How to Channel Water Under a Second-Story Deck

Water collects under second story decks, and needs to flow away.
Water collects under second story decks, and needs to flow away. (Image: Jupiterimages/Polka Dot/Getty Images)

The space underneath a second story deck is usually at least ceiling height, making it comfortable for people to work underneath -- such as when you're doing grading work against a foundation after installing an overhead deck or building a new home and deck together. Optimally, grade work should be completed before a deck is constructed, but excavation for posts and other construction activity can disturb the ground and potentially disrupt your drainage or rainwater channeling pattern. Water can pool around posts and the house foundation as a result. Properly channeling water under a second-story deck is typically a straightforward matter of restoring the drainage, using the slope of the yard outside the disturbed area as a guide, and adding gravel for stability as necessary.

Things You'll Need

  • Wheelbarrow
  • Shovel
  • Rake
  • Well-drained gravel, crush or road mix
  • Water hose and spray gun attachment
  • Plate compactor

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Use a shovel and wheelbarrow to remove any soft soil or organic matter from the area underneath the deck. Replace with well-drained gravel, using the shovel and wheelbarrow.

Add gravel against the foundation wall of the house -- if the backfill was disturbed during construction -- within 8 inches of the top of the wall, still using the shovel and wheelbarrow. Slope away from the house with a minimum 5 percent slope, or a 3/4-inch drop over 1 foot, for at least 5 feet.

Follow the slope of the yard for the rest of the ground under the deck -- beyond the 5 feet of slope from the foundation -- which may be less than 5 percent. Make sure the ground under the deck slopes a minimum of 2 percent, or a 2½-inch drop over 10 feet, from the high point of the slope to the low point of the slope, filling depressions with gravel, or cutting high spots down, as necessary, with the shovel or rake. Complete the work so that it's slightly below the high point of the grade adjacent to the ground under the deck, and slightly above the adjacent grade on the low side.

Slope away from the base of the deck posts. Rake the gravel to a uniform plane, with no dips or bumps. Wet the gravel with a water hose and spray attachment, and compact with a plate compactor.

Fill low spots with gravel, and smooth or cut high spots. Compact again if necessary.

Tips & Warnings

  • A skid steer loader can save labor with the shovel and wheelbarrow, if there is room under the deck for equipment to maneuver.
  • Water trapped in soil causes it to shift and settle. Crush or road mix gravel lets water drain through, and compacts well, creating a stable surface.
  • Take care using the compactor next to the house or deck posts to avoid damage to the structures.
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