How can I tell if my deck or subfloor is rotted?

1. The bounce test

Walk slowly across the deck or the suspect part of your floor. Pay attention to your feet. Healthy framing transfers your weight evenly to the ground. Rotted framing flexes — you'll feel a slight bounce, a give, or a "trampoline" effect, especially in the middle of the span.

For interior subfloors, a spongy feeling near a toilet, shower, sink, or exterior door is a classic sign of long-term water damage that's eaten the OSB or plywood below the finished floor.

2. Dark stains and mold

Wood that's been wet for years discolors. Look for dark brown or black streaks at the base of posts, where deck boards meet rails, around door frames, and at any spot where water tends to sit. On indoor subfloor, dark spots typically show through laminate or vinyl flooring eventually — that's the water reaching the surface from below.

Visible black or green mold is a clear escalation. Mold needs moisture and food (wood is the food). If you see it, the rot is already established.

3. The screwdriver test

This is the test contractors use. Take a regular Phillips-head or flathead screwdriver and press the tip firmly into the suspect wood. Healthy structural wood resists — the tip dents but doesn't sink. Rotted wood gives way. If the screwdriver pushes in like the wood is soft cheese, it's gone.

On one of our recent O'Fallon-area projects, the homeowner's subfloor was soft enough we could push a #2 pencil through it with light pressure. At that point the only thing keeping the floor from falling into the basement was the laminate on top.

4. The tap test

Tap the deck boards or floor with a hammer or a hard knuckle. Solid wood gives a clear, ringing sound. Hollow or rotted wood sounds dull and thuddy. Compare suspect areas to areas you know are healthy — the difference is obvious once you've heard it.

5. Crumbling fibers

Inspect cut ends of deck boards, the base of every deck post where it meets the ground, the ledger board where the deck attaches to the house, and around every fastener. Look for wood that's crumbling, flaking, or has a stringy fiber texture instead of a solid grain. Pick at the fibers with a fingernail — if they peel off easily, the wood is decomposing.

What it usually means

One sign in isolation may just be cosmetic. Two or more signs together almost always mean the rot has spread beyond the visible damage. By the time a deck board feels soft, the joist underneath is often rotted too. By the time a subfloor feels spongy, the rim joist or floor joist is usually compromised.

The cause is almost always water — failed flashing, a leak from above, ground contact with no barrier, or no roof venting on a bathroom dumping moisture into the framing. Until the water source is fixed, replacing the rotted wood is just a clock reset.

What we do when we find it

• Open up the area to see how far the rot extends.
• Identify and fix the water source (flashing, vent, ground barrier, etc.).
• Cut and replace the rotted framing — joists, rim joists, posts, ledger, subfloor.
• Add insulation and flashing where there was none.
• Quote the additional work BEFORE doing it. No surprises on the invoice.