, you’ll notice information stamped on the side of the hanger and on the bottom; this is crucial. When a hanger is properly installed using the correct nails, the load amount shown in the manufacturer’s manual is the approximate amount of weight each hanger can hold.

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