Build a Straight Deck Box Beam From Bowed Lumber for Picture-Frame Decking

The Problem

Picture-frame decking and handrails both need a straight, flush foundation to sit on, but lumber for a box beam — two joists with a center 2x6 sandwiched between them — is almost never perfectly straight out of the pile. A bowed or wavy board in that assembly telegraphs into every board that lands on top of it.

The Technique

1. Pin the box beam's boards together at both ends first, with screws, so the assembly holds its overall length and position while you work the middle.
2. Sight down the length of the beam to find where it's low or high relative to flush.
3. For a low spot: drive a screw in at that point and use it to lever the board upward — lifting it by roughly a quarter inch or whatever it takes to bring it flush — then drive the screw home once it reads flush, releasing the lifting pressure as you go.
4. For a high spot (a hump): change your angle of approach and use a hammer to tap the board back down flush before fastening it off.
5. Work down the length of the beam, checking and correcting each section the same way, until the whole assembly reads flush.
6. Once it's straight, cover the beam in joist tape to protect it before the picture-frame decking and handrail posts go on top.