For an exact answer you need to know what type of wood (including grade) for the old and new).
This then allows you to use any of the various nail loading tables to determine what is needed to create a joint as strong as the original material.

Mastic has no structural rating in framing.

'Fine Home Building' had a detailed article a few years ago on developing nailing schedules.

Bolting schedules get a little tougher.
Going over 3/8 inch rarely adds anything since the point loads created start to exceed the compressive rating of wood.

There are lots of 'rules of thumb' based on joist height.
Some are closer to reality tan others.

You can get generally maximum nail capacity by going through by art least 0.5 inches with 16d nails and then bending the nail over on the far side.

A nailing pattern similar to the arrangement of 5 on a playing card dos well.
Repeat the pattern for at least 3-4 times the height of the joist (both sides if a splice).

If the sills are gone at this point with no major sagging of the joist ends they are not heavily loaded anyway.

How can I determine the type of wood and grade of the existing joists? The home was built in 1967
I note you didn't mention screws, are they not a good way to link one joist to another? Are bolts better than nails?
When deciding the length of the sister would you say 8' is enough considering that the joists have almost no sag and the joist ends are my main concern, the rest of the joist is in good shape.

"How can I determine the type of wood and grade of the existing joists?"

Look for grading stamps. They should still be there.

Nails have published load ratings, screws not so much.
8 feet should be fine for 2X6.

That should be a major portion of the allowed span.

Why not add joists that are capable of supporting the floor and ignore the old ones?

It can be pretty hard to find joists of the grade and quality in older ones.

You end up having to use taller joists to meet the codes with modern materials.

My house has 3 inch (net) rough cut (you can see head saw marks on the sides) that are 11 inches tall.

I would hate to lose any more ceiling height using modern lumber (especially since the basement is already short).

LVL's would certainly be stronger than the original joists and if the original joists cannot rest on the end supports they're not going to add much strength to the new joists.

Can a 2x6 really be a floor joist?

Ceiling joist of upper level OK for attic storage but not for active living space.

"Can a 2x6 really be a floor joist? "

If the span is small enough ...

LVL's would have the additional advantage of not shrinking. I recommend avoiding the use of new dimensioned lumber (2x's) in an old structure.

"LVL's would certainly be stronger than the original joists and if the original joists cannot rest on the end supports they're not going to add much strength to the new joists."

That sort of depends on what problem you are trying to solve.

If you sister 2x6 to 2x6 you have doubled the stiffness.
The floor would not be as likely to feel 'springy' unless it is already so over-spaned the short single sections at the end cannot take the load.

I have put a lot of 1.25 inch wide #10 steel straps on the bottom of joists to stiffen them.
Done correctly it is like adding a railroad tie to the bottom of the joist.
Needles to say, the straps do not have to extend to bearing surfaces since that is not how they function.

I must have misunderstood the OP. I thought the 2x6 joists were water rotted and/or damaged by termites at the ends where the joists lay on the 2x4 sills and that the sills had rotted too. That doesn't sound like anything worth reinforcing; it sounds more like it requires new joists.

Without an accurate description of the existing conditions it isn't really possible to offer useful solutions.

"I have put a lot of 1.25 inch wide #10 steel straps on the bottom of joists to stiffen them. "

Brickey - where are such straps to be found?

Welding shop / steel supply should have these with no issues.

Sheet metal fabricators can usually handle stuff that thick.

Or you may need an actual steel yard.

For maximum stiffness, would one install the straps on the underside of a joist from sill to sill or from the centerline of the joist run half way to the sill in either direction of the centerline?

The longer the better.

You are making a composite beam with the wood and the metal strap.

The holes for the screws in the metal need to be tightly matched to the screw shank diameter.

A slight interference fit (a few thousandths of an inch) is actually better than a clearance fit.

For the joist to sag the bottom surface must get longer as it flexes to a curve.

The steel and screws prevents that from happening, but only if the slightest movement is transferred to the steel immediately.

It is more effective than a plain flitch beam with metal attached to the side of the joist.

Flitch beams have the same issue with hole clearance on the fasteners, and add buckling of the steel away from the joist to the issue.
One of the reasons a classic flitch beam sandwiches the metal between two joists.

The fasteners than go all the way though both joists and the steel between them, limiting chances of buckling even with relatively thin metal (and the double joist immediately adds more stiffness).

The flitch can be a PITA for wiring and plumbing though.

It is much harder to drill through the typically hardened steel plate often used for the metal.

This post was edited by brickeyee on Sat, Apr 13, 13 at 14:39