Is Sagging Floor a Major Problem?

A newspaper ran the following article recently. It caught my eye because the subject concerns one of the most common problems I address in my engineering practice.

The columnist provided some information that fit in the limited space allotted but not nearly enough to really address the issues. For example, s/he says there are four reasons a floor can sag, exactly four:

The unfortunate truth is that there are more than four causes. Here are, in my experience, the most common:

* Improperly compacted fill material that when the house was new appeared solid, but instead, over time, compacted, taking part of the house down with it.

* Soil with a lot of organic material that over time rots and loses volume. Organic material can be roots, stumps, or anything else that once was living.

* Saturated clayey soils that change volume as they lose or gain water. Clayey soil is like a sponge, when it’s dry it wants to absorb water and get larger. When it’s wet, water can be squeezed or evaporated out resulting in shrinkage.

* Landslides, which generally only happen on sloping sites. This is where a big chunk of earth develops a failure or slip plane underground and then moves downhill. Any structure built on top will go along for the ride. Landslides can travel slowly (fractions of an inch per year) or rapidly (many feet per second.)

* Wood under attack from pests. When a portion of floor is attacked by microorganisms, termites, or other wood-eating critters, said wood will disappear taking down whatever is on top.

You can see that my list is mostly about bad soil. The author doesn’t mention soil at all.

Also, are we talking about sagging floors or sloped floors? To me a sag is a low spot in or near the middle. That’s quite a bit different than a floor that heads down but doesn’t come back up. The writer says, “I feel that I’m walking downhill…” So is that downhill to the middle of the room then back uphill? Or downhill all the way across the room?

Floors, especially 1^st-floors, rarely sag. And when they do it’s usually because bad soils are present, not because of some framing deficiency.

Let’s say, however, there really is a sag caused by bad floor framing, i.e. the foundation and soils are competent. How much sag is too much? For floors, the maximum amount of deflection (sag) per code under full load is l/240. The “l” is the member’s span in inches. So for example, if a joist’s span is 16-feet (192-inches), the maximum deflection would be 192/240 = 0.8-inch. Any 16-foot joist that deflects more than that is not in compliance with code. Now, if the joist is only 6-feet long (72-inches), the maximum code-allowed sag is 72/240 = 0.3-inch. That would hardly be noticeable.

Why wouldn’t I expect 1^st-floor joists to sag? Most span only 6 – 8 feet. These relatively short joists, even when significantly undersized, are not likely to sag much. A heavy load, such as a pool table or waterbed, will likely be spread over several joists and a couple beams, which typically wouldn’t span more than 6 or 8-feet either and also are not prone to significant sag. So as long as the soil supporting the posts and piers is competent, I wouldn’t expect much sag in a typical 1^st-floor system.

[For Gearheads Only: To make sense of the above paragraph you need to know what “undersized” really means. There are three criteria that engineers check in the design of any beam or joist: shear strength, bending strength, and deflection. A member’s adequacy has mostly to do with span. Short undersized members are more likely to fail in shear. Longer members typically have trouble with bending and deflection.  A shear failure commonly results in rupture at or near a support. Bending and deflection problems are indicated by sagging and bounciness. So long span joists (typically greater than about 14-feet) are more prone to sag or bounce, and short-span ones, if overstressed, are more likely to crack or split near a support. Going back to the above paragraph, most 1^st-floor systems are made up of short span joists and beams that if distressed would likely experience shear cracking not deflection or sagging.]

Let’s look at the author’s four causes one-by-one.

1. Improperly sized floor joists. Floor joists should be 2×10 Yellow Pine or Douglas Fir.

I just about fell out of my chair when I read this. It is a bad statement on several levels.

A. Improperly sized floor joists rarely sag. They may well be bouncy, but a sag indicates permanent deflection – a rarity. I’ve seen many more sagging ceilings than floors, mainly because ceiling joists are more likely overspanned than are floor joists.

