What You Save, and What You Give Up
What you save is the tear-off labour and the disposal — genuinely a meaningful line, and the reason the question comes up at all. An asphalt tear-off produces several tons of waste per average roof.
What you give up is more consequential than it first appears.
- Nobody sees the deck. This is the single most useful thing a re-roof reveals — rot at the eaves, delaminated sheathing, old board decking with gaps, previous repairs done badly. You find out now, or you find out in fifteen years when it is a bigger job.
- You lose the ventilation opportunity. The moment to correct ventilation is while the deck is open. If ice dams are any part of why you are re-roofing, this is the step where actual improvement happens, and going over the top skips it.
- Eave detailing gets harder. Ice-and-water membrane wants to go on a clean deck. Building up over an existing roof complicates the eave geometry and the transition where it matters most in this climate.
- The eventual removal is twice the job. Whoever takes this roof off in fifty years takes two roofs off.
- Trapped moisture is a real consideration. Asphalt under metal in a freeze-thaw climate, with any moisture in it and imperfect ventilation, is not an assembly anybody designed on purpose.
You are saving the cost of the one hour in the whole project where somebody looks at what your roof is actually made of.
That Decide It
How many layers are up there already?
Rules on the permitted number of roofing layers vary by town in New Hampshire, and a building department is the authority rather than a contractor. If there are already two, the answer is usually settled for you.
What condition is the existing roof in?
Flat, sound, well-adhered shingles on a deck with no known problems is one situation. Curling, lifting, patched, or a roof that has leaked is another entirely — you would be covering evidence rather than a substrate.
Do you have ice dams?
If yes, going over the top is close to the worst available choice, because it forgoes the ventilation fix while spending the money. Deal with the cause, tear off, detail the eaves properly.
How long will you own it?
A short holding period changes the calculation — though if the horizon is genuinely short, the more coherent decision is usually asphalt rather than metal over asphalt.
Where it is genuinely defensible: a single sound layer, a deck with no history of problems, adequate existing ventilation, a simple roof form, and a purlin or batten system over the top that creates a ventilated cavity rather than laying panel straight onto shingle. That is a real specification and it is a different thing from “we can just go over it”.
Questions
Will it void the panel warranty?
It can, depending on the manufacturer and the detail. Ask specifically, get the answer in writing, and check whether the installer's workmanship warranty treats it differently too. This is a reasonable question and a manufacturer's technical department will answer it.
Is the weight a problem?
Rarely, because metal is light — a standing seam roof over one layer of shingle is typically still lighter than two layers of asphalt. The permitted-layers rule is usually about the roofing assembly rather than the structure, but your building department is the authority.
Does it make the roof quieter or warmer?
Marginally quieter, since the shingle layer adds mass under the panel. Not meaningfully warmer — the insulation doing that work is below the deck, not above it. Neither effect is a reason to choose this approach.
What do you recommend?
Tear off, in most cases. Not because we want the extra line on the invoice, but because the hour spent looking at your deck and fixing your ventilation is where a fifty-year roof is actually made. If your situation is one of the defensible ones, we will say that too.