reroofing over metal

[Chris]

I have a low pitch metal roof that I am going to go over with new rafters and ridge beam to add pitch/overhangs. Can I leave the old tin roof in the "new" attic space, add rafters and insulate right on top instead of tearing it off? It isn't a trailer house but would be similar to adding a shingled roof over such-just wider(22'span).

[Rich]

I'll leave this one for Grumpy :)
Just one question though - Are you going to increase the pitch of the roof?

[grumpydasmurf]

Building ontop of the old roof may make the new structure un-even. Also I can forsee possible condensation problems build up on the top surface of the old roof.

When in doubt, tear it out. I think you won't be saving much time by keeping it there. If I were you I'd yank it.

[Chris]

yes...and no. I have a dual pitch roof 3/12 and 5/12. I will be making both pitches 5/12 using 2x12s and a ridgeboard. This will give me space to add much needed insulation and venting. Presently the peak is offset due to the dual pitch. The 5/12 pitch side has the shorter rafter length (about 10feet) By stacking on top of the old using 14 foot rafters I can center the peak on the house and get @2ft overhangs. This new peak will then be a overhanging/cantelivered a couple feet above the old 3/12 roof and rafters will be hung from there for that side. Not sure about collar ties or actually attachment methods.

This is a company house and many in town were built identical (early 50's). Others have used 2x6s and insulated with R-15 and airspace while keeping the same pitches. A couple of homes in town have done similar to what I will be with 2x6or8s. Nobody really knows much of what is in their redone roofs. I'd rather get some real insulation and pitch.
The home has vaulted cieling with 2x6 rafters and load bearing walls to the peak.

[Rich]

One thing to think about..
If you are increasing the pitch of the roof and you don't want to remove the existing roofing material - you will still need to cut the tails off the existing rafters/trusses otherwise the old lower pitch rafters will extend over the new steeper pitch. The main reason for this is that you'll want the new rafters bearing on the walls and not on the roof.

[grumpydasmurf]

If your increasing the roof for venting, the old metal roof will trap air.

[Rich]

Yep.. my recommendation follows Grumpy's - get rid of it. It'll save you many headaches down the road.

[Chris]

I guess I'll remove the old tin roof and sleep at night.

I hadn't planned on raising the new rafters above the old on the 5/12 pitch side-just lay 'em on top. The old rafters have 4" overhangs which would be totally enclosed by the soffits of the extended new rafters. No goofy looking ends would show. I guess I could run a 2x4 along the ridge and wall to lay the rafters to keep them off the existing roof. But, I don't see why other than to prevent thermal bridging. Any thoughts?

[Rich]

The only thing I would worry about is the bearing for the new rafters if they aren't sitting on the walls.. especially in Minnesota. Just a recommendation though :)

[Chris]

Tore off the old metal roofing. And, to keep the added rafters off the old I plan to nail an additional plate (2x4) above the outside walls (this will keep the load bearing on the walls ). On one side of the roof I plan to use adjustable metal connectors which will allow for the increase in pitch while securing to the now tilted "top plate". However, on the other side where I plan to keep the 5/12 pitch if I attach a 2x4 top plate above the outside wall it will have the same pitch as the rafter. Can I simply toenail the rafter to the plate without a birdsmouth since they will be flush with each other?