Cracks in solid wood floor joists and beams are not uncommon. Some do not even matter much.
This is especially true of ones that are the result of the natural drying process of the wood. These “checks” as they are called, are especially common in larger timbers---like the 6x6 support posts you might see in your basement or at other support locations. This solid wood 4x10 beam has a large check right in the center over the length of the picture.
These checks are rarely an issue structurally, in fact they are quite impossible to eliminate entirely. Checks like this are rarely very deep. They are as “to be expected” as shrinkage cracking of concrete floors.
At an inspection a while back I took this picture of a large crack in a joist that actually is problematic.
It is a problem because the furnace is hanging on the joist in two locations (blue thingamabobs). If you look at the top right part of the picture, where the gas pipe has been “let-in” to the top of the joist, you can see that it has settled on the right side about an inch, and of course the notch for the pipe is the weak point where the joist has split from.
Repairs won’t be difficult for a qualified party, but repairs are necessary.
Charles Buell, Real Estate Inspections in Seattle