Home built in 1975. Moved in a year ago. I’ve been hearing very loud cracking sounds periodically a night, and also sometimes when walking around the main floor. I went down to my basement to look at the joists and discovered many of them to have horizontal cracks many feet along the joist. Also two header joists are cracked all the way horizontally. The third header joists along that same wall seems to be uncracked. Probably 10-15 joists in all that are cracked. I assume this a major problem. I have a structural engineer coming next week to look. Any ideas on what’s the cause and what it’s going to take to fix? I’m very worried what it's going to cost me.
Horizontal cracks may or may not be a problem. That it is happening in several places is not normal and might be a sign that the wood was not adequately dried at the time it was sold. As mentioned in the comments, vertical cracks are a lot more alarming, and I would take a good look at every joist (especially around any knots) to make sure there is not vertical splitting.
If necessary, the joists can probably be reinforced by sistering or otherwise adding framing to support the weakened sections. The cracked header is more concerning, and there is no way to know for sure exactly what will be needed without a more complete understanding of the structure and the extent of the problem. It could be as simple as filling the crack with epoxy, or as complex as requiring the structure be lifted using jacks to allow more extensive (expensive) repairs. The engineer should be able to give you a pretty good idea what you're in for. Good luck!
Joists develop horizontal splits for several reasons: 1) wood drying out, 2) extra large loads on relatively short spans, 3) longer spans, 4) defects in wood grain, 5) combination of items
As wood dries out over time, it shrinks causing horizontal splits along the grain of the wood. Depending on the species and moisture content it may take the grain several years to dry out enough to split. The split is not always in the middle of the joists, depending on defects in the grain. It won’t split from top to bottom (vertically) because the grain is stronger in the vertical direction. That’s why notching joists is dangerous.
When an extra heavy load is applied to a relatively short span (usually less than about 10’ in residential construction) it creates extra horizontal stress in the middle of the joist because the stress on the grain is greater in the middle of the joists than the compression in the top or tension in the bottom. (There’s no waterbed, refrigerator, stacks of books over 2’ high, etc. on the floor above, is there?)
Likewise, for longer spans the stress on the grain in the top and bottom is greater than the stress on the grain in the middle.
Wood (grain) is not perfectly uniform across the joists. If there is a defect that runs diagonal across the joists, then it’s likely it will split at the defect.
Often splits are caused by a combination of the above items. Because the joists have been there awhile, I doubt it’s caused by drying out, unless it’s been there all along and you just noticed it.
I doubt you have a problem unless you’ve moved furniture around lately or done some remodeling lately.
Btw, it’s not clear if the two joists on either side of picture #1 frame into the joist spanning across the picture or if the horizontal joists (its divided into two in the picture) is actually blocking. Blocking or diagonal braces are required by code for joists that have a ratio 6:1 or greater. If it’s blocking… then no problem. If not, there are other problems.
You may also have a problem with the direction the plywood is installed over the rear portion of the first picture. If you look closely, you’ll see the plywood is installed parallel to the joists, if the joists extend back into the space (with blocking between the joists).