Crawl space structural guide
Sistering and replacement are not two prices for the same job. They fix different problems, and picking the wrong one either wastes money or leaves decaying wood carrying your floor. One physical test decides most cases.
The test that decides it
Almost every repair-or-replace argument comes down to one physical question, and you can answer it yourself with a screwdriver.
Sistering works by bolting a second member face to face with the first so the two share the load. That sharing depends entirely on sound wood holding the fasteners. It is a stiffness repair, not a substitute for missing material.
So the question is never really “how bad does it look?” It is whether enough solid wood remains to bolt to. Press a screwdriver or an awl into the worst area with firm hand pressure. Wood that resists is structurally present, even if it is stained, dark, or has surface growth. Wood the tool sinks into has lost its section, and no fastener pattern recovers that.
Check the bearing ends as carefully as the middle. The ends are where load actually leaves the joist and enters the sill plate or beam, and they sit closest to the damp perimeter, so they decay first and matter most.
Probe, do not stab. Driving a screwdriver hard into a joist tells you little and can damage sound wood. And do not lean on, cut, or load a member you suspect is decayed — it may be carrying more of the floor than it looks.
Decision table
Read the condition you can see, then the direction it points. A contractor confirms it in person; this tells you whether the recommendation you receive matches the damage.
| What you find | Usually points to | Why |
|---|---|---|
| Split or cracked along the grain, wood still hard | Sister | The wood has lost stiffness, not substance. A full-length sister restores the section without disturbing the floor above. |
| Sagging or bouncy, no decay, joist undersized for the span | Sister | The original joist is sound but was never deep enough. Sistering adds depth, which is what controls deflection. |
| Surface mould-like growth, wood hard underneath | Sister (after drying) | Growth on the face is a moisture signal, not section loss. Confirm the wood is hard with a probe, correct the moisture, then decide. |
| Soft, spongy, or crumbling wood a screwdriver sinks into | Replace | Decayed wood carries no load and holds no fastener. A sister bolted to rot transfers load into material that is already failing. |
| Rot at the bearing end, over the sill or beam | Replace | The bearing point is where the load actually leaves the joist. Damage there cannot be bridged by a member that has nowhere solid to sit. |
| Insect galleries or frass through the section | Replace | Hollowed wood looks intact from outside. Pest treatment stops the activity but does not restore the section. |
| Notched or drilled through for plumbing beyond code limits | Either | A properly sized full-span sister can restore it, but if several joists were cut the same way, replacement plus rerouting is often cleaner. |
| Charred from a past fire | Depends on depth | Shallow charring can retain most of its capacity; deep charring does not. This is an engineer's call, not a visual one. |
Two of these deserve emphasis because they are where quotes most often go wrong. Growth on the face of a joist is a moisture signal and frequently sits on hard wood, so it does not on its own justify replacing anything — see mould-like growth on floor joists. Rot at the bearing end is the opposite case: it can look like a small patch and still be the one condition sistering genuinely cannot solve.
Where sistering fails
Sistering is a good repair used in the wrong place more often than it is a bad repair.
Rot at a bearing point. The sister has nowhere solid to land, so the load still passes through failing wood at the sill or beam.
A short scab instead of a full-span member. A sister only carries load where it is supported at both ends; a patch over the damage stiffens the patch.
Decay left in place and sealed over. Encapsulating or spray-treating around soft wood hides the problem without restoring any capacity.
There is a fourth, quieter failure: sistering onto wood that is still wet. Fasteners in damp lumber loosen as it dries and shrinks, and the moisture that softened the original is still working on both members. Drying the crawl space is part of the repair, not a follow-up task.
