I can't lay my hands on the book right now because I don't even remember which one it is, might be the big one from OHJ, but one of my old-house-repair books discusses replacing rotted rafter tails. Basically, they had you cut the rafter tail back to good wood and attach a replacement piece, right to that cut end, using dowel pegs, exterior-grade glue, and screws (from underneath) with wooden plugs to fill the screw holes. Fill the seams with an exterior-grade waterproof filler, sand, prime and paint. I remember it said to prime the cut ends that were going to be butted together before assembling just in case any water made its way in there. For simplicity's sake, I would figure out which rafter tail is the worst and use that as the measurement for creating the new pieces, just cutting back all the rafter tails to that same point and "mass-producing" the replacement pieces all of the same size. I'm not even sure why it stuck in my mind except perhaps that our house originally had open soffits and exposed rafter tails but a PO cut 'em off and closed in the soffits, and someday if there's ever money to do so it would be nice to reverse that alteration.

Another random thought - some decorative rafter tails were originally just separate pieces grafted onto the ends of the actual rafters. Take a look to see if this is the case with yours.

There's also the option of the epoxy wood consolidators and fillers - Abatron is probably the best known manufacturer. Dig out the loose rotted material, saturate with the liquid consolidator, "rebuild" with the filler and finish from there. Spendy though, and would probably take longer.

A quick Google search also notes that some information on restoring rafter tails is in the National Park Service Preservation Brief #45, Preserving Historic Wooden Porches, which is not available online at this time (just 1-44 are online). Send your snail-mail address to nps_hps-info@nps.gov to request a copy.

I had the rafter tails on my bungalow repaired this past summer. Mine had been hacked off when aluminum trim was put up over my soffits. I pulled down the aluminum two summers ago. I've put a link to my blog below....look for the label "soffits" in the lower left to pull up the posts/photos on the replacement. The carpenter I used cut new tails...used long screws to attach them to the ends of the existing tails....with WoodEpox in between and on the surface to fill in the gap. I am happy with the results. Mine were not rotted, but I have had very good luck with LiquidWood on restoration of other exterior wood trim.

Here is a link that might be useful: Rafter Tail Repair

I have used a Kreg jig to create pockets in the side of the new tail with the screws going into the end of the old tail.

Some Minwax High performance wood filler fills in any gaps in the joint.

You should also treat the end grain if the new tails that will be exposed with Minwax wood hardener.
It is resin in solvent and soaks into the end grain filling it up.
Paint it on repeatedly until a slight gloss appears on the wood from the build up.
It prevents water from being absorbed into the end grain extending the life of the wood even better than just paint.

Yikes, I mean't the label "rafter tails" not "soffits"

SuzyQ2, our rafters were similarly hacked off while new gutter was installed by preivous owner. would love to see how you did it - tried to access your blog link but it was accessible to invited readers only. could you share the blog post in other ways, or could I be invited as your reader?