Build a Flush Deck Box Beam
Straightening bowed or wavy lumber in a deck box beam by levering low spots up and tapping high spots down before locking each section flush.
Double-Block Deck Miter Corners So Debris Washes Through
Under a mitered deck corner, one solid diagonal block turns the required expansion gap into a debris trap — two blocks with a gap between them support the miter while letting water and debris fall through.
gunk collecting in deck corners, leaves stuck in deck gaps, corner joint keeps getting dirty
The Problem
Composite decking instructions call for miter joints to be gapped — an eighth of an inch, per the brand in this video — so the boards can move as they expand and contract with temperature. The standard framing under that corner is a single diagonal block centered under the joint. Put those two details together and you've built a trap: the gap collects needles, seeds, and grit, the solid block underneath stops anything from falling through, and the debris sits there wet, growing moss and mold in the most visible joint on the deck.
The Technique
Mike Guertin's framing fix, shown for Fine Homebuilding:
- Instead of centering one diagonal block under the miter, slide it back about half an inch from the joint line.
- Add a second diagonal block on the other side, leaving a gap between the two blocks directly under the miter.
- Run the mitered boards over the pair. Each board end is still supported about an inch back from the cut — plenty — but the expansion gap now opens into the space between the blocks.
Water drains, and the debris that lands in the gap washes through with it instead of accumulating.
Pro tip: This is a framing decision, not a decking one — it costs one extra block per corner during the frame stage and can't be retrofitted without pulling boards. If your deck plan has mitered corners or a picture-frame border, decide the blocking layout before the first board goes down.
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