I have a large (80 years old) Redwood Tree in my front yard, approximately 10 feet from the house. My house is a ranch style house and I can see the roots of the tree go under the house in the front and come out on the other side. How much do roots grow in thickness, as opposed to length, and do I need to worry about them lifting up the house at some point?
Yes, they can lift and break the foundation. I had similar issues with a London Plane tree root starting to do damage, it was 6 inches in diameter. Some work with an axe and exploratory trenching eliminated the issue. Feeder roots will have to be searched out every so often and chopped out.
The root system on a Redwood is huge so I'd get a tree surgeon in for some quality consulting time, along with a foundation expert before something gives way.
Damage to sidewalk and part of road from Sequoia. Not pictured are the sinuous lumps out in the street where the blacktop is being broken up.