Moisture movement is usually the starting point
The Building America Solution Center, a U.S. Department of Energy resource run by Pacific Northwest National Laboratory, explains that wood products swell when they take on moisture and shrink as they dry out, and that uneven wetting and drying will cause wood to warp. The National Park Service's guide to the care of wooden objects describes cupping as a deformation across the width of a board, and notes it can happen when opposite surfaces sit in different conditions, such as a sunny top drying faster than the shaded side underneath. The same guide describes checking, where uneven shrinkage separates wood along the grain, usually at the end grain or near the surface. That guide is written for museum furniture rather than decks, so we use it here only for how wood moves and the terms that describe it.
Look for a wetting-and-drying pattern
A deck board can take on water from more than its walking surface. Sherwin-Williams, a coatings manufacturer, says in its exterior troubleshooting FAQ that when only the top of a deck is painted, the uncoated sides and undersides can absorb moisture from rain and from the soil under the deck, and that this moisture is the main reason paint peels off decks. A fresh-looking top coat therefore doesn't show that the board underneath is staying dry.
Before asking for repairs, note whether the damage affects scattered boards or a whole area. Does it cluster beside planters, under a roof drip or a downspout, or in a corner that stays damp? Photograph the same spots after rain and again in dry weather. Those notes give whoever inspects the deck useful questions to look into; they aren't meant for you to settle the cause yourself.
Installation and storage history matter too
The same Building America guidance says pieces installed while swollen may not align well and will shrink and leave gaps as they dry. It also recommends keeping lumber off the ground and covered before installation so it doesn't soak up water from rain, soil or a slab. So the way the boards were stored and installed matters, not just the deck's age or finish.
Questions worth asking the installer or the person inspecting the deck: Was the lumber dry when it went down? What spacing and fastening instructions apply to this particular decking? Are the cracks concentrated at board ends or around fasteners? Treat those locations as clues to investigate, not proof of a specific installation mistake, and don't pick a one-size gap or screw pattern without knowing what the decking is.
Decide between repair and replacement by condition
NADRA, the North American Deck and Railing Association, includes split or decaying wood in its Check Your Deck consumer checklist, and tells homeowners to pay special attention to areas that stay damp, are regularly exposed to water or are in contact with fasteners. The same checklist lists loose fasteners and says a deck shouldn't sag, sway or move. If boards are deeply split, badly distorted or loose, or you suspect decay, ask for an assessment of replacement rather than a cosmetic-only proposal. Our guide to when deck joists or posts need a professional inspection covers the structure underneath.
Keep two decisions separate: which boards need replacing, and whether the framing under them is fit to reuse. Trex, a decking manufacturer, lists inspecting the frame and making any needed repairs as steps before new boards go down in its guide to resurfacing a wood deck. New boards on top don't replace that check. Our article on replacing deck boards without rebuilding the frame explains what that scope usually looks like, and our deck repair permit guide for Philadelphia and Bucks County covers when the work may need a permit.
Make the repair address the cause
A useful repair proposal names the affected boards, explains the suspected moisture or installation problem, and separates replacement that's needed from refinishing that's optional. Ask what will change so new boards aren't exposed to the same conditions.
Treat stain or sealer as maintenance, not as proof that damaged wood is safe. NADRA's checklist says a fresh waterproofing coat can help prevent split and decayed wood, but it doesn't make an already-damaged board sound. If a board moves underfoot, has a raised edge people could trip on, or looks decayed, keep people off that area until it has been looked at, and don't test it by jumping on it or loading it with weight.
Two of the sources above come from product manufacturers, Sherwin-Williams and Trex. We used them only for the general points cited and don't recommend any brand. This is general information, not a structural inspection of your deck.