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How to Fix Sagging Floors

Understanding the common structural shortcomings that cause floors to sag and what to do about them.
By Steve Jordan

    Underfloor diagramOne of the most common complaints of old-house owners is sagging floors. In my own house, for example, every floor pitches toward the center stairwell. Although generally only an annoyance, sagging floors can be an indication of worsening problems. Here’s a quick review of the most common problems and a few of the typical remedies.

    Investigate the Problem

    Typically, floors settle near the center of the house because the perimeter walls are constructed over a sound, deep foundation and settle very little. Major support beams within this perimeter, though, are often supported by makeshift posts.

    Support posts should not sit on dirt floors, but instead be upgraded to concrete pads with footings that spread the load.

    Support posts should not sit on dirt floors, but instead be upgraded to concrete pads with footings that spread the load.

    If your house is built over a basement, first inspect all of the basement support beams and posts where they meet the floor. Be suspicious of wood posts set on dirt floors or wood posts with concrete poured around the post bases. As the posts slowly rot and melt into the floor, the house settles accordingly, bottom to top. As a test, firmly push a metal probe or screwdriver into the post at the floor line. If this area is mushy, punky, or rotten, you may have found your problem. Also look for floor joists that have been cut improperly to install pipes, wiring, or HVAC ducts. If you’ve had a chronically damp basement or crawlspace, look for indications of insect damage to structural members. Powderpost beetles leave joists and beams riddled with small holes, carpenter ants are usually apparent at the first sign of warm weather in the spring, and termites usually leave telltale mud tunnels on foundations and posts. Then solve moisture problems around and under the house and repair deteriorated or compromised structural members.

    Improper holes and notches from alterations and running service lines are a major source of weakened joists. Generally there should never be any cuts or penetrations in the middle third of any joist, or anywhere along the bottom of the joist. Notches at the end of a joist should not exceed 1/4 of the joist depth. Center notches (B) should not exceed 1/6 of the joist depth. Holes should be a minimum of 2″ in from the top or bottom of the joist, and no larger than 1/3 the depth of the joist.

    Plan the Remedies

    Depending upon the conditions, it is possible to strengthen or repair existing framing members, such as floor joists or roof rafters, by adding reinforcing material. Sandwiching the member on either side with plywood is sometimes worthwhile, but the plywood must be installed correctly for greatest strength. A better option is sistering, where identical lumber is bolted to the member. Better still is sistering with a flitch plate, a 1/4″ to 1/2″ piece of steel or plywood. Two flitch plates may also be used to repair localized damage. Where these repairs are not sufficient, also consider shoring up joists or beams that were cut, drilled, or notched for pipes, wires, or ducts.

    Strech a string across the floor to evaluate the amount of deflection, then use it as a benchmark for improvement when jacking.

    Strech a string across the floor to evaluate the amount of deflection, then use it as a benchmark for improvement when jacking.

    One of the good things about floor deflection is that it is repairable. The bad news is that it often takes a long time. The solution to sagging floors, or the damaged sills and joist ends that contribute to them, often involves jacking. A common scenario is to install temporary jack posts and support beams, then permanent posts and beams over new footings. A taut string stretched across the floor will show the amount of deflection and improvement. Posts set on dirt floors should be upgraded to concrete pads with footings. Place wood posts on metal post supports to create a waterproof barrier between the post and the footing.

    Jacking must proceed slowly; it took a long time for your floor to sink, so you can’t push it back up quickly without causing cracks and stress in the building. As with other structural repairs, jacking must also be done appropriately. You cannot simply put a screw jack under the lowest spot and start turning. Ideally, someone with experience will assess the problem and set up the posts and any necessary beams. You can then screw the jacks up a turn or two each month. Expect some cracked plaster along the way, and aim not for perfection, but simply stability and improvement. After all, if perfectly level floors and pristine walls were important to us, we wouldn’t live in old houses, would we?

