Why Does My Deck Feel Spongy in Some Spots But Not Others?
A spongy deck spot means the wood there has stayed wet long enough to rot. Rot needs wood that stays soaked, which is why it shows up where water sits and can’t dry, and not across the whole deck.
The real question is whether the rot is in a single deck board or in the frame under it: a joist, the ledger board on the house, or a post. The location of the soft spot is a useful clue. Below is a table that maps location to cause, what the USDA’s wood scientists say about how much strength rotten wood has already lost, and what a sound repair has to include.
Why rot is patchy
The USDA Forest Products Laboratory (FPL), the federal wood research lab, says serious decay happens only when wood moisture is above the fiber saturation point, about 30% on average. FPL says humid air alone won’t wet wood enough for significant decay.
Wood that has dried out usually sits at 20% moisture or less, and FPL says it won’t decay. Even rot that’s already started stops advancing if the wood stays dry.
So rot maps where water lingers on your deck:
- Board ends and seams. FPL says exposed end grain at joints soaks up water more readily than the face of a board.
- Around screws and nails. FPL says the area around all fasteners, including drill holes, needs extra on-site preservative treatment, as do cut ends.
- Gaps packed with leaves and dirt, which hold water against the board edges and the joist tops.
- Under planters, grills, and rugs that never move.
- Along the house, where the ledger board sits against the wall.
- Low, enclosed decks with little air moving underneath. The industry standard for above-ground treated lumber assumes there’s enough air circulation for the wood to dry, the Western Wood Preservers Institute says.
Temperature matters too. FPL says decay is slow below 50°F and above 95°F, and nearly stops at 35°F. Rot advances mainly in warm, wet weather.
Soft wood has already lost much of its strength
FPL’s laboratory tests found that wood can lose 6% to more than 50% of its toughness, its ability to absorb a sudden impact, by the time rot has eaten away just 1% of its weight.
By 10% weight loss, most strength measurements drop by more than half. At that stage, the decay is visible only under a microscope.
FPL’s conclusion: wood with decay you can see has lost a great deal of strength in every respect. If a board feels spongy, it’s well past the early stage. And the wood next to it that only looks discolored may already be weakened.
That’s why screwing down a soft board isn’t a fix: the screw would be held by wood that has already lost much of its strength.
Where the soft spot is, and what it means
| Where it’s soft | What has likely failed | What to check | DIY or pro |
|---|---|---|---|
| Middle of one board; boards on either side feel solid | That deck board | Look at the underside and the joist below once the board is up | DIY board swap, if the joist is sound |
| At a seam where two board ends meet, or along a line of screws | Board ends, and possibly the top of the joist below them | Pull both board ends and probe the joist top | DIY if the joist is firm; frame repair if not |
| Several boards in a row give together | The joist under them | Probe the joist from below, especially where it sits on the beam and near fasteners | Structural: get a deck contractor |
| Along the house wall | The ledger board, and possibly the house framing behind it | Probe the ledger; check bolts and flashing (list below) | Pro. Keep gatherings and heavy loads off until it’s checked |
| Near a post, at the stairs, or where a railing post attaches | Post, stair stringer (the notched board stairs sit on), or rim joist | Wiggle railings and posts; probe at the base and connections | Pro, and soon: these are structural parts |
| A little soft everywhere | Widespread decay, or decking that’s too thin for the joist spacing | Probe several joists; measure joist spacing | Pro inspection before spot repairs |
How to probe
Push an awl or a flat screwdriver into the wood at an angle. Sound wood resists and lifts out in long splinters. Rotten wood lets the tip sink in easily and breaks off in short, crumbly pieces.
Probe more than just the spot that feels soft. FPL says early decay often shows up as discoloration: abnormal mottling, unnatural brown or bleached patches, or wood that looks water-soaked. Probe those areas too, along with joist tops under seams and the wood around every bolt you can reach.
A flashlight and a few pulled boards tell you more than standing on top. Wear eye protection when prying boards. If the deck is high enough, look underneath.
The ledger: what “correct” looks like
The ledger is the board bolted to your house that carries one whole side of the deck. Water that gets behind it can rot both the ledger and the house framing, out of sight.
The American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (known as DCA 6, based on the International Residential Code) sets out what an attached deck needs. Use it as a checklist from below:
- Fasteners: ½-inch lag screws or bolts with washers, in a staggered pattern. Nails alone don’t meet it, and the guide prohibits lead anchors.
