What should I do about roots and trees in the main sewer line. Every time I use anything upstairs (shower of flush the toilet) the toilet in the den overflows and whatever is flushed upstairs comes out of that toilet

Rent (Home Depot) or have someone clear the lines.

There is probably a main cleanout inside and possibly one outside.

If not, there is always access by removing the toilet. Always snake in the direction of flow.

Snaking from the roof is always a possibility, but I don't think you have a stopped vent at this point.

Sppedball1 has some ideas about root treatment.

Roots in the drain means you have at least a small crack or bad joint. After rodding with a cutter head on a sewer rodder you should flush 1/2 lbs of copper sulphate down the drain. You can buy Root X Root x Root Killer Sewer Cleaning - eBay (item 180413751901 end time Dec-28-09 14:40:35 PST) (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180413751901) or go to a garden center and ask for their much cheaper bag of copper. After rodding and treaing if you continue to get problems it will be time to get a plumber to use a SeeSnake camera to see what's going on, could be your line or at the riser by the city sewer line.

How deep is your sewer? The root treatment is just to keep you going until you can either replace or reline the pipe. You're going to hafta find out what's wrong down there and have it fixed. A sewer cam is your next step. Good luck, Tom