I am wanting to put a shed dormer, full length (28ft) on the back of my 11/2 story farmhouse. I was wondering if it would be wise to sister the front roof rafters (2×4) with 2x6s and extend them 2ft above the exsisting ridge. I want to get some more headroom and be able to have a acceptable pitch on the dormer. The current roof is 8/12 and has a 8ft doghouse style dormer on both sides,the walls are 2×6. The width is 24ft
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There is a whole long list of things that would be wise to do if you take this on, primary amoung them being that since you are totally changing the way the roof loads bear, thrust, and transfer loads, you might have to totally change the roof structure itself, with good engineering planning. There are hundreds of roofs with tilting walls and swaybacked ridge lines caused by people sticking in hed dormers without knowing how to account for this change with a ridge beam and point loads to foundation.
"I want..." does not always translate to "I did this well" or even " It can be done"
Yeah, rather than do a hack job, it's probably better to remove and replace the entire roof. Get it properly architected/engneered rather than trying to do it by the seat of your pants.