Cost difference
Planning ranges for comparing scopes, not quotes. Access and the number of members move these more than the method does.
| Approach | Typical planning range | What drives it |
|---|---|---|
| Sister one joist | $150-$500 per joist | Access, member length, whether the floor needs supporting while it is fitted. |
| Replace one joist | $350-$1,000+ per joist | Temporary shoring, removing the old member, plumbing or ducts routed through it. |
| Room or bay of joists | $1,000-$20,000 | Number of members, crawl height and access, whether the floor is being levelled. |
| Moisture correction first | Varies by source | Drainage, a vapour barrier, or a plumbing repair, depending on what caused the damage. |
Note the overlap. A single replacement can cost less than sistering three joists, which is why the count matters more than the method once damage spreads. Full figures and what moves them are on the floor joist repair cost guide.
Reading the quote
A scope that names the member and the cause is one you can compare. One that says only 'repair joists' is not.
If two quotes disagree on sister versus replace, that is usually a disagreement about how much sound wood remains — ask each to show you the spot they probed. The quote analyzer checks whether a proposal documents these things clearly enough to compare.
Related pages
Where the damage came from, how the work is done, and what it costs.
The symptom side: what decayed joists look like and how far it usually goes.
Read moreHow sistering and replacement are actually carried out, step by step.
Read morePlanning ranges by scope, access, and number of members.
Read moreWet rot, dry rot, and the moisture conditions behind both.
Read moreWhen the floor moves but the joists themselves may not be the cause.
Read moreFix the moisture first, or the new wood follows the old.
Read moreFAQs
Short answers to the questions homeowners ask before approving structural work.
Sister the joist when the wood is still hard and the problem is a crack, a split, or a span that was always too shallow. Replace it when decay, insect damage, or rot at the bearing end has destroyed the wood itself. The deciding test is simple: press a screwdriver into the damaged area. If it sinks more than about a quarter inch, there is not enough sound wood left to fasten a sister to, and replacement is the honest answer.
Not properly. Sistering works by sharing load between two members bolted face to face, and that transfer depends on sound wood holding the fasteners. Rotted wood will not hold a bolt under load, so the sister ends up carrying everything alone while the original keeps decaying beside it. If the rot is limited to a short section well away from the bearing points, a contractor may cut out the decayed length and splice in new material, but that is a replacement of that section, not a sister.
As planning ranges rather than quotes, sistering commonly runs about $150 to $500 per joist, while replacing one typically runs about $350 to $1,000 or more. Replacement costs more because the floor above usually has to be supported temporarily, the old member removed, and utilities routed through it dealt with. Whole-room work of either kind more often lands in the $1,000 to $20,000 band depending on access, the number of members, and whether moisture repairs come first.
It should run bearing point to bearing point wherever access allows, because a sister only carries load where it is supported at both ends. A short scab bolted over the damaged spot stiffens that spot and does very little for the span. Crawl spaces often make full-length sisters awkward to manoeuvre, and that practical constraint is one of the more common reasons a contractor recommends replacement instead.
Yes, and it is the step most often skipped. New lumber decays on the same schedule as old lumber when it sits in the same conditions. If the cause was standing water, a plumbing leak, ground vapour through bare soil, or humidity that stays above about 60 percent, correcting that comes first or the repair is on a timer. Ask any contractor what they believe caused the damage before you accept a scope that only replaces wood.
Some homeowners do, on a single accessible joist with sound wood, where nothing structural depends on lifting the floor. It stops being a DIY job quickly: temporary shoring, gradual lifting, correct fastener patterns, and bearing details all matter, and getting a full-length member into a crawl space is genuinely difficult. Anything involving several joists, a beam, a bearing point, or a floor you intend to level should be planned by a professional.
There is no code number, but the economics shift when a whole bay or room is affected. Once several joists in a row need work, the temporary support, access, and labour are largely shared, so replacing the run can cost little more than sistering it and leaves no decayed wood in place. Widespread damage also suggests the moisture problem was long-running, which is worth investigating before choosing either.
Sources
This page is educational and cannot assess your floor. Structural decisions need an in-person evaluation, and sometimes an engineer.