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I’m looking to build a 2-car detached garage that’s 24ft wide with a loft above for storage. I’ve been searching and I can’t seem to find any lumber that spans 24ft. If I had 2x6 (or 2x8 or 2x12) that are 20ft long (longest I can find) can I stagger them and attach two together? There would be 20ft of overlap that’s attached and 2ft on either side that’s just a single board. Is this safe? Or do I have to do more of a truss system to support the beams?

  • I'm guessing this would best be answered by a structural engineer on-site, but let's see. – Daniel Griscom Oct 1 '20 at 2:29
  • You should be able to get engineered lumber that length. Engineered lumber is like gremlins... don't let it get blasted by sunlight, don't let it stay wet... and never feed it after midnight! – Harper - Reinstate Monica Oct 1 '20 at 2:38
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I suppose there must be a way you could splice dimensional lumber and make a longer span, but... why?

Have a look at wood I-joists. Boise Cascade (BCI), LP Building Products (SolidStart) and Weyerhaeuser (TJI) are three options widely available in the US. (photo below from lpcorp.com)

wood I-joists

As a single point of reference, a BCI span table shows spans up to or exceeding 24 ft with joists 11-7/8, 14, or 16 inches deep with spacings of 12, 16, 19.2, or 24 inches on center. You'll have to consult the table yourself to see the effects of joist family, spacing, allowed deflection, and loading.

  • I went a bit bigger. 20" deep and 24" on center. Works perfectly. I upped the loading to reflect my actual planned use. – Ecnerwal Oct 1 '20 at 3:53
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I’d use trusses made with an open area that you can stand up in, in the middle third of the span.

Consult a truss manufacturer or a lumber yard. They can design a void in the middle third so you can use it as a storage room or an extra bedroom.

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