Rotted deck: repair it or replace it?

Many rotted decks don't have to come out. Boards and railings can be replaced on a sound frame. Rot in the ledger, the posts, the beam or the joists is a different job: that's the structure, and in Madison replacing posts, beams or joists goes through plan review. Check each part with a screwdriver, then use the checker.

Parts of a deck · Where decks rot · How to check · Checker · Part by part · When to rebuild · Composite on an old frame · Permits by town · Old lumber

The parts of a deck

• Ledger: the board bolted to the house. It holds up the house end of the joists.
• Flashing: the metal strip over the ledger that sends water out past it instead of behind it.
• Joists: the boards set on edge that run out from the house and hold up the deck boards.
• Beam: the heavy member, often two or three boards fastened together, that carries the outer end of the joists.
• Posts: the upright supports under the beam.
• Footings: the concrete under each post. In Wisconsin they go at least 48 inches down, below the frost.
• Boards (decking): what you walk on.
• Railing: the posts, top rail and balusters around the edge. The code calls it a guard.
• Stringers: the notched boards on each side of the stairs that carry the treads.

Where decks rot first

Wood decays only when it's wet: above about 30% moisture. Air-dried wood is usually at 20% or less, and existing decay stops spreading while the wood stays dry. (USDA Forest Products Lab) So rot starts wherever water sits and can't dry out:

• The tops of the joists, under the boards. Water collects where joists meet the decking and dries slowly. An engineer who studied 10-year-old decks found most of the decay there. (JLC)
• The ledger, the board bolted to the house, especially without flashing above it. NADRA calls it a common source of deck failure. (NADRA)
• Built-up beams, in the gaps between the boards that make them up. (JLC)
• Stair stringers, at their cut ends and where they sit on the ground. End grain soaks up water much faster than the sides.
• Post bases, and anything else touching soil.

Pressure treatment only goes about 1/4 inch into the lumber, which is why Madison's deck handout says to treat every cut end. (City of Madison) A cut that was never sealed is often where the rot starts.

Decks built after 2003 can rot sooner than older ones

Before 2004, decks were commonly built with CCA-treated lumber. CCA left the residential market at the end of 2003, (EPA) and one deck engineer reports that store lumber for joists and beams shifted from ground-contact to the lighter above-ground treatment, with builders seeing framing rot in about 10 years. (JLC) The wood-treating standard now says joists and beams that are hard to replace and critical to the structure should be ground-contact grade, and posts and ledgers should be too. (AWPA) If your deck is 10 to 20 years old, check the framing, not just the boards.

How to check it yourself

Use NADRA's probe test: push an ice pick or screwdriver into the wood. If it goes in 1/4 to 1/2 inch easily, breaks off in a short piece without splinters, or the wood feels soft and spongy, it may be decayed. Small holes can mean insects. (NADRA) Work through the deck in this order:

1. Ledger. From underneath, probe along its length. Check that it's held with bolts or lag screws, not nails alone, and that there's flashing above it. Look for rust stains around the fasteners: corroded fasteners damage the wood around them.
2. Posts. Probe at the ground and at the top where the beam sits.
3. Beam. Probe the bottom edge and the joints between plies.
4. Joists. Probe the top edges you can reach between the boards, and the ends at the ledger and beam.
5. Boards. Soft spots, splits, and boards that flex underfoot.
6. Railings. Push on the posts. They shouldn't move, and probe them where they're bolted to the frame.
7. Stairs. Probe the stringers at the bottom and at the cut notches.

Then stand on the deck and shift your weight. It shouldn't sag or sway. (NADRA) Wood loses a lot of strength early in decay, before it's visible, so a part that's visibly rotted has much less strength left than it looks. (USDA Forest Products Lab)

Repair or replace, part by part

Madison's own lists: replacing surface deck boards, handrails or guardrails is an online repair permit; replacing posts, footings, joists or beams is structural and goes through plan review. (City of Madison) Repairs are charged $11 per $1,000 of value, $25 minimum. (City of Madison)

When the whole deck should go

Once the frame is failing in several places, a repair replaces most of the structure anyway. Plan on rebuilding when:

• The ledger is soft, or it's attached in a way Wisconsin's deck appendix doesn't allow: to brick or stone veneer, to a house overhang without engineering, or with fasteners other than the ones it lists. (SPS 325 App. B)
• The beam or more than one post is soft.
• Many joists are soft along their tops.
• The deck sags or sways.
• You want a different size, height or shape anyway.

A rebuilt deck is built to today's code. In Wisconsin that means footings at least 48 inches down, a 40-pound-per-square-foot live load, guards on any side more than 24 inches above the ground, ground-contact lumber where it touches soil, and G-185 galvanized or stainless hardware. (SPS 321) (SPS 325 App. B) The 24-inch guard rule is stricter than the 30 inches in national deck guides, so a guide written for another state can be wrong here.

Composite boards on an old frame

Composite boards need closer joists than many old wood decks were built with, and they go on a frame that has to last as long as they do:

• Joists no more than 16 inches apart, and 12 inches for diagonal boards. A wood deck on 24-inch joists needs a joist added between each pair. (Trex)
• Check the frame first for soft spots, splits, moisture, corroded fasteners and how it's attached to the house. Run a string line across the joists and fix high, low or crowned ones. (TimberTech)
• Joist tape on the tops of clean, dry joists keeps water out of the spot where framing rots first. (TimberTech)
• In Wisconsin, composite decking has to carry an ASTM D7032 label. (SPS 325 App. B)

With the old boards off, every joist top is exposed. That's the easiest time to replace or sister weak joists.

Deck repair permits by town

• Madison. Every deck needs a permit. Boards under $500: none. Boards, handrails, guardrails: online repair permit. Stairs, joists, beams, columns: a permit or a check with staff. Posts, footings, joists, beams: plan review. Decks 3 feet or higher must meet setbacks and need a site plan; lakefront lots need a setback survey. Most new decks under 250 sq ft are $75. (City of Madison) Call City of Madison Building Inspection, 608-266-4551.
• Fitchburg. Decks 24 inches or more above the ground need a permit; uncovered decks lower than that don't. Maintenance repairs not otherwise listed are exempt up to $2,000. (City of Fitchburg) Call City of Fitchburg Building Inspection, 608-270-4240.
• Sun Prairie. Replacing deck boards needs no permit. Replacing or altering a deck needs one, with a plan. Rear setback is 8 feet for a deck up to 3 feet high and 24 feet above that. (City of Sun Prairie) Call City of Sun Prairie Building Inspection, 608-825-1184.
• McFarland. Decks go under the village's General Permit, with a site plan, 3 to 5 business days before you start. (Village of McFarland) Call Village of McFarland Building Inspection, 608-838-3154.
• Middleton, Monona, Verona, Shorewood Hills and Maple Bluff don't publish deck repair rules. Call before you start: City of Middleton Building Inspection, 608-821-8370; City of Monona Building Inspection, 608-222-2525; City of Verona Building Inspection, 608-848-9940; Village of Shorewood Hills, 608-267-2680; Village of Maple Bluff Building Board, 608-244-3048.

Anywhere you dig new footings, call Diggers Hotline at 811 first.

Old deck lumber

A deck framed before 2004 is probably CCA lumber, which contains arsenic. The EPA doesn't require removing it. If you tear it out: don't burn it, don't use it as mulch, wear goggles and a dust mask when cutting, and wash your hands after. It can usually go in household trash, but check local rules. (EPA) Madison's handout says treated scraps go in regular garbage. (City of Madison)

Frequently asked questions

Can I just replace the deck boards?

Yes, if the frame under them is sound. Probe the joist tops, the ledger, the beam and the posts first. In Madison, replacing boards costing under $500 needs no permit, and above that it's an online repair permit with no plan review.

How do I tell if deck wood is rotted?

Push an ice pick or screwdriver into it. If it goes in 1/4 to 1/2 inch easily, breaks off in a short piece without splintering, or the wood feels soft and spongy, it may be decayed. Check the ledger, post bases, the tops of the joists and the bottom of the stair stringers, where water sits longest.

Do I need a permit to repair a deck in Madison?

Not for deck boards under $500. Replacing boards, handrails or guardrails above that is an online repair permit. Replacing stairs, joists, beams or columns needs a permit or a check with Building Inspection, and replacing posts, footings, joists or beams goes through plan review.

Can I put composite boards on my old deck frame?

Only if the frame is sound and the joists are close enough together. Trex and TimberTech both call for joists no more than 16 inches apart, and 12 inches for diagonal boards. A wood deck on 24-inch joists needs a joist added between each pair.

How deep do deck footings have to be in Wisconsin?

At least 48 inches below the ground next to them, or below the frost line if that's deeper, under the Uniform Dwelling Code. Madison inspects the holes before the footings go in.

When we look at your deck

Tell us which parts failed the probe test when you ask for a quote. Boards and railings on a sound frame are a repair. A soft ledger, post or beam usually means rebuilding the frame, and we'll tell you which one you're looking at. See deck building and repair.

Get a free quote for your deck.