raised rafter plates, deepened rafters for insulation

[colchu]

Hello,

I am planning to frame a new roof over an old roof. The idea is to tear of the existing roofing, place deeper rafters on top of the old ones (edge-to-edge; bearing directly on, or suspended above) to allow for more insulation and a proper cold roof. If bearing directly on, then they would be attached with gussets. The roof would be strapped with 2x4's and then metal roofing.

Perhaps I could tie the new rafters to the floor joists with metal strapping to prevent spreading, kind of like in attached diagram.

The attic is finished... and there are windows in each gable end. 8:12 pitch. 2x4 rafters 24" o/c.

Would I need a structural ridgebeam or could I just have a ridgeboard?

Alternatives would be to sister the new rafters to the old, but this would require heavier, wider rafters.

Any thoughts on the feasibility of this?

Drawing attached.

Many thanks,
C

[Richard A Hetzel]

If you are tearing off the old roof, why not just tear off the old rafters also, and install new deeper rafters. You will still need collar ties no higher than the bottom of the upper third of the roof height, otherwise you'll need a structural ridge, which could be a pretty big chunk of wood (like double 16-inch LVLs maybe) and then you'll need a complete load path from the ends of the ridge to the earth (such as posts extending all the way to the foundation).

[Don_P]

I'd agree that its probably better to just remove and replace. It will be stronger and better connected. If there is good connection to the floor joists (there's the rafter tie) then a ridgeboard is fine. I'm not sure I agree with high "collar ties", gussets under the ridge are fine.

[colchu]

The attic is finished. There is a kneewall at each side, then a sloped ceiling (drywall attached to rafters), and then a little bit of flat ceiling at the ridge. I would really like to leave the finished attic in tact.

My understanding is that the floor joists serve the same purpose as would collar ties. There is a small horizontal member attaching the rafters just below the ridge (keeping the rafters tight to the ridge).

What I wonder is whether I can frame my new rafters above or on top of the old ones, bearing them on the floor joists, and strapping them back to the floor joists with metal straps, like in the drawing.

Or, can I have some combination of this with a partially-structural ridgebeam. I cannot have a ridge-to-earth point load at each end of the ridge because there are small windows in the gable ends. However, the headers are pretty substantial and I think the gable wall could bear some of the load, and the triangle created by the rafters and the floor joists could bear some of the load. But I'm no engineer either.

The whole structure is 25' x 25'

Thanks,
C

[Jack Rafter]

Just what do you think the drywall is going to look like after you remove the roofing and sheathing?

[Richard A Hetzel]

The floor joists absolutely do NOT "serve the same purpose as would collar ties."

The joists resist outward thrust caused by the rafters, but the collar ties tie the tops of the rafters together. And there is no magic to carrying the point loads from the ends of a structural ridge to the earth. It goes straight down. If it encounters a header on the way down, the header must be investigated for the ability to carry the load. There could be a beam above the attic floor which would divert the loads around the header, but now you need two paths to the earth, not one.

And Jack Rafter makes a good point. Whenever I have tried to save a ceiling when it was exposed to the weather, the contractors tore it down and replaced it when everything else was weathertight. At the very least, someone is going to put a foot through it.