Replies

1. Expert Member
   MALCOLM TAYLOR | Mar 13, 2022 06:32pm | #1

   Trickworm,

   I'm sure others will have some ideas too, but I'd be inclined to just use a tapered 2"x4", and support it a couple of times at mid-span either with vertical chords (much like a truss) , or pony walls sitting on the existing rafters. To me that's easier than screwing through 2"x12"s and adds a lot less weight to the roof.

   You won't get any appreciable cross ventilation from the drilled holes. If that's important a better solution is to strap the top of the roof with 2"x4"@24"oc.

2. 
   user-723121 | Mar 13, 2022 06:48pm | #2

   I have used scissor trusses and gone over an existing roof leaving the interior finish and fiberglass insulation intact. I removed the first few feet of roof deck and tied the new rafters in to the top plate. We then blew cellulose over the original roof deck to bring the ceiling R-value to code. By doing this the roof pitch was raised from 2/12 to 5/12, a lot better working pitch in MN. The roof is vented at the eave and ridge.

1. 
   Davidw2022 | Mar 15, 2022 08:28am | #4

   Doug,

   This is exactly something I am looking to do. Do you have any pictures of the process that you could share? What did you do for truss uplift? On new construction your have to either use hurricane clips or screw in from below.

1. Expert Member
   MALCOLM TAYLOR | Mar 15, 2022 12:13pm | #6

   David,

   Interesting question. As the truss manufacturer will be supplying the engineering, I'd ask them for a solution. I suspect something like a Simpsons Strong tie A33 installed with structural screws would work.

   Just for clarity: truss uplift is a separate problem involving seasonal movement of the bottom chord.

2. 
   user-723121 | Mar 15, 2022 12:35pm | #7

   We used rafter ties as Malcolm has described, the lower portion of the roof deck was open so the ties could be fastened through the wall sheathing into the double plate. The original ceiling was vaulted with full fiberglass batts so we added additional cellulose on top of the existing roof deck. We used energy heel trusses to get the height we needed to get over the original roof deck and allow for adequate insulation near the outside wall. The ceiling is independent of the new roof system so truss uplift is not in play.

3. 
   kbentley57 | Mar 15, 2022 07:30am | #3

   Trickworm,

   I'd consider what you're doing to be a lot of work. Going from a 2.25 to a 3 : 12 pitch isn't really all that much of a gain, and still limits you to not using shingles in the future by alot of manufacturers. On top of that, those 14 inch screws will cost you about as much as the rest of the roof, and drive you insane trying to drill straight through a yellow pine board that has tight growth rings. The bit or screw will follow the rings and blow out the side, or split the wood if not done exactly right.

   You're going to be more satisfied and money/time ahead just to reimplement the roof using the 2x12's that you would have bought anyways. You'll end up with better performance too, as an R38 batt will fit there nicely and still give you a ventelation channel. Without all of the intermediate layers and roof for gaps and air leaks, and if a water leak happens, you won't be able to pinpoint it with any confidence.

4. 
   luke_p | Mar 15, 2022 08:47am | #5

   I'd build tapered pony walls and mount them on the existing rafters. It gives you your cross ventilation and you can do it with 2x4s and nails.

   Also consider optimal roof overhangs as well the depth of insulation over the exterior wall in your design choices.