Water Drainage on Metal Decking

Design

• Metal decking typically snaps together using an interlocking design. The deck boards go together quickly, each fastened on one edge to the joist underneath and locking on the other edge to the next board. The final deck presents a smooth, solid surface that remains waterproof underneath.

Considerations

• Interlocking metal decking, the main type sold, lacks the gapping found between composite and pressure-treated deck boards to allow for water draining. This raises the issue of how to address water ponding on the surface of metal decking. Ponding leaves standing water after rainstorms that attracts dirt and leaves the surface with a smudged appearance after the water evaporates.

Slope

• Manufacturers recommend a downward slope of at least 1/8 inch per foot in the direction that you want the metal decking to drain. This slope typically runs away from the house on a deck attached to your back door. On a roof deck, the slope runs toward existing gutters at the edge of the roof. FSI Products, which makes Lock-Dry aluminum decking, recommends an even greater drop of 3/16 to 1/4 inch per foot, if possible. The maker of AridDek agrees on the 1/8-inch slope as a bare minimum, noting that that this pitch is not visible to the naked eye. Note this slope yourself or ask your architect to include it in your deck-plan drawings. Confirm with the worker framing your deck that the joists need to slope downward so that the applied decking doesn’t suffer from ponding.

Alternative Drainage

• Manufacturers deal with the issue of puddles and ponding by adding channels between the metal deck boards to collect water. These channels allow water to drop below the surface and roll to the deck’s rim and into a gutter. Versadry, a product made by Versadeck that looks like a continuous-plank deck from above, hides this drain system from view. FSI Products offers an alternative to its interlocking Lock-Dry design called Nextdeck, an aluminum plank held by clips that does not interlock and allows water to drain between the planks. Nextdeck acts more like wooden deck boards that allow water to drain between them.