I have a couple of sags and slopes in my house currently and the one I need to address most is in the kitchen where we are planning a renovation. At a minimum the renovation will include a tile floor and at most it will be tile with a quartz/granite counter.
The house was built in 1990. The floor joists are 2x10 Spruce Pine Fir on 16" centers which spans about 15'2". From what I have read this is ok for 20dead/30live and 10dead/40live, but its already sagging with the current appliances. I would really like to beef it up and I can't add a beam as the basement is also the garage. Some of the ideas I got from reading and speaking with people:
1) Add new 2x10s to the existing 2x10s (but get both ends up on the supports). So essentially sister them but also get them on support and glue and nail them to the existing joists.
2) Add new 2x10 joists in between the current joists essentially creating 2x10s on 8" centers.