Ask the Builder: Joist hangers, beams and columns
Q: Tim, I’ve never worked with metal joist hangers. How long have joist hangers been around? When I hold one in my hand, I question whether it’s actually strong enough to support all the weight that will be resting in it! Are they safe? Are there any best practices when it comes to using them both indoors and outdoors on decks? What about the beams that joist hangers are attached to? How do you size those correctly? What about the support posts? —Lisa K., Hartford, Conn.
A: If you're about to dip your toe in the magic waters of rough framing, I can tell you that it’s immensely satisfying to transform a pile of lumber into a structure. My customers for years would marvel at how we would put all the different pieces of lumber together to create a deck, room addition, or a house. Joist hangers are safe if installed correctly.
Hanging joists is by no means new. You typically need to hang a joist if the top of the joist needs to be in the same plane as the beam that supports it. There are all sorts of reasons and conditions on jobs where this is necessary.
I happen to have in my garage an example of how carpenters did it 150 years ago. Back then there were no fancy metal hangers. The carpenters of old hung joists from beams by cutting a tenon on the end of a joist and then they created a matching square or rectangular mortise in the beam that would support the joist. They used joists and timbers that were much thicker than the dimensional lumber you buy today, so the method was structurally sound.
It’s important to realize you’re dealing with people’s lives, and death and serious injury can result when you install joist hangers, beams and support columns the wrong way. Don’t hope things are being done correctly. Get the actual written installation instructions from the joist hanger manufacturer. My go-to source of information for all things dealing with joist hangers and other metal structural connectors is Simpson Strong-Tie.
The best things to use to attach the joist hangers to the beam are structural screws or bolts. Never use roofing nails. I’ve seen people do this. You can buy structural nails that are rated for the weight, but trust me, the structural screws or bolts are far better. Don’t overdrive the fastener, stripping out the wood if you use an impact driver tool.
It’s extremely important that you use the correct joist hangers, or other metal connectors, and fasteners that are rated for outdoor use. Modern treated lumber has a higher copper content, and when this copper gets wet and then comes into contact with the galvanized metal connectors and fasteners, it can corrode the metal. Once again, don’t hope you have the right things. Make sure everything you use is rated for extreme outdoor conditions, especially if you are working near the ocean, where salt spray adds to the corrosive brew.
Beam sizing is not to be taken lightly. All too often I hear of homeowners who just ask some builder what beam to use. Beam sizing is complex because there can be concentrated loads on beams. What’s more, the type of material used for the beam makes a huge difference. Beams should only be sized and specified by registered structural engineers.
The posts and columns that support beams are just as important. Here in central New Hampshire I see carpenters just balance beams on top of a square cut post. They install a few nails and hope the beam stays on top of the post. I’d never walk out onto a deck constructed this way. There are all sorts of ways and metal connectors made to properly connect a beam to a column. Once again, an engineer can specify exactly what to do and what to use.
The material used for the post or column is important. I recently walked through a new home being built near my home and saw a massive beam in a garage supported by regular lumber. Termites are not a big issue here, but I’d be more inclined to install treated lumber to support a beam in a garage where water and insects seem to have a better chance of coming in contact with lumber.
Keep in mind that hollow steel round or square columns used to support beams can fail in a fire. Steel can’t easily burn, but once it gets hot it can bend and twist with ease. Most engineers specify that hollow steel columns or posts be filled with dry sand to prevent failure in a residential fire. It takes a little bit of work to do this, but it’s worth it.
I’ve got an interesting video on my website showing you just one trick I’ve used to install joist hangers. The tips are pretty helpful when it comes to solid lumber that might be cupped and for joists that are not always the exact same height. Use this URL and be sure to type in the word “GO”: GO.askthebuilder.com/joisthangers.
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