We recently had to cut down a very large cottonwood tree in our front yard. We're now looking at adding onto the house in the area where the tree's roots would have been. I'm concerned about the ground sinking in that area as the remaining roots rot away and was wondering if anyone has any experience with having removed cottonwoods. This was a VERY large tree, the stump is over three feet across. One of the builders we've talked to suggested digging up the area to get rid of the roots and then having the area re-compacted. Any thoughts?

Suckers coming up will be a much more significant issue than any ground subsidence.
Our yard had a number of large poplars (a commonly-planted tree here for it's hardiness and rapid growth but generally a bad decision to plant on a city lot, due to suckering habit, very large size and other negative aspects such as fluff produced by female trees (though that's avoidable by planting male clone cultivars)) that, fortunately, were cut down by the previous owner before we purchased the property. We come across old roots even 17+ years later* (we've lived here 17 years) but ground subsidence is not a factor. Fortunate also, for us, is that the previous owner dealt with the suckering, and we did not have to.

*Edit: To clarify, it's not as though the roots decay and disappear immediately, or even that they take up such a volume as to create a potential for ground subsidence.

Ah, thanks for the input. I was concerned about it because when I lived in SoCal, we cut down a small fig tree and a few years later, we ended up with a noticeable sinkhole where the roots had been. Unless that was just coincidence.
From what I've seen of the roots of these trees, they seem to be fairly small, but widespread. We're thinking of doing a slab foundation, rather than a raised foundation, and I was concerned that it might end up breaking from subsidence. There must be a LOT of roots, tho, because the trees (there are two more, farther away from the house) are just huge! And we'd be building very close to the base of the cut tree, probably less than 6 feet. (Can't get accurate measurements yet, because the remains of the tree are still scattered over the yard....grrrrr!)

What is standard building practice for slab on grade construction in your area?
Given your in z5 will a perimeter foundation be installed? How deep?
Here in the NE, typically soil is removed under where the slab will be, then back-filled with gravel +/or sand and compacted. To eliminate/minimize the amount your slab will settle proper sub-soil construction is essential.

All very good questions, which I shall ask my builder. I don't believe that is common practice in the area here, tho, just because they didn't do anything like that when we added a large garage with a slab to our barn. The soil here is very hard clay. I have a good deal of confidence in our builder. He has done a lot of work in the area (built some VERY nice homes) and several jobs for us, too.

The fact that your soil is very hard clay can also have challenges for a slab. In the summer during drought times the clay dries out severely and shrinks away from the foundation which is supposed to be around the foundation to support it. With the clay shrinking, it can cause your slab to settle/crack. Some people go out and water their house around the foundation to prevent this. Sometimes the type of slab used for this kind of soil is a Monolithic slab. It has a footer poured every so many feet to support the span. Of course chat and a supportive type of metal (sometimes re-bar) is also used. Whatever you do I would for sure dig the roots out around the area and fill w clay and pack this before pouring over it.
I am just going out on a limb here and assuming some things about your barn. I assume that it is a pole barn of sorts? Just because those are very popular and affordable. The thing with a pole barn is that the concrete is poured as a floor only, it is not intended to support any walls. The walls are free standing of themselves. Many many pole barns are put up on just dirt- dirt floor. Of course if the building is intended to house vehicles or tractors ect, then the concrete would have to support the weight of those. I'm just saying, an actual footer to support the building is not necessary in most cases. So the slab design for your barn could be very different than what you need to support a home's walls/weight.
Since your post is a month old, how is your project coming along?

Frillylily said:In the summer during drought times the clay dries out severely and shrinks away from the foundation which is supposed to be around the foundation to support it. With the clay shrinking, it can cause your slab to settle/crack. Some people go out and water their house around the foundation to prevent this. S

This happens when certain types of clay are present in the soil, aptly referred to as "swelling clays", often the result of deposition in ancient lake beds. The crystal structure of these particular clay minerals (montmorillonite, smectite, etc.) causes an expansion in volume when the clay takes up water, and shrinkage when it dries out again, just as you described. Swelling clays can be hell on basements where they occur, but, fortunately, they're not everywhere.

I don't think we have that kind of clay here (and we do have a basement that has had no issues), and our builder has been putting up houses in this area for a long time, so I'm sure he'd be aware of the problem if it existed. He says there will be a 3 ft deep footer around the perimeter. He doesn't think the tree roots will be a problem, and of course a lot of them will be dug out when they dig down for the footer. Come to think of it, there used to be another of that same type of tree closer to the house; you can see where the stump used to be, and it's demise hasn't appeared to impact the house at all. I'd forgotten about that.
They're supposed to start construction around the first of April, so soon!!

I'd just plant shrubs around the old stump then and forget it's there. Sounds like it isn't going to be in the way at all. Any roots that would be a problem will be dug out when they do the footer anyway. I would be too cheap to pay the extra to have it removed

That's pretty much my plan. We left the stump a couple of feet tall for interest and I plan to put a small fountain and a couple of potted plants on top, then make a nice flowerbed around the base. I've ordered some hardy ferns for the base of the stump and hope to get some astilbe going in that same area.