One think I am unsure of with mulch is whether it would ultimately cause the same problems for the tree as piling on soil. I guess it would at least initially be more permeable, but it's something I'd like to hear expert advice on. Of course, it's hard to know just where the roots go. There are no doubt feeder roots all over (at least where the woodchuck hasn't gnawed through them). The one main root that goes into the area I'm looking at is at surface level, so far as I can see, as the trunk is actually 2-3' up a little bank, and this root is trailing down. That part of the "shade room" area is also the highest. Other areas are depressed 1-2'. So perhaps the lower areas can be filled with gravel, with only some sort of topcover (whether mulch or crushed stone) over the area with known roots? I'll look up KwikDek, though a surface that has to be moved around every so often is probably not a good bet for me. And I'm not totally keen on decks anyway, as the one I have needs annual waterproofing. (I think if I ever install more decking, I'd want to go with one of the new composites like Trex, but they seem beastly expensive by comparison right now.)

So do you have shade under those trees? Is it dense shade or dappled? The most important roots most trees have are those near the surface of the ground. Cut too many and the tree will eventually die or be susceptible to falling over after heavy rain/wind. Since you like the trees (and I would too!), I would suggest you choose a ground cover or ground covers that can handle those conditions. Usually under trees the conditions are very dry, so you need something that tolerates the sun/shade plus dryness. This link will take you right to a site where you can choose the conditions you have and see what your options are. Then you can plant the entire slope with one plant or do groupings or whatever you prefer. In sunnier spots you can choose plants that want more sun, or just choose a plant that does both. Consider the annoyance that some plants could cause by being invasive. Ivy would cover the slope, be a beautiful green, and hold the soil quickly, but it's not going to stop invading areas beyond the areas you want covered, so you have to be willing to vigilantly clip it to contain it. Here is a link that might be useful: Ground cover search

I have been living in my house for 3 years and have a septic system and drainfield. This is the first septic system for me and when we started landscaping I did lots of research on the matter. Unfortunately, you are limited in what you can plant near the tanks and on the drainfield. I would recommend against a small tree. We first planted a small Japanese Maple, but our septic service people said to move it. Now I have a drought tolerant native wildflower meadow planted on the drainfield that should be looking beautiful this summer. And, over the tanks, I have planted various perrenials (18" - 30"H), low growing ground covers, and the largest is a Cape Fuchsia (which I'm a little unsure of, but planted it anyway). I also planted a bunch of Evergreen Huckleberry shrubs that won't get larger than 2'-3' tall. Whatever you do, you don't want to damage your septic tank due to landscaping choices. There are so many beautiful ways to fill in an area. And, if you really want a small tree, get a large pot and plant it in that. Then, there won't be any roots to worry about. :-)

Root destroyer is a great product, assuming roots are your problem. Having had limited knowledge of your problem due to your short term habitation, you must be sure first that roots are it. Until a plumber chases a cutter down your pipe or uses a camera to inspect, roots may be suspected but not the cause. Any number of reasons including broken, sagging, plugged pipe could also be the culprit. If it turns out to be roots or you just wish to throw some money down the drain, via chemicals, remember that root destroyer takes time to work. If any water is used or flushed, more treatment is required. It is best to add at night when going to bed and avoid using any water for 8 hours or more. Also, many times basement drains are not tied to the sanitary and often to the storm sewer. If so and not a root problem, you may have other issues. Not necessary a fix but a way to prevent future flooding would be to install a backwater check valve or flood-guard like one made by General http://www.generalpipecleaners.com/specialties/floodguard.html Here is a link that might be useful: Root Destroyer