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Ask the Builder: Can trusses beat solid wood joists?

Ask the Builder: Can trusses beat solid wood joists?

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These are fantastic floor trusses in my daughter’ s home.

Last week I was downloading some photos from my fancy digital camera and I stumbled across hundreds of forgotten photos I shot while my daughter’s new home was being built on Mt. Desert Island in Downeast Maine.

I was there every day for the first few months capturing the framing of the home, as well as installing all the plumbing, radiant floor heat and electric in the home. I did my best to capture each aspect of the construction. It was snowy, cold and windy, but that just reminded me of all the cold days I had worked on jobs during my building career. We didn’t have heated jackets back then and kept warm by constantly moving.

One of my favorite aspects of the construction of the home was the use of floor trusses instead of floor joists. I specified floor trusses for quite a few reasons, not the least of which is that they produce amazing flat floors with no bounce. My daughter wanted tile floors throughout the home because of a dust allergy, and flat floors with no bounce are a must for large-format tile.

You may not even realize you can get floor trusses. I’m certain you’ve seen common roof trusses, so you’re familiar with how smaller dimensional lumber like 2x4s can be cobbled together in such a way as to be as strong as or stronger than common dimensional larger lumber.

Floor trusses borrow the same engineering principles bridge builders have employed for decades. There’s a very good chance you’ve driven across a Pratt truss bridge in your lifetime. This bridge is designed just like a floor truss, or vice versa. It’s got a flat top and bottom chord with lots of triangles in between.

Just as a bridge can span a river resting on two piers, one on each bank, floor trusses can do the same in your new home or room addition. The ends of the floor trusses rest on the parallel exterior walls. There’s no need for interior bearing walls in the center of your home. Imagine the possibilities of an open floor plan when you get silly bearing walls out of the way!

Let’s talk about the humble traditional floor joist for a moment. I’ve installed thousands of them in my lifetime. I used them in the last home I built, a stunning Queen Anne Victorian home that had a maze of bearing walls throughout the house because of the traditional design.

Floor joists are sawn from logs. They often have inconsistent crowns to them. A crown is a hump in the floor joist. Humps and floor tile don’t play well together. You have to be careful with the size and placement of holes you might drill into a joist to run pipes and ducts. Even if you use the strongest and highest-grade lumber floor joist, you can’t come close to the distance you can span using a floor truss.

Trusses do have at least one drawback: how they act in a house fire. Firefighters prefer floor joists because it takes a while for a fire to burn the lumber to the point when it’s unsafe for them to be inside the building trying to knock down a fire. Floor trusses fail faster. If a fire is raging to this degree in a home, there’s a good chance burning plastics in the house that create poisonous toxic smoke have already caused fatalities.

Building departments should work with fire departments so that while the truck is on it’s way to a fire, the crew leader in the passenger seat can look up to see if the house at the given address has joists or trusses. He/she can then decide to not go inside to fight the fire. It just becomes a slightly larger insurance claim in the event firefighters do the right thing by extinguishing the fire by staying outdoors and staying safe.

Another reason I favor floor trusses is that they make it easier, and thus cheaper, for plumbers, electricians and HVAC people to install all the necessary utilities in your new home. The open web design allows copious room to run pipes, ducts and cables with ease. Electricians will think they died and went to Heaven. They don’t have to drill any holes!

Drywall hangers love floor trusses too. The drywall sheets meet on the wide dimension of a 2x4, not a narrow 1.5-inch-wide floor joist. There’s no need to install silly 1x3 furring strips as is common in some parts of the USA. Those things are a waste of money and time as far as I’m concerned.

Realize that you can work with the truss designer and make the floors as stiff as you want so there’s no bounce. To minimize bounce, the truss height is usually increased a few inches.

If you couple floor trusses with factory-built walls, you can minimize construction time and get your house under roof faster. The precision of these factory-built components is remarkable. I can assure you, the workmanship is far better than you’ll get from most carpenters stick-building your home.

I recorded a wonderful video showing these magic floor trusses. You can view it on my website. Just type:

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