The short answer

Cupping is a movement pattern, not a complete repair diagnosis. Look at the board’s edges, fasteners, gaps, underside ventilation, joist tops, shade, drainage, and the condition of nearby boards. Re-fastening a sound board may differ from replacing split or decayed wood, and neither addresses water held below the deck.

Survey the pattern

Observation Possible explanation Useful next step
One board cups Board defect or local water exposure Check fasteners and joist contact.
A whole shaded area cups Drying and ventilation pattern Look for trapped debris or moisture.
Ends split near fasteners Fastener placement or movement Do not reuse damaged holes blindly.
Boards stay wet underneath Poor airflow or drainage Clear debris and investigate structure.
Joist tops are soft Concealed decay Stop cosmetic work and assess the structure.

Photograph the deck dry and after rain if safe. Do not sand or coat a wet surface, and do not walk an unstable deck to inspect its underside. Keep the board spacing and drainage paths clear of leaves and rugs that hold moisture.

Ask for the full scope

Separate board replacement, re-fastening, joist repair, flashing, ledger work, cleaning, sanding, and finish. A proposal that promises to flatten boards with screws or stain should explain what happens if the wood or supporting structure is already compromised.

Limits

This guide does not certify deck safety or select a finish. A moving, soft, split, or elevated deck needs qualified assessment.