Hi everyone,

  We have a house dating from 1895 in Westchester New York. The house has a slate roof which also covers the wraparound porch. The porch roof is due for replacement (original). 

  The porch is partially enclosed and is now finished space. For this portion of the roof we plan on adding continuous insulation ( rockwool). We will make up the height difference with the porch area not enclosed by building up the rafters/ roof deck to match insulated area. I am struggling to find a viable detail for this construction. 

  Typically they recommend rough sawn plank roof deck for slate as it is so long lived. But adding continuous exterior insulation complicates this. I would like to vent the roof. Therefore the best I can come up with is….

. Demo existing roof
. cut off rafter tails
. redeck roof with 5/8” plywood 
. cover ply with WRB
. add rockwool insulation 
. add new monopoly framed rafter tails
. place battens over rockwool fastened to deck below for ventilation 
. Install rough sawn plank above battens
. Add 30lbs felt ( traditional underlay) and flashing 
. slate roof

   With the slate having to be attached to a solid wood deck the insulation causes a conundrum. We cannot make up for R-value needed on interior as rafters are true 2”x6”. If anyone has experience with this or can point me in the way of a resource it would be greatly appreciated. I have tried a few other websites etc. None cover this. I spoke to National Slate Association, they recommended their book which does not cover this situation. 

  Any help would be greatly appreciated I’m truly at a loss. 

Regards,
Denis