If a section of your floor bounces when you walk across it, or a door in that part of the house has started sticking, the joists underneath are probably the reason. We get calls about this every week. Most homeowners don’t think about floor joists until something feels wrong underfoot, and by then the damage has usually been building for a while. This page covers what causes joist problems, how we fix them, and what it typically costs.
What Floor Joists Actually Do
Floor joists are the horizontal wood beams that run under your subfloor and carry the weight of everything above them: furniture, appliances, people, walls. In most homes built in this region, they’re spaced 16 to 24 inches apart and rest on a foundation wall, a sill plate, or a support beam. When one joist fails, it doesn’t fail alone. The load shifts to the joists next to it, and the sag spreads.
Here’s the part nobody likes to hear: a sagging floor is rarely just a floor problem. It’s usually a symptom of moisture, pest damage, or a foundation issue that’s been developing underneath the house for years.
Common Signs of Joist Damage
- Springy or bouncy spots when you walk across the room
- Visible sag in the floor when viewed from a crawl space or basement
- Cracks in drywall near the floor line
- Doors and windows that stick without an obvious cause
- Musty smell coming from a crawl space, often a sign of rot
- Gaps opening up between the floor and baseboard
Termite tunnels and water staining on the wood are the two most common causes we find in crawl spaces around here. North Carolina’s humidity does joists no favors, especially in homes without a proper vapor barrier.
How We Repair Joists
The right fix depends on how bad the damage is. There are three approaches we use most often, and each one solves a different problem.
| Repair Method | Best For | Typical Cost per Joist |
|---|---|---|
| Sistering | Minor cracking or moderate rot where the joist still has structural value | $100 – $300 |
| Full Joist Replacement | Severe rot, termite damage, or a joist that has lost load capacity | $100 – $2,500 |
| Supplemental Support Beam | Joists undersized for the span, or spans that were never adequately supported | $1,000 – $5,000 |
Sistering means bolting a new piece of lumber alongside the damaged joist so it shares the load. It’s less invasive and usually the cheaper option when the joist hasn’t rotted all the way through. When we do sister a joist, we typically use pressure treated lumber or an engineered product like Weyerhaeuser’s TimberStrand, and we connect it with Simpson Strong-Tie or USP structural hardware rather than just nails, because nails back out over time under repeated load.
For jobs that need adjustable support instead of a permanent beam, we sometimes install screw jacks or steel columns from manufacturers like U.S. Jack or Foundation Supportworks. Their IntelliJack and PowerBrace systems let us fine tune the lift gradually, which matters because raising a sagging floor too fast can crack drywall and tile upstairs.
What This Runs, Roughly
Nationally, floor joist repair averages somewhere between $4,000 and $12,000 for a full project, though a single sistered joist can run as low as a few hundred dollars. In the Raleigh area, older homes with crawl spaces, particularly ones built before the 1980s, tend to land on the higher end of that range once you factor in moisture remediation and access difficulty. A structural engineer’s assessment, if one is needed, typically adds $300 to $800 to the project.
We don’t quote a number over the phone for this kind of work. We need to get under the house and see what we’re actually dealing with.
What Happens If You Wait
Joist damage doesn’t fix itself, and it doesn’t stay the same size either. A joist that’s 20% compromised this year can be 50% compromised in three, especially if the moisture source hasn’t been addressed. Waiting also tends to turn a sistering job into a full replacement job, which costs more and takes longer.
If your floor has started to feel different underfoot, or you’ve noticed any of the signs above, we’ll come take a look and tell you honestly what we see. No pressure, no upsell on work that isn’t needed.