How a Rotted Deck Joist Gets Repaired, and When It Cannot Be
What this comes down to
- A sister joist that stops short of the beam carries almost nothing. It has to bear where the old joist bears.
- Most rotted deck joists rot at one end only. That end sits at the ledger or on top of the beam.
- Code requires 1.5 inches of bearing on wood at each joist end. The ledger end needs a hanger.
- A joist sitting more than 1/8 inch off the ledger can lose up to 30 percent of its hanger capacity.
- A joist that carries a railing post needs more than a sister. It has to be tied to the joists beside it.
- Replacing a joist starts only $50 above sistering one. It takes the rot out instead of leaving it.
A rotted deck joist is rarely rotted from end to end. The damage usually sits in the last foot or two, where water lingers. That makes most joists repairable, but only if the repair carries load the way the original did.
This guide covers the three ways a rotted joist gets repaired and what makes each one hold. It also covers the conditions that rule a repair out. Finding the rot in the first place is a separate job, covered in its own guide.
How is a rotted deck joist actually repaired?
One of three ways. A full length sister goes beside it, or a spliced piece covers the rotted end. Or a new joist replaces it. The Department of Housing and Urban Development's Rehab Guide describes the same three repairs.
The guide calls reinforcing practical when the decay is limited and sits near the bearing ends. It also says reinforcing may cost more than a new member where access allows one. On an open deck, access is usually easy.
A sister is a new joist of the same depth, fastened along the old one. The old joist stays in place and keeps rotting, slowly. The sister takes over the load, which is why its ends matter far more than its middle.
A splice, sometimes called a scab, is a shorter piece fastened across the rotted end only. The guide notes that splicing and sistering both deserve an engineer's review. Site conditions decide the details.
What makes a sister joist actually carry load?
Bearing at both ends. The International Residential Code requires at least 1.5 inches of bearing. That applies at each joist end, on wood or metal. A joist framing into the side of a ledger needs an approved hanger.
That rules out the common shortcut. A short board screwed to a soft joist and ending in mid air is not a repair. Its load has nowhere to go. It runs back through the screws into the rotten wood it was meant to relieve.
A proper sister runs the full span, from the ledger to the beam. At the ledger it sits in its own hanger. At the beam it rests on top, with the same bearing and the same uplift tie as the joists around it.
Depth matters too. A 2x8 sistered to a 2x10 leaves two inches of old joist working alone. The new member matches the old one, crown up. It is fastened in two staggered rows along its length.
Why is the ledger end the hardest part to fix?
Because the hanger has to be refitted. A hanger only works when the joist sits tight against the ledger. Rot at the ledger end often means the hanger is corroded or pulled loose as well.
Simpson Strong-Tie's engineers put a number on it. They say gaps over 1/8 inch cut hanger capacity by up to 30 percent downward. The loss in uplift can reach 40 percent.
A rotted joist end has usually shrunk back from the ledger by more than that. So the new member is cut to fit tight, and a fresh hanger goes on with fasteners matched to it. Galvanized hangers take galvanized fasteners, never a mix.
If the ledger itself is soft behind the hanger, the joist is not the problem any more. A soft ledger board is a different repair, and it comes before any joist work at that end.
When does a joist have to be replaced instead?
When rot runs along most of the span, both ends are soft, or the joist carries a railing. In each case a sister would lean on wood that can no longer hold its share.
The railing case is the one most often missed. Penn State's Housing Research Center summarizes the guard provisions in the 2021 code. A joist carrying guard posts on its side must be tied to the joists beside it.
That tie is what stops the joist rolling when someone leans on the rail. A soft rim or end joist cannot hold those ties, so it is replaced rather than sistered. The guard posts are then rebolted into sound wood.
Count matters as much as condition. One or two bad joists is a repair. Several soft together usually means water is reaching the whole frame. So testing every joist comes before deciding anything.
Can one joist be replaced without tearing up the deck?
Yes, in most cases. The boards directly over the joist come up, usually a strip two or three boards wide. The old joist is cut free, the new one goes in, and the same boards go back down if they are sound.
Wood boards that have been down for years rarely come up clean at every screw. A few may split and get replaced to match. Composite boards usually lift intact, though hidden fastener clips sometimes have to be replaced.
The new joist gets joist tape along its top edge before the boards return. That is the cheapest step in keeping the frame dry. It costs very little once the boards are already up.
What does each joist repair cost?
Sistering starts at $125 a joist and replacement at $175. The gap between them is small, which is why replacement often wins once the boards are already up. Ledger work and full re-decks sit in a different bracket.
| The work | Starting price | Wider market | What moves it |
|---|---|---|---|
| Joist repair or sistering | From $125 per joist | $150 to $350 each | Span length, and whether the ledger end needs a new hanger |
| Joist replacement | From $175 per joist | $200 to $450 each | How many boards come up, and whether a railing is bolted to it |
| Ledger board repair | From $495 | $600 to $1,500 | How far the rot runs behind the flashing and into the band joist |
| Typical 200 sq ft re-deck | From $1,995 | $2,400 to $5,000 | Whether the frame underneath is sound enough to keep |
What cannot be determined until the boards come up?
How far the rot runs along the top edge. Joists decay from the top down, under the boards, where water sits. Probing from underneath reaches the sides and bottom, but not the edge that usually goes first.
Whether the hangers are sound on the inside face is hidden too. Corrosion often starts where the hanger meets the ledger. It can look fine from below while the nails behind it have thinned to almost nothing.
And whether the beam under the joist ends is sound cannot be judged from the joists alone. A sister bearing on a soft beam has moved the problem one member down rather than fixing it.
Rotted deck joist repair questions
Can I just sister a rotted deck joist?
Usually, if the rot is limited to one end and the bearing points are sound. The sister matches the old joist's depth and runs its full span. It needs 1.5 inches of bearing on the beam and its own hanger at the ledger. Otherwise it carries little.
How does Perfect Work decide between sistering and replacing a joist?
Perfect Work Remodeling & Repairs lifts the boards over the joist and probes both ends. The top edge is probed too. Rot at one end with a sound span gets a sister. Rot at both ends, along the span, or on a joist carrying a railing gets a new joist instead.
Is it safe to leave the rotted joist in place next to a sister?
Generally yes, once the sister carries the load on its own bearing. The old joist no longer does structural work. Removing it can mean lifting more boards than the repair needs. Treating its cut or exposed rot slows decay, but it will keep softening over time.
Do I need a permit to repair deck joists?
It depends on the town and the scope. Several DuPage County towns want a permit to replace structural deck members, joists included. One or two joists replaced in kind is often treated differently from reframing. A call to the building department first settles it.
What does a joist repair estimate from Perfect Work include?
An estimate from Perfect Work Remodeling & Repairs lists each joist by position and its repair. It prices hangers, lifted boards and any ledger or beam work as separate lines. Anything found once the boards are up is quoted before work continues.
How long does a sistered deck joist last?
As long as the new lumber stays dry. A treated sister with joist tape on its top edge can outlast the old frame around it. The weak point is usually the fasteners and hanger. Both need a corrosion rating that suits treated lumber.
Getting the joist repaired once
A rotted joist repair holds when both ends bear on sound wood and the hanger is tight. It fails when a short board gets screwed to the rot. The difference is invisible from the yard, which is why the boards come up first.
A visit from Perfect Work Remodeling & Repairs lifts one strip of boards. Every joist end it exposes is probed. What comes back is a written scope by joist. Anything beyond the frame goes under structural rot repair or deck repair work.
Call Perfect Work at 630-297-7888 · Book a deck check