How do you jack up a sagging floor joist?

You jack up a sagging floor joist slowly, with a temporary screw jack or post under the low spot, then you put permanent support in place so it holds. A common fix is an adjustable steel column, a lally column, set on a proper concrete footing under the beam or joist. Floor support runs about $1,300 to $2,500. Never crank a sagging floor flat in one day. You lift a small amount at a time over weeks so the framing and finishes above can adjust without cracking.

That last part matters. A floor that sagged over years took years to get there. Lift it too fast and you trade a sagging floor for cracked drywall, stuck doors, and popped tile.

Here is how the work actually goes. We figure out why it sagged first. Sometimes a joist is undersized or split. Sometimes the beam it sits on is sagging between supports that are too far apart. Sometimes a wood post rotted at the bottom because it sat on damp basement or crawl space dirt. The fix depends on the cause, so we look before we lift.

For the lift itself, we set a temporary jack and a beam to spread the load under the low run. Then we raise it in small steps. Once the floor is back where it should be, we install the permanent support. Most of the time that means a lally column on a footing, or a few of them under a beam. If a joist is cracked or rotted, we sister a new joist alongside the old one. If a main carrying beam has failed, that is a beam replacement, which falls under sill and beam work.

What drives the cost is how much support you need and what shape the wood is in. One lally column under one beam is the low end. A long sag that needs several posts, new footings, and a sistered joist or two costs more. If the beam itself is shot, sill and beam replacement runs about $7,000 to $40,000. We give you the number after a free inspection, not before, because guessing a price on a floor we have not crawled under would be guessing.

Here is where we tell you to slow down. Not every soft floor needs piers or a full beam swap. A bouncy floor over a long span often just needs a post or two added in the basement. You may not need the big structural job a salesperson three towns over tried to sell you. We will tell you that to your face if it is true.

Now the part people miss in New Hampshire. A lot of sagging floors trace back to water and rot under the house. A post or a beam end that sits in a damp basement or a wet crawl space rots from the bottom up. You can jack the floor and add steel all day, but if the space stays wet, you are fighting the same battle next decade. So we check the moisture too. If your basement takes on water, the long fix is interior drainage and a sump, the Forever Dry System. If it is a crawl space, that is crawl space encapsulation with a vapor barrier and a dehumidifier. Dry wood holds. Wet wood gives.

One more local note. NH frost heave moves foundations, and water pools against the house because the backfill around it is loose, disturbed soil that drains slowly and holds water. Both can push framing out of level over time. Fixing the support without fixing the cause is half a job.

Lally columns and floor support carry a 10-year warranty from us. Helical and push piers, if your job needs them under a settling foundation, carry a 25-year engineered warranty. We self-perform this work with our own crew. No subcontractor showing up you have never met.

If your floor dips, bounces, or your doors started sticking, let us crawl under and look. We will tell you what it needs and what it does not.

See our structural and foundation repair page, our supplemental support options, sill replacement, and what we promise on every job in the 603 guarantee. If water is part of the story, start with basement waterproofing or crawl space services.

For general guidance on what soil moisture does to foundations and framing, the U.S. Department of Energy’s Building America program is a solid neutral reference (basc.pnnl.gov). For how foundation and structural issues interact with homeowners insurance, the Insurance Information Institute (iii.org) is worth a read.

Ready to fix that floor?

Call us at 603-610-1770 or book your free inspection. Free estimate, and you get your quote within 24 hours.

Related questions

Can I jack up a floor myself? You can add a temporary post to stop a bounce, but a permanent fix needs a footing sized for the load and a controlled lift over time. Done wrong, a fast lift cracks walls and ceilings. Floor support from us runs about $1,300 to $2,500 and carries a 10-year warranty.

How long does it take to jack up a sagging floor? The lift is done in small steps over days or weeks, not in one afternoon, so the framing above adjusts without cracking. The install of permanent posts is usually quick once the floor is back to level.

Will my homeowners insurance pay to fix a sagging floor? Usually not, because most policies treat settling, age, and wear as maintenance, not a covered sudden loss. The Insurance Information Institute (iii.org) explains what home policies generally cover. Check your own policy.

Do I need helical piers for a sagging floor? Often no. A sagging floor inside the house usually needs a post or beam support, not piers. Helical or push piers are for a foundation that is settling into the ground. We will tell you which one you actually have.