Deck drainage

Board gaps. Decking is spaced approximately 1/8 inch apart. That gap is the primary drainage system, and it stops working the moment leaves pack it. Clearing gaps every fall is the single highest-value maintenance task on a deck.

The ledger connection is the critical one. House siding must be removed before the ledger board goes on, approved corrosion-resistant flashing is required at any ledger board connection to a wood-framed wall, and the door threshold must be carefully flashed and caulked to prevent water intrusion from splash off the deck or from melting snow and ice. Flashing must be corrosion-resistant metal at least 0.019 inch nominal thickness or an approved non-metallic material.

Aluminum warning. Aluminum should not be used in direct contact with lumber treated with copper-containing preservatives such as ACQ, Copper Azole or ACZA. That includes flashing. It's a common, quiet mistake that corrodes over years.

Under-deck drainage systems capture water below the boards to make the space beneath usable. They're a real product category and a real add-on cost. The caution is the same as with skirting: don't create a sealed, unventilated damp cavity against the framing, and keep an access route for inspection.

How to choose

Clear the board gaps every fall

Roughly 1/8 inch of gap is the whole drainage design. Packed debris defeats it.

Insist on flashing at the ledger

Corrosion-resistant metal at least 0.019 inch thick, or an approved non-metallic material, with siding removed first.

No aluminum against copper-treated lumber

ACQ, Copper Azole and ACZA all corrode aluminum in direct contact. Ask what flashing material is being used.

Check the door threshold detail

It must be flashed and caulked against splash and melting snow. It's the leak that shows up inside the house.

Keep the under-deck space ventilated and reachable

You need to be able to look at the framing. Rot repair runs $300 to $5,000 and catching it early is everything.