Symptoms
- Boards curved upward at the edges (cupping) or twisted
- Uneven walking surface
- Gaps opening or closing unevenly between boards
Likely causes
- CommonUneven moisture exposure (wet underside, dry top)The underside of a board often stays damp longer than the sun-exposed top, causing it to expand more and cup.
- PossibleBoards installed with the wrong grain orientationGrowth-ring orientation affects how much a board cups as it dries and wets repeatedly.
- PossiblePoor airflow or standing water underneath the deckTrapped moisture below accelerates uneven swelling.
What to do
- Check whether cupping is minor and consistent (often normal for wood decking) or severe and worsening.
- Improve airflow under the deck by clearing vegetation, debris, or storage that blocks ventilation.
- Make sure the ground under the deck slopes away and doesn't hold standing water.
- For minor cupping, sanding the high edges can sometimes even out the walking surface.
- Flip a board end-for-end or replace individual badly cupped boards rather than sanding heavily, which weakens them.
- Apply sealant or stain to all sides of any new or flipped boards, including the underside, to balance moisture absorption.
- Reseal the whole deck on a regular schedule to slow future warping.
Tools and parts
- Sander
- Pry bar
- Drill/driver
Deck boardsSearch: “pressure treated deck boards”
Deck sealantSearch: “deck sealant all sides”
When to call a pro
Call a deck contractor if warping is widespread or the deck surface has become uneven enough to be a trip hazard.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.