I am rehabbing a ranch house.   One of the problems I have wrestled with is how to beef up the roof rafters which are undersized for the snow loads and current codes.   

I thought about leaving them alone but have decided to vault one side of the house and make a great room that is 14′ wide by 40′ long.  The other side is two bedrooms and a bath.  The house has been gutted and is completely dried in with siding and roof in place.  I know I should have done the rafters before the roof but didn’t realize how bad they were at the time.  I was only addressing the engineering of the vault by running a load wall the entire length of the ridge board.  

I have been reading and researching the best way to address beefing up the rafters.    I have very little deflection at this point (about 1 inch) on true 2 x 6 rafters that were milled on the property.   This is what I found here.

These Old Rafters Need a Lift – Fine Homebuilding

The article referenced covers all the known options.   I decided to dig a little further.     What I found is I can take advantage of both the support at the ridge board and a full tight fitting heal cut.   

The solution is to make a scarf joint.    But cutting the full-length rafter and near 2/3 to the top creating a 4 to 1 scarf joint.   The key is even simpler by using composite shims as the joint is a match from the same piece of wood.  

On the cold side of the house, I did this with 2x8s since this will provide some additional strength on the side where snow does not melt due to the sun and can stay for some time building up.  It will also allow me to put in more insulation giving me an additional R4. (R 54 total – R28 above the roof deck) The entire house has unvented attic and crawl space.    On the vaulted side I will use 2×6, function of not having to trim down for the seat cut at the exterior wall.  

What I am wondering is if my logic makes sense.    Scarf joints at a 4 to 1 are tested to maintain up to 65% of thier flexural strength.   To me that is saying not only do I get the value of the sister but also a substantial increase in flex strength by the full length of the scarfed sister.    

Does this make sense and if so, why isn’t there discussion about scarf joints when beefing up and repairing rafters?