    Published in: Old-House Journal November/December 2007

    { 14 comments… read them below or add one }

    Mike August 2, 2011 at 11:49 am

    good way to fix sagging second floor ceiling/floor with sister studding old
    1920′s construction to bullon constructed walls to level floor?

    Metal brackets or bracing types ?

    Michaele Dodrow September 22, 2011 at 10:32 am

    Everyone who has been here says this is sorta what I need . A structural engineer was out and said need to support to keep from getting worse, and need to support a few other areas. My problem is noone seems to want to , or has experience doing the jacking. So In sitting here getting dizzy from the rolling floor. It got worse over extremely hot and dry summer. House is 76 yrs old 2 story full basement. with sunroom over crawlspace. Some floors are pulling away from baseboard. Engineer made like it was no big deal. I even have cracks in mud under tile, some tile came loose and were removed, he wasnt concerned said push grout into crack when putting old tiles back down. Mud under kitchen floor sags now, he said just put support under joist. What do you think? Thank You Michaele

    jennifer January 31, 2012 at 2:02 pm

    My house was built in 1928 in Phoenix AZ, built on floor joists with crawl space- no basement. One corner of the house is shifting, cracking. What is the solution for that? I am thinking of adding gutters to that corner to redirect the rain water, and add dirt to create a slope away from the house. Is there some repair I should do to the joists? Please help…..

    David July 18, 2012 at 11:57 am

    My house is 20 years old, has a history of moisture in the crawlspace. (We bought it as a foreclosure). For whatever reason, the family prior to us elected to remove the TYVEK sheeting off the crawlspace dirt. At some point, the home settled in the middle and split the bathroom ceiling upstairs. The stairs are in the middle of the home, along with the air handler for our heat pump heating system which is hanging from the floor joists by threaded rod and Unistrut. Once the joists got a little soft, the weight of the air handler and stairs did the rest and now I have a pretty good sag in my living room right where the air handler is hanging. To make matters worse, the driveway wasnt paved, and erosion took support away from one front corner of the house and the cinderblock wall cracked vertically in 3 places 3 feet apart, but thankfully not in a corner. My question, is can I jack up and build a support frame seated on the ground in my crawlspace and then jack the floor joists to make permanent support beams? This is right where my ductwork for the air handler is, so not a lot of access to the floor joists. F.Y.I., the moisture has been mitigated and a dehumidifier is now running to maintain the RH at 50%.

    Marty kessler September 4, 2012 at 10:59 am

    I just purchased a 12 year old ranch with a finished basement. the previous owner installed granite counter top (3-4 years ago). We have an Island with a 5 by 8 piece of granite and I started to notice a sag in the floor. The kitchen living room is an open area. I also notice when Im in the kitchen and someone walks by or one of my kids run thru the room you can feel the floor move…. Not sure how to correct … 1) remove the grant piece from the island and see if that helps ..2) keep counter top and put in some support beams in the basement (the basement is finished)… Not sure who to turn too for advice as well.

    any suggestions or comments

    Sinai Construction September 8, 2012 at 8:45 am

    @jennifer adding drainage and gutters may help in stabilizing your house but the root cause of your problem may be the settlement of the house foundations itself into the soil – this usually manifests itself through the cracks in the perimeter foundation. check to see if the cracks are propagating or widening [old cracks tend to accumulate dirt and deposits] and if yes, have it checked by a structural engineer.

    @david just like jennifer, the root cause of your problems may be settlement. but in this case, the settlement of the staircase into the ground. before you install and support here and jack up the surrounding structures, address the possible settlement problem of the staircase first as this may result in the same situation in the future. you may have to install underpinning for this section in order for it to be supported by load bearing soils.

    Maria November 13, 2012 at 10:56 pm

    I live in an old row home. The basement walls are the old stone and plaster. I just noticed that in the corner of mu home in the basement, the wall is crumbly causing the joist to sag. Apparently someone had previously “shimmed” it, but I am noticing that my living room floor sags in the area above that joist. I notice that there is at least a 1 inch sag compared to the other beams. It is a very tough corner to get to being that the heater is installed in that corner. Money is very tight. Any suggestions to try to correct this and to keep it from sagging more. Thanks!

