Floor joist repair - and who actually cut them

What is floor joist repair?

Floor joists are the repeating members that span between beams and walls and carry your floor. Individual joists fail for a small number of reasons: they were cut, notched or bored through by another trade running services; they rotted at the end where they bear; insects ate them; or water from a leak above got into them. The first of those is the most common and the most overlooked, and unlike the others it usually has an identifiable person who did it - which matters when you are deciding who should be paying.

Signs you need floor joist repair

• A section of floor that flexes, drops or feels different underfoot from the rest of the room, often in a line rather than a patch
• Visible notches, drilled holes or square cut-outs in joists where a pipe, duct or cable passes through, particularly near the ends or on the bottom edge
• A joist cut clean through with the services running where the wood used to be - most often under a bathroom, kitchen or a later-added HVAC run
• Joist ends that are dark, soft, crumbling or fungal where they sit on the sill plate or bear into the foundation
• Hollowed or galleried wood, mud tubes on the foundation wall, or frass - the signature of termites and other wood-destroying insects
• Joist hangers that are rusted, pulled loose, or where the joist has dropped out of the hanger and is no longer fully seated
• Staining, streaking or active drips on the underside of subfloor and joists below a bathroom, laundry or kitchen

Seeing something not on this list? Browse structural repair problems by symptom →

How floor joist repair actually works

1. 1

   Go and look - this damage is almost always visible

   Unlike foundation movement, joist damage is generally right there on the underside of the floor, in the basement or crawl space, in plain sight. Take a bright light and a phone. Photograph everything you find with something in frame for scale, note which room is above each area, and pay particular attention to where plumbing stacks, duct trunks and cable runs pass through the framing. That photographic record is the beginning of every subsequent conversation.

   Watch forInsulation stapled up between the joists. It hides exactly the damage you are looking for, and inspections routinely stop at it. If it comes down anywhere, take it down under the wet rooms.

2. 2

   Identify the damage type, because they have different remedies

   Cut, notched and bored joists have lost section in a specific place and the remedy depends on where. Rot at the bearing end is a moisture problem needing a repair at the support and a fix to the water. Insect damage needs the insects dealt with by a licensed pest professional before any carpentry, and needs an honest assessment of how far the galleries actually run. Water damage from a leak above needs the leak found first. Treating all four as one job is how scopes get wrong.

   Watch forA single line item reading "repair damaged joists". Four different causes, four different remedies, four different follow-up trades - and a lump sum hides which ones are being skipped.

3. 3

   Find out who did it and when

   This is the step nobody tells you to take. If a joist was cut to run a waste pipe, a duct or a circuit, somebody cut it, and often it can be dated - to a bathroom remodel, an HVAC replacement, a basement finish. If that work is recent, was permitted, or was performed by a licensed contractor still trading, the repair may not be yours to fund. If it happened during a purchase you were not party to, it may be a disclosure question. Establish the timeline before you pay for anything.

   Watch forRepairing it quietly and quickly because the floor is annoying you. Once it is fixed, the evidence is gone and so is any recourse against whoever caused it.

4. 4

   Get the remedy specified by someone not selling it

   Whether a compromised joist can be reinforced or has to be replaced depends on how much section was lost, where along the span it was lost, and what the joist carries. That is a determination. Where several joists in a row are involved, or where the damage is at or near a bearing point, an engineer's written specification is what makes competing quotes comparable - see the referral path.

   Watch forA remedy that reroutes nothing. If the pipe or duct that caused the damage stays exactly where it is, the repair has to work around it, and working around it is what compromises the repair.

5. 5

   Move the services, or design the repair around them

   Frequently the honest answer is that the plumbing or ductwork should be relocated so the framing can be made whole. That means a second trade, more money, and sometimes opening a floor above. The alternative - reinforcing around a permanent obstruction - is legitimate when specified properly and a bodge when it is not. Which one you are getting depends on whether anybody sized it.

   Watch forA carpenter quietly notching new wood to clear the same pipe. The new member now has the same weakness as the old one, in the same place, for the same reason.

6. 6

   Execute the repair - sistering, replacement, or work at the bearing

   Adding a member alongside the damaged one is the common approach and has real limits, which are set out on sistering joists rather than repeated here. Full replacement means taking load off that bay temporarily and threading a new member in past ducts, pipes and wiring. Rot at the end is often addressed at the bearing itself, together with whatever is keeping that area wet.

   Watch forSistering used as the default because it is quickest. It is a good method inside its limits and a cosmetic gesture outside them.

7. 7

   Fix the cause, then close up

   Rot means water: a leak upstairs, ground moisture in the crawl space, a bearing detail that stays damp, or drainage delivering water to the foundation. Insects mean a pest treatment and, usually, a moisture correction as well, because the two travel together. None of that is carpentry, all of it is necessary, and it should appear as named lines in the scope rather than as advice given on the last day.

   Watch forA vapour barrier or new insulation going back before anyone has inspected the connections and hangers. Cover it and verifying it costs money again.

What does floor joist repair cost?

One accessible joist reinforced in a walk-in basement is a small job. A run of joist ends rotted along a sill, in a low crawl space, with plumbing to relocate and a moisture source to correct, is a different order entirely. Price the pest, plumbing and moisture work separately rather than inside one lump sum.

See the full floor joist repair cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →