locating load bearing walls

I bought a ranch house last summer built in the early 1970's that has a 4:12 pitch gable roof with 28 pairs of 16 foot long 2x6 rafters 24 inches on center, a 2x6 ridge board, and 2x6 joists. There are a few flimsy collar ties (looks like 1x4's ?), maybe averaging every 6 rafters apart, but no other supports to this roof frame. The roof was sagging in a few places and had developed an unrelated leak, so it was replaced last fall, including approximately 20 new 2x6 rafters and all new 5/8 inch plywood sheathing (original was 1/2 inch and much of it rotted) (have a Cobra ridge vent now and am in the process of increasing soffit ventilation at every rafter bay). Some of the old rafters that were removed were so curved they looked like giant rocking chair legs. The new rafters improved the sagging areas of the roof greatly, but not to the point where it's perfectly flat. Since all of this happened, I have learned more than I ever wanted to know about roofs, including that 2x6's are not supposed to span 16 feet, no matter what grade of wood, without some support along the way. I also learned not to use the same home inspector again (he is supposedly an engineer no less!) and that the veteran roofers on the crew that did my roof made no mention that 2x6's wouldn't be strong enough to span 16 feet without extra support. My question is what is the best way to strengthen the roof frame from within the attic to prevent any further sagging of the rafters? The house only appears to have 1 interior bearing wall, running directly under or close to and parallel to the ridge. The supports below are concrete block piers and doubled up 2x10's in the crawl space. Would 1x6 or 2x6 cross ties on every pair of rafters be useful in stopping further sag? If so, how far down from the ridge should they be attached? Also, if the rafters don't perfectly match up at the ridge, can or should a cross tie be used at all? Also, how many/ what type of nails or screws should be used to attach them? My other idea to supplement the cross ties was to have 2x4 knee studs (not sure if this is right term) attached to the side of each rafter, going straight down, and attached to the top of a flat continuous horizontal 2x4 running perpendicular to and on top of the joists. Since there would be no bearing wall directly under these knee studs, I thought of placing them closer to the eaves where I would think (but certainly don't know) the joists are not at their peak load (i.e. closer to the exterior wall and not at mid-span). I'm not sure if this is a sound idea or if I would just end up with sagging joists too. I figure this cross tie/ knee stud combo could split the 16 foot rafter span in 3 sections and as long as they are all less than around 8 feet, I should be pretty safe in not overloading the 2x6's to the point of sagging. Then again, I'm not even sure if cross ties prevent rafters from sagging at all. Some sources say they do, but most don't mention that as the reason to use them. The 1 thing I really don't want to get involved with is purlins and tying them to the bearing wall. I'm no engineer, but I think the single row of block piers and double 2x10's in the crawl space are under enough of a direct load, supporting the floor joists above. I just don't want to risk anything with them. Sorry for being long winded, but I wanted to provide as much information as possible. Any advice on this subject would be greatly appreciated. Thank you very much.

Hello - I need to create a larger access to my basement, so that I can build a wooden canoe in there and get it out when completed. Currently have basic 12" x 30" windows. I don't think I can do a Bilco door at this time, so I'm considering installing a 36" h x 48" wide sliding window, and removing the sash when I need to pass the boat through. This will require a large window well, bigger in all three dimensions than the current one. a) Does this sound like a good plan? b) Bilco ScapeWel is $800 - what is an alternative, build-it-yourself way to create window-well walls? c) Are there other issues that are going to arise, i.e. drainage, protecting the foundation wall, etc.? We live in Burlington Vermont. I have some professional building and carpentry experience, but have not done a project like this. Thank you, Robert.