B. Floor joists need not be 2×10, they could be anything: 2×6, round poles, 2×12, 4×8, any of dozens of manufactured I-joists (TJI is a trade name for one brand), etc. Saying a floor joist should be a 2×10 is like saying baseball is Derek Jeter. For me, baseball is the Mariners, and little league, and Willie Mays, and bringing your mitt to a ballgame, and home runs, etc. The right sized floor joist depends on loading and span, both of which can vary wildly.

C. Floor joists need not be Yellow Pine or Douglas Fir, they could be Hem Fir, Cedar, Redwood, Oak, a manufactured I-joist, you name it. There is no wrong material for a floor joist, you generally pick the most cost-efficient, code-compliant one.

Selecting the right floor joist is a pretty simple matter with good software. This link from my company’s website shows an example of how it’s done: http://www.constructioncalc.com/blog/wp-content/uploads/2009/05/example-floor-joist-9-13-051.pdf  Note that the Results Section shows both sawn lumber and I-joists at the same time – very handy.

2. The floor joists’ span between supports may be too long.

It’s true that an overspanned joist can cause problems but not likely a sag, especially with a 1^st -floor system.

3. Decay or infestation can weaken the floor joists.

Sure, but what about the subfloor, beams, and posts? Why limit decay or infestation to just joists? I think most seasoned home inspectors will tell you that rot or decay is as likely to affect the other structural components as joists.

4. Improper loading, for example a pool table or waterbed might weigh more than the floor was designed to support.

I suppose a heavy load could cause sagging but it’s not likely. While it’s true that waterbeds and pool tables are heavy, they’re not much, if at all, heavier than the code-prescribed loads to which floors should be designed. For example, a high-quality slate pool table might weigh 800 lbs, which if spread over its approximately 23 square feet, yields a unit load of about 35 pounds per square foot, easily below the code prescribed minimum of 40 psf. A waterbed that has about 8” of water in it would weigh about 42 pounds per square foot, including the wood frame. Add people and you’re at about 50 psf. While this is 20% more than code allows, it shouldn’t cause a problem with a code-compliant floor system. Why? Because wood structures built in conformance with code have a 2.5 +/-  factor of safety included. That means a 40 psf code-compliant floor could theoretically take up to 40*2.5 = 100 psf before sustaining serious damage. A waterbed is half that.

But let’s say the floor isn’t code-compliant – that if calculated would only support 20 psf. Now would we have a sag? Maybe, maybe not. It would depend on the span of the floor joists (see, For Gearheads Only, above.)

What about a sloping floor? This is one with low point(s) at perimeter wall(s) and would give the feeling of walking downhill across a room. This is a far more common problem than a sag. Almost always the cause is settlement of a perimeter footing due to bad soils – but bad soils in isolated places, not under the entire structure. The engineering term for this is “differential settlement.” A rule of thumb for older homes is that differential settlement of more than about an inch is cause for concern. Whether or not it really is a problem depends on how much more than an inch and your tolerance for walking downhill (and of course uphill when you go the other direction.) In my book GREEN FRAMING – AN ADVANCED FRAMING HOW-TO GUIDE there’s a picture of a house I call the Titanic. It’s differential settlement is about 16-inches and yet people still live there. That would be like a perpetual Stairmaster workout . Compared to such a house an inch or two differential settlement might seem like nothing.

Let’s say your house has 2-inches of differential settlement and you don’t mind the feeling of uphill and downhill. And you don’t mind trimming doors so they shut correctly, nor pencils rolling across countertops or uneven utensils in cupboards. Should you spend the money to have it fixed? The answer depends on whether the settlement is done or not. I.E., is the bad soil finished being bad?  That’s an awfully hard question to answer unless you know what’s causing the settlement. If, like in the case of the Titanic, the problem is a lot of organic material buried under one end of the house that’s been rotting for 50 years and probably will continue to rot for another 50, then a fix is in order. If on the other hand the problem is that the original builder didn’t  properly compact some of the soil when he built but that soil, over the life of the home, has self-compacted to an equilibrium point, then no, you don’t really need to fix it.

