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Question DetailsAsked on 5/15/2016

Sagging in floor joints are typically caused by

Why is that sagging is caused?

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2 Answers


Oh, let me count the ways - what is that, Shakespeare ?

1) normal loads cause sag, sometimes visible in wood framed floors and roofs, because the materials supporting roof and floor bend as they pick up the structural loads - just like the cushions on the sofa deflect when you sit on them. Normally not visually objectionable except in smooth finish ceilings but can be enough to let a marble start rolling across the floor if you set it down. Generally not more than 1/2 inch or so in new construction.

2) above sag gets more pronounced with time because the materials (especially wood and plastic, to limited extend in concrete, very minimal in steel construction) creep with time under load, so you can easily get an inch or so of floor joist/ceiling sag in a typical 12-20' span over several decades of time - I have seen 300 year old houses with 3-5 inches of floor sag even though the wood was in very good shape. Of course, longer spans like 30-40 feet, especially with dimensional lumnber as opposed to built-up plywood joists or trusses, can develop 2-5 inches of mid-span sag over a long period. High humidity (seasonally or always) promotes wood sag, so old (say 50 year+) deep south/tropical houses will have a lot more roof/floor sag and even house swaybacking than a comparable aged northern or temperate climate house.

3) soaking from leaks of course aggravates the above and doesnot go away after it dries out, and can get very severe if wet or dry rot sets in, with wood fungus eating away at the structure and weakening the members so they sag and eventually fail. Common issue under bathrooms with leaky showers or plumbing.

4) can be caused by overloading of the surface - excessive roof load (commonly more than 1 foot of normal type snow in low or no-snow load design areas, more than 2-3 feet generally in snow design areas, though rain-on-snow or melting and compacting over a season can make as little as as about 3-4 inches of icy snow or ice glaciering/ice damming reach critical load. Floor loading can cause sag too - though normally takes pretty substantial tall bookshelves or similar located away from edges of the floor which are supported on walls to do this. Common causes of excessive floor loading are waterbeds and significant stacks of heavy (usually books or paper files) storage boxes or file cabinets, especially if over a few feet high or pretty much wall-to-wall.

5) defective structural materials - notches or slots cut in them wrong places or during utility installation through them (plumbing and HVAC ducts are common causes of this, electrical generally not a problem if done through ceneter area because holes tend to be small), or cracks or significant defects in the mateials or construction or design, making it too weak in the first place

6) foundation settlement - sometimes wall foundations especially if settling dramatically, but more common cause is loss of support of mid-span piers or posts under floor joists in basements or crawlspaces - due to rot, insect damage, digging by rodents under foundation, wetting of the ground causing support pad settlement

7) insect (termite, carpenter ant, post beetle, etc) damage to the structure

8) improper modifications by contractors or homeowners, removing load-bearing elements in a remodel

If you want to describe your situation - what is sagging, about how much, is it an even swaybacking curve or a distinct location like cracking, if a supporting post or pier is loose or missing, etc I can respond back more definitively maybe. Use the Answer This Question yellow button right below your question to reply back so it stays in this same question "thread" - and you can use the leftmost yellow icon at the top of the "Your Answer" text reply box which comes up when you click that to attach photos if you want. JPG, TIFF, couple of other photo file formats allowed - click it and it should tell you allowable formats.

Answered 3 years ago by LCD

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