Metal Roof Over Shingles Build-Ups, From Bare Underlayment to Nailbase

Grey synthetic underlayment rolled over old brown asphalt shingles, with the first charcoal metal panels fastened at one end of a brick ranch roof

Direct Over One Layer With New Underlayment

The lightest recover rolls synthetic underlayment straight over the old shingles and fastens the metal through both into the deck. It needs a flat shingle surface and a deck with no soft spots. Screw-down panels and some nail-strip standing seam can go on this way when the panel maker's instructions allow it, which is the code's first test for any recover. Curled or cupped tabs show through wide flat pans as bumps, so on a lumpy roof a roofer may steer you to strips instead.

Rows of new 1x4 wood furring strips screwed across weathered grey shingles on a split-level house, a stack of silver ribbed panels resting near the ridge

Over Furring Strips or Purlins

Here 1x4 strips are screwed through the shingles into the framing in rows across the slope, and the panels fasten to the strips. They flatten out a wavy old roof and leave a shallow air space between asphalt and steel. R-panel and 5-V crimp are the usual match. Strip spacing comes from the panel's span and wind tables, and it should tighten at the eaves, rakes and corners where uplift is highest.

Over a Vented Nailbase

Nailbase is rigid foam insulation bonded to a plywood or OSB face; the vented kind has a built-in air channel between the two. It's fastened through the shingles to the framing and gives the new metal a solid deck, a layer of insulation and an air path from eave to ridge. It costs more than strips and raises the roof edge, so fascia, drip edge and gutters all need new trim. It's worth pricing on a house with a shallow attic where the ceiling bakes all summer.

Roofs That Have to Be Stripped First

Some roofs can't be recovered at all, and the code's reasons are listed under R908 below. On older houses the usual one is a second layer of shingles already in place; the next most common is a soft deck turned up by a test opening. Either one means stripping to the wood. The shingle tear-off job itself is on metal roof installation.

Gloved hand holding a tape measure at the eave of an old shingle roof, with charcoal, silver and red metal panel samples leaning on a ladder

Why Jackson homeowners call Jackson Metal Roofing Pros

One independent metal roofer gets your request, not a call list
A free measure, deck check and written estimate
Ask for panel system, gauge and paint finish in writing
The roofing permit is part of the scope: ask who pulls it
No charge to the property owner for the referral

Metal Roof Over Shingles and Section R908 in Jackson

Jackson builds to the 2018 International Residential Code; adoption and the city roofing permit are covered on metal roof installation. Its reroofing section, R908, decides whether the shingles may stay:

Allowed
A new covering over an existing one, installed per the maker's approved instructions. Standing seam systems that carry their loads straight to the framing, without leaning on the old roof, don't require the old covering to come off at all.
Not allowed
A recover where the roof already has two or more applications of any covering, where it is water-soaked or too deteriorated to serve as a base, or where the old covering is slate, clay, cement or asbestos-cement tile.
Framing
R908.2 says the roof structure has to carry the new covering plus the material and equipment loads during the work.
Wood shakes
Metal may go over shakes, but if that leaves a combustible concealed space, R908.4 calls for covering the whole surface with gypsum board, mineral fiber, glass fiber or another approved layer first.

A lot of Jackson's houses date from the 1950s through the 1970s (the numbers are on metal roof replacement), so plenty have been reroofed before, and owners who bought later rarely know how many layers are up there. A good roofer will lift a tab at the eave or cut a small test opening and count before quoting a recover.

Metal Roof Over Shingles Savings and Trade-Offs

The saving is the tear-off. HomeGuide's national cost guide puts removing an existing roof at $1 to $3 per square foot, so on 2,000 square feet of roof a recover skips roughly $2,000 to $6,000 of labor and dumpster fees. Weight is rarely the issue: the Metal Roofing Alliance says metal weighs about half as much as asphalt. The same trade group says metal roofs are 100% recyclable and contain at least 25% recycled material, depending on the metal, and leaving the old shingles in place keeps a truckload out of the landfill.

The costs are harder to see. Nobody walks the bare deck, so a soft sheet of plywood or a rotted rafter tail stays hidden under new steel. Screws through shingles need solid wood behind them, so the deck matters more than the shingles. Humid air trapped between asphalt and cool steel can also condense on the underside of the panels; how underlayment handles that in Jackson's weather is on metal roof installation.

Metal Roof Over Shingles Bid: Lines Worth Getting on Paper

Layer count
How many layers are on now, and how the roofer checked.
Build-up
Direct, furring strips or nailbase, with strip spacing in the field and at the edges.
Fasteners
Screw type and length, and how far each one reaches into solid wood through the old shingles.
Underlayment
The product going over the shingles, by name.
Edges
New drip edge and rake trim, plus any fascia extension the added height needs.
Penetrations
R908.5 requires existing flashings, edgings, vents and similar parts to be replaced where they are rusted, damaged or deteriorated, so ask which ones the bid replaces.
Permit
Who pulls the city roofing permit, and when.

Metal Roof Over Shingles in Jackson