Hey everyone. I'm brand new in the boards but I've been around my share of tools. I thought hearing a few opinions on my situation might help me make a decision. Here's my deal: My attic has 2 x 6 Douglas Fir #2 floor joists spanning 13'. I realize that this is just about the max. span for these joists. I plan on using the attic for some light / medium storage (some boxes of baby clothes & toys). To keep the joists from flexing I'm considering sistering the joist where the storage will be and the path used to crawl to the boxes.

Please give me your thoughts on using 3/4" plywood (glued and nailed with staggered seams vs. dimensional lumber).

Thanks in advance,

Solan

If, by flex, you mean deflect you're not going to be able to stop it. All beams will deflect under any load (including it's own selft weight). It's just a matter of how much and what is acceptable to the finishes and human perception - the actual amount is dependent on the beam properties E & I. L/360 LL deflection and L/240 TL deflection is common.

Since plywood is typically made from lesser quality lumber AND will be thinner, I would use dimensional lumber. It is thicker, and therefore has a greater I, and the modulus of elasticity is typically higher.

I would run the sister the full length. If you only put it where you intend to place the boxes you run into two problems. First, the sister joist could end up providing no net benefit if the fasteners are sized and spaced properly to prevent bearing deformation issues at the fastener holes. Second, increasing the I for a small portion of the span isn't going to help the overall deflection very much.