I am purchasing a mobile home with a shingled roof. The shingles need to be replaced (>15yrs old) and with the cost of heating going up each day I thought I would added some of that 2" thick rigid insulation. I would place this right on top of the existing roof and then place the firring strips for the metal roof right on top of the rigid insulation panels (which are 4x8'). By using 3" galvanized sheet rock screws I will get a good solid attachment. Then I will put the metal roofing on to the firring strips as normal. Does anyone see any downside to doing this? Has anyone tried this before?
I agree with the soundness of every part of your plan EXCEPT not removing the original shingle roof. The shingle roof is already at the end of its usable life. There’s no benefit to covering up old problems, and they will most likely affect the durability of the new roof. Plus, removing the old roof gives you the opportunity to inspect and repair any issues with the roofing deck. Your new insulation and metal roof will attach better and perform better by removing the old shingle roof.
The only downside I see is your miscalculation about the length of the screws need. Lets see, 2 inches of insulation, 3/4 inches of furring, leaves you with a solid 1/4 inch of bite, and we all know that the first 1/4 inch of the screw is meant for piercing, not holding. So plan on 4 inch screws, because you still need to penetrate the old roof and bite the structure. Don't forget to plan your drip edge which now will have 2+ inches of roof thickness to carry the rain water over into your gutter.