- Flashing: corrosion-resistant flashing over the ledger, so water can’t reach the house’s band joist (the framing the ledger is bolted to).
- Ledger size: at least a 2x8 of pressure-treated lumber.
- What it’s attached to: the guide prohibits attaching a ledger to brick or stone veneer, hollow masonry, a cantilevered floor overhang, or a bay window. In those cases the deck can’t hang from a ledger under DCA 6, so a deck contractor has to design other support.
A soft ledger, rust-streaked or missing bolts, a nailed-only ledger, or any gap between the ledger and the house means a call to a deck contractor. Our guide on when a repair is worth calling a pro draws the same line.
Making the repair last
Replacing rotten wood with the same setup just restarts the clock. These details come from the sources above.
Pick the right treatment for the job. Treated lumber is sold for two main exposures. Above-ground (UC3B) lumber assumes the wood gets wet but has the air circulation to dry quickly. Ground-contact (UC4A) lumber is for wood touching the ground, and the Western Wood Preservers Institute says it’s also meant for above-ground parts that are critical to safety, hard to replace, or face ground-contact-level wetness. Joists on a low deck with poor airflow can fit that description. Check the lumber’s label for its use category before you buy.
Treat every cut and hole. FPL recommends applying copper naphthenate (1% to 2% copper) to cut surfaces, and says exposed end grain and the area around all fasteners, including drill holes, need on-site treatment too. DCA 6 requires field treatment of all cuts.
Use the right fasteners and hardware. DCA 6 calls for hot-dip galvanized, stainless steel, silicon bronze, or copper fasteners and connectors in contact with treated lumber.
Decking basics from DCA 6:
- decking must be 2-inch-nominal lumber or a decking product rated for the span
- each piece must rest on at least three joists
- fasten with two nails or screws at every joist
- space boards about ⅛ inch apart
Clear the gaps. Run a putty knife through the board gaps whenever debris packs in, and move planters and rugs now and then so the wood under them can dry. Fix drainage too: if water pools around the posts after rain, new posts can rot the same way. DCA 6 sets 6x6 as the minimum post size.
Before digging new post footings, call 811 or use call811.com a few business days ahead so buried utility lines get marked. It’s free in the US.
Spongy versus bouncy
A deck can flex without any rot. If the whole surface bounces evenly and the probe test finds firm wood everywhere, the decking may be too thin for how far apart the joists are. DCA 6 requires 2-inch-nominal lumber or decking rated for the span. Measure the joist spacing from below and compare it with the decking’s span rating or the manufacturer’s maximum before assuming the wood is failing.
FAQ
Can I sister a new joist onto a rotten one?
Sistering means bolting a new joist alongside the damaged one. It’s a structural repair, and the rot in the old joist doesn’t stop on its own if the wood stays wet. FPL says decay that’s visible means the wood has lost much of its strength, so have a deck contractor decide whether sistering is enough or the joist has to come out.
Is a spongy deck dangerous?
It depends on where the soft spot is. A soft deck board in the middle of the deck, over a sound joist, is a repair job. Soft wood at the ledger, posts, stairs, or railing posts is a safety issue, because those parts carry the deck and the people on it. DCA 6 requires a guard (railing) on any deck more than 30 inches above the ground, so a railing post that wiggles at its base is part of that safety system. Keep people off that area until it’s fixed.
How often should I check for soft spots?
Walk the deck, press on seams and the boards along the house, and probe the posts on a regular schedule. Pair it with the fall yard cleanup, when leaves start packing into the board gaps.
Sources
- Wood Handbook, Chapter 14: Biodeterioration of Wood (USDA Forest Products Laboratory) — 30% moisture threshold, 20% safe level, temperature range, strength loss in early decay, signs of early decay
- Wood Handbook, Chapter 15: Wood Preservation (USDA Forest Products Laboratory) — end grain and fastener holes; copper naphthenate on cuts
- Prescriptive Residential Wood Deck Construction Guide, DCA 6 (American Wood Council) — ledger fasteners, flashing and prohibited attachments; decking, fastener, post and guard requirements; ground-contact lumber
- Preservative Use Categories (Western Wood Preservers Institute) — above-ground vs. ground-contact treated lumber
- Call 811 — calling before digging new post footings
Written and reviewed by peter.bae for Hearth & Habit · Last reviewed October 11, 2026
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