Do I need blocking between 2x8 joists with an 8-foot span in a kid's fort?

Question

About to embark on a DIY project to build my kids a fort. It will be a deck using 4x4's for legs, holding up a box of 2x8's. Three interior joists for a total of 7 2x8's will make up the frame for the floor. The 2x8's will be attached to each other using 3 1/2" long, either 3/8" or 1/2" thick galvanized lag screws. On top of this, I will be building walls 5' high out of 2x4's and cedar fence boards, with a simple gable roof.

My question is: should I use blocking between the floor joists? I know this prevents cupping, but is there a structural need, or is this overkill?

Answer 1

Not really. 8 feet is a fairly short span, and you're unlikely to see substantial movement even with some warpage. It certainly won't be enough to reduce the capacity of the floor to a dangerous degree. (No deck I ever recall seeing has had joist blocking.)

Minor point of clarification: Blocking doesn't reduce cupping (curvature of lumber across the grain). It prevents displacement (curving) of the joist along its length. That's not typically referred to as "cupping".

One further suggestion... Typically flush beams (rim joists acting as beams) are sized larger than the common joists because their load is much larger. I'd either double yours, increase the rim size to 2x10, or reduce the common joists to 2x6, depending on your stiffness standards. 2x6 joists on 16" centers are adequate for a playhouse of that size, IMO.