    Amy December 6, 2012 at 12:53 am

    I have a 60 year old house that now shows some rot in the floor joist. My problem is that I absolutely can’t afford to have the floors in the house gutted in order to replace these. The floor expert said that although his company would not do it (because they guarantee their work for 2 years and without good joists they can’t jack up the floor) he would recommend that I find someone who will go in my crawl space and add an extra beam the length of the room and then add 4 supports. This would be in my dining room area where the floor seems to be starting to bow in the center. There is another, more troublesome area under a bathroom and hallway area that houses my HVAC. He told me that in this area the joist are in worse shape with some of them having some “chunks” missing. Is there any kind of product that can be sprayed or brushed on the rotted wood and then have stabilizing wood placed on either side of the joist? I have found someone to go under my house and look at the situation. He is certain that he can deal with the dining room floor problem but really needed to see under the bathroom area. Does anyone have any helpful suggestions.

    Virginia January 9, 2013 at 1:24 am

    I am looking at purchasing a 102 yr old 2 story (plus small attic and basement) house that had been moved in the sixties. The foundation looks perfect, dry no cracks, but there is severe bowing and obvious buckling of the lathe and plaster living room wall located in the middle of the house. Previous owner had 1,000s of pounds of music organ and piano and very heavy ornate furniture in this area. The window sills are level believe it or not, but the middle of the first floor is sagging badly. Damp cold can be felt at this level but all other floors are dry. Elect and heat and plumbing need to be completely brought up to code, but my concern is for structural repair cost. A framed wall of sorts was attempted in the basement and shims are loose in areas and the 2×4 are actually bowing!

    Can anyone give me an idea of cost of structural fix? If I just leave it, pay cash and do all the modernizing, a bank will mostly never loan on this home. Is there a cost effective fix? (I like this home and don’t want to find something else)

    Second question: Is it reasonable to figure about 20K for all the basic elect/plumb/HVAC just to move in and not worry about the elect heater burning it down !

    Dolly Hanley March 26, 2013 at 2:43 pm

    We had a built in back porch we use it to enlarge our kitchen The floors are not even what can we do to even them up?

    Barb Villeneuve May 17, 2013 at 9:45 pm

    We are purchasing a house that was built in 1900. A previous owner cut a support beam for a heating duct to run through and now we are trying to fix the sagging floor. The basement is dirt and stone. Any ideas???

    Maria dp Nguyen October 30, 2013 at 5:25 pm

    My house is located on oceanfront it’s six years old, 3 floors, floor tiles on second and third, the house is sitting on the piles. One of contractor took some pictures that showing under the house look likes the joist are compressed or sagging down along with the girders . It also appears the ledger board is bowing down also. Please give any suggestions to correct this, and to avoid the problem happen. Many Thanks.

    tim allen May 8, 2014 at 1:57 pm

    In 1998 I purchased a one story 1920s rental property with a slanting floor in one bedroom. It has a raised foundation and I had guys go under the house to look at what was going on. Everyone who looked at it talked about a sinking foundation and how I had to get mechanical devices to raise the house, etc, etc. I was not interested in spending that much money and the tenants were okay with it. Fast forward to this year, I decided to redo the bathroom next to the affected bedroom and the contractor found three rotten beams, one of which was cracked under the bathroom floor. The beams were 24 feet long and went under the bedroom as well. Replacing the beams took some strong guys and a lot of hammering to get the new beams in place. Short story – slant is effectively gone and it was all done for very cheap.

    Wilma Conway May 17, 2014 at 10:35 pm

    We plan to sell our 2 story home of 40 years. Inspector found a rotting rafter in the crawl space. We do not have a water problem, but he said the ventilation was not adequate. Is there an inexpensive fix for this?

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