Dear Builder’s Engineer,
With green framing all the rage I have been tempted to start using studs at 24-inch spacing rather than 16, as I’ve always done. How much will I really save doing this, and is it safe? I’m especially concerned with wavy drywall and siding.
Bob M., Houston, Texas
Tim Garrison is an author, public speaker, and professional engineer. He welcomes correspondence via his blog at ConstructionCalc.com
Savings will come in several ways. First, you’ll use far fewer studs. Here’s an easy way to determine approximately how many you’ll save: number of studs saved=lineal feet of wall*0.25.
As an example, say you’re building a 50-foot-by-50-foot, one-story house. The total length of exterior wall is 50*4=200 lineal feet. Let’s assume there are 100 lineal feet of interior wall. The total length of wall is therefore 200+100=300 lineal feet. If all of that is framed with studs at 24 inches rather than 16, the number of studs saved will be approximately 300*.25=75. That’s approximate because it doesn’t take into account doors and windows, but you could subtract out that lineal footage for more accuracy. If this were a two-story home, the number of studs saved would double: 75*2 floors=150.
The second form of savings is more indirect. All that wood left out creates additional space in walls for insulation. With our example home, at 1.5-inches in width multiplied by 75 each, that equates to 112-inches, or about 9.3 lineal feet of wall that will be filled with insulation rather than wood. This will, in time, translate to energy savings.
Here’s a third form of savings: labor. Fewer studs will mean less labor to haul, layout, and nail studs in place; less labor in nailing sheathing and drywall to studs; less mudding and sanding for drywallers; and less labor for electricians who route wire through holes they drill in studs. No single item here is a back breaker, but it all adds up.
Now for the downside. What about strength? Studs are the skeleton of any stick-framed house, so it stands to reason that removing a few bones might weaken the structure. That’s undoubtedly true. But the better question is, is what’s left still strong enough? A leg with two femurs would be stronger than a leg with one, but would it be better? Probably not. Let’s run some numbers.
In my neck of the woods, nearly all exterior studs are 2x6. What’s the capacity of a 10-foot long, 2x6 stud at 24-inch spacing? Where snow and wind loads are moderate—most of the U.S.—a No. 2 or better, Hem Fir, Spruce Pine Fir, or Doug Fir stud in the first story of a two-story house calcs with plenty to spare; it makes it by some 21%.
If a 10-foot-long stud calcs, a shorter one with the same parameters will too, and in fact will make it by a larger margin. (Those of you interested in a simple explanation of the theory behind this should check out my previously published article on the Spaghetti Test.)
What about 2x4 studs? A 2x4 stud with the same loading as above but spaced only at 16 inches does not calc. Shorten that stud to 8 feet long and it makes it, but barely. To get a 2x4 to calc at 24-inch spacing, it can be no longer than 9 feet and support only roof load. Most of the trouble with 2x4 studs is their weakness in resisting wind load. But interiors don’t experience wind, so using 2x4s at 24-inches there makes sense. Take away the wind load and a 9-foot-long 2x4 at 24 inches calcs fine supporting a floor plus roof; it makes it by 19%.
To restate the strength conclusions, 2x6 studs are great at 24-inch spacing, plenty strong even up to 10 feet long, supporting several stories. However, you have to be careful with 2x4s – they can make it at 24 inches but with restrictions. An ideal situation could be 2x6s on the first floor and 2x4s on the top floor, with both floor’s interior studs being 2x4s.
What about wavy siding? Most nail-on siding these days is made assuming stud spacing of 24-inches. There are probably war stories out there, though none that I’ve heard, concerning properly installed siding showing waves due solely to studs at 24-inches. If you know of such a case, please let me know via my blog.
The same goes for wavy drywall. If the studs are of good quality and the drywall is installed correctly, I’ve not heard of problems due to 24-inch stud spacing. And that goes for ½-inch drywall, not just 5/8-inch. Again, let me know if you’ve experienced differently. You might wonder if code even allows ½-inch drywall on studs at 24-inches. You bet it does. In fact IBC Table 2306.7 includes earthquake-resisting shear walls made with ½-inch drywall over studs at 24 inches.
To summarize, studs at 24-inch spacing will save money and resources, and are safe. With 2x6s, it’s a no-brainer for most residential construction—they’re plenty strong in nearly any configuration. With 2x4 studs you can also get by with 24-inch spacing, but walls should be 9 feet tall or less and the loads modest.
Tim Garrison is an author, public speaker, and professional engineer. He welcomes correspondence via his blog at ConstructionCalc.com.