Here’re some more dubious points from the article.

The fibrous materials in wood are somewhat elastic and will stretch under tension.

Wood is very elastic and under normal loading springs right back to its original shape when live loads (people, furniture, etc.) are removed. The problem occurs when permanent loads are too great, causing overstress. In this case wood can deflect permanently, called “plastic deformation” or  “creep”. As stated before, this is most common in overspanned joists. You also see it frequently in overspanned roof rafters.

Regarding tension, tension is only part of the stress story. In any bending member there is also compression and shear. To mention one without the others is kind of like the ying without the yang, a cheeseburger without the cheese, the One Stooge.

Typical 2-by-10 floor joists, spanning 16-feet, can be expected to sag about 1/8 to 1/4 of an inch.

As we discussed earlier, a joist spanning 16-feet can be expected to sag, under full load, 3x – 4x that amount (0.8-inch) and still be in compliance with code. The maximum allowed deflection (sag) has nothing to do with the joist’s size, it is a function only of span; 0.8-inch applies to 16-foot long 2x10s, 4x6s, I-joists, whatever.

The floor system can be repaired…

What would a “fix” be? There are several ways to fix settlement. If the problem is an interior sag due to bad soil, usually the most practical remedy is to jack up the low spot and shim the post to beam connections. This may or may not be permanent depending on whether the bad soil is done settling. Meaning that you may have to repeat the process in a few years. But it’s pretty cheap, and it might work for many years. Certainly if the soil under a few posts and piers is obviously bad, it should be removed and replaced with compacted granular soil, and the footing replaced. Such operations are usually challenging because most crawl spaces don’t have a lot of room to maneuver. Still, however, these types of repairs are made regularly by nimble, unclaustrophobic people.

The best fix for any settlement involves underpinning: the installation of piles, usually helical anchors or pin piles. These are small diameter steel piles screwed or pounded into the bad ground to a depth such that they reach good soil and or develop sufficient strength to permanently hold up the structure. Installing them under a perimeter footing isn’t too big a deal as long as heavy equipment (small excavator usually) can access the settled area. You can see, I think, that attempting such a repair on a sagged portion of floor would involve driving an excavator into the house, which is not recommended, nor safe. Even small excavators weigh 3 -5 tons. Underpinning can easily cost $15,000 or more depending on the extent of lifting needed.

If a sag is caused by inadequate framing, i.e. undersized joists and or beams, usually the best remedy is to add more support in the form of new beam(s) and or new post(s). If done in the crawl space, this shouldn’t cause any ancillary problems, though as mentioned above, doing it presents certain confining challenges. If the floor in question is a 2^nd floor, new beams and posts will definitely have an effect on the living space below. Another option is to add extra joists. This can be done on any floor but if it’s the 2^nd floor you’ll have to tear off the ceiling to gain access.

The floor may need to be raised using jacks over several months before adding a new beam.

I think the author is talking about fixing plastically deformed (permanently sagged) members here. That’s the only explanation I can think of for taking several months. I suppose you could attempt it, but who has several months to drag out a repair? And even then, it’s doubtful you’d recover all of the creep. A more realistic approach would be to jack up as much as possible without causing damage then do the repair. If you don’t get it back perfectly level, so what? The house is over 50 anyway. (I’m over 50 for crying out loud.) Then again if you’re that persnickety, you probably could afford to tear the bad section out and replace it.

In summary, sagging or sloping floors can be tricky things with many possible causes and potential fixes. An excellent first step for anyone considering purchasing such a home is to hire a competent building inspector or engineer and have them give you the full scoop.

Tim Garrison of ConstructionCalc.com, is a professional engineer, author, and software producer for the building industry. Check out his latest book, “Green Framing – An Advanced Framing How-To Guide” at www.constructioncalc.com

This entry was posted on Tuesday, August 3rd, 2010 at 6:42 am and is filed under Structural Design. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.