Someone has told you your roof has one layer of shingles on a dry deck and offered to lay the new shingles over the old ones for less money. Nothing in the state code stops that, and the code repays reading in full, because it turns on a word most people get wrong. SPS 321.28(6) says new roof coverings may not be installed over existing roof coverings where “(a) The existing roof or roof covering is water-soaked or has deteriorated such that it is inadequate as a base for additional roofing. (b) The existing roof is wood shake, slate, clay, cement or asbestos-cement tile. (c) The existing roof has 2 or more applications of any type of permanent roof covering.” Applications, not shingle layers, and any type of permanent covering, so an old roll roof buried under the asphalt has already spent one of the two. That is how a roof reading as a single layer from the driveway turns out to be at the state’s limit.
Whether the offer is legal on your roof is a second question, because the same chapter makes the shingle maker’s instructions binding, and on some shingles they say remove. We tear off on every house, so we do not sell the option this page weighs. That is the bias. The rest is the ledger behind it.
Can I put new shingles over my old ones in Wisconsin?
The code hands the details to the manufacturer. SPS 321.28(3)(a)3 says shingles “shall be installed in accordance with the manufacturer’s recommendations,” and a note under the same section applies the roofing rules to any alteration of a dwelling, permit or no permit.
The Asphalt Roofing Manufacturers Association is the fairest voice for the roof-over. Its reroofing bulletin says that “if a roof has only one layer of shingles that lay flat and the decking is in good condition, a tear-off may not be needed,” and that the old layer provides “a secondary back-up roof for the new shingles.” It adds that some roofers insist on replacement anyway because it ensures a completely new roofing system goes on.
CertainTeed’s applicator manual allows a roof-over over one layer when the deck provides “a good nailing base” and the combined weight stays inside “the rated carrying capacity of the deck.” Its inspection list carries the line that decides most roofs here: “If the old roof consists of architectural, lock-type, dutch lap or wood shingles other than sawn square-butt style, remove the existing roofing.” The word that matters on a house is architectural, the laminated shingle with shadow lines and depth; 3-tab is the older single flat layer cut into three tabs. Most roofs reaching replacement age around here are architectural, and where one is, that line plus SPS 321.28(3)(a)3 makes remove the binding instruction. By the book, a CertainTeed roof-over is a live option mainly over old 3-tab.
GAF’s 2012 Timberline instructions (the current sheet is on GAF’s site) let old asphalt stay if every “loose, curled or lifted” shingle is nailed down or replaced, the old surface is dry, and the nails still reach at least three-quarters of an inch into the wood deck, which on a roof-over means longer nails than a single layer needs.
Before any of that, CertainTeed’s inspection list starts underneath, and it is the one step a homeowner can run alone: from inside the attic, check the rafters and sheathing for sag and look for dry rot, fungus, or carpenter ants. Sagging sheathing between the rafters, or widespread rot, and the manual says the roofing comes off. A flashlight and an hour is the closest anyone gets to the deck without tearing off, and it is the look a roof-over asks you to skip.
What a roof-over gives up
| The second layer keeps | The second layer gives up | |
|---|---|---|
| Ice barrier | Whatever the old roof had at the eaves, or did not have. | A membrane bonded to the wood. CertainTeed’s WinterGuard instructions: “Do not apply over shingles.” Its roof-over instructions: “Use no underlayment over the old roof.” |
| The deck | The old shingles as ARMA’s “secondary back-up roof.” | Any look at the wood; soft sheets and rot stay covered. CertainTeed’s warranty excludes “failure or movement of: the base material over which the shingles are applied, the roof deck.” |
| Flashing | The old step flashing, the L-shaped pieces where the roof meets a wall, and the chimney flashing, bent to fit. | New flashing on the deck. CertainTeed’s manual: replace step flashing when re-roofing “unless the flashings are in like-new condition,” and replace any that will not “last at least 20 years.” |
| CertainTeed warranty | Over old 3-tab, the manufacturing-defect coverage: the 2026 warranty carries no roof-over exclusion for it. | The 130 or 160 mph wind coverage in the first fifteen years, which applies only when the shingles “are not applied over existing roof shingles (roof-overs are not permitted).” Over architectural, CertainTeed’s own applicator manual says to remove the old shingles, so a roof-over there departs from the installation instructions the warranty is written against. |
| GAF warranty | The base coverage; the 2013 edition “remains in effect” even when installation departs from GAF’s instructions; read the current edition on GAF’s site. | Payment for “improper fastening” or for application that departs from GAF’s instructions, and for ice damming beyond the leak barrier. |
| Weight | The old layer, on the rafters for the life of the new roof. | Framing margin, which the next section works out in tons. |
| Shingle life | Nothing. | Years. InterNACHI: roofs with new shingles over old “will not last as long,” the shingles “dissipate heat more slowly,” and where they bridge the butts of old shingles, the thick bottom edges, they are “unsupported” and “more vulnerable to impact damage.” |
Two code lines sit behind the first row. SPS 321.28(2) requires No. 15 felt, the asphalt-saturated paper rolled onto the deck, or an equivalent “under shingles,” and says nothing about a second layer; CertainTeed puts none over the old roof, so a roof-over rides on whatever felt went under the first. SPS 321.28(4) requires ice dam protection on a shingled roof over heated space at 4:12 or flatter, four inches of rise per twelve of run, “in the form of sheet metal or a product labeled as meeting the requirements of ASTM D 1970.” The membrane is the form everybody uses, and in a roof-over it has no deck to bond to. So on a low-slope roof that never had one, the only code-compliant ice barrier left is the sheet metal the statute also names, which is not how these roofs get built.
The deck row is why a tear-off estimate carries a decking allowance; the decking replacement guide covers what the allowance is for and what turns up under the shingles by house era. Metal panels over one shingle layer are a different question; the metal roof over shingles guide answers it.
How much does a roof-over weigh, and does my roof care?
CertainTeed’s Landmark data sheet lists 211 to 225 pounds per square (100 square feet), depending on the plant. Old 3-tab ran 200 to 230 pounds a square on CertainTeed’s 2007 product sheets, and Landmark over old 3-tab is the roof-over the manual leaves open. On the 2,500 square foot single-story ranch that prices as a one-day job, call it 25 squares, the low case, since a pitched roof with overhangs covers more than the floor plan: 25 squares of old 3-tab is 5,000 to 5,750 pounds already on the rafters, and adding 5,275 to 5,625 pounds of Landmark means the roof-over carries 10,275 to 11,375 pounds of shingle before the first snow.
That is not an overload by itself. InterNACHI runs the same arithmetic on 15 squares of two 250-pound layers, “nearly 4 tons,” and reports that “most jurisdictions allow only two layers of shingles to be applied to the roof due to the load considerations,” with more than two “likely to be overloaded.” So the second layer sits inside what the codes contemplate; it is the third that puts the framing in question. Where weight disqualifies a roof-over outright is at the heavy end: CertainTeed requires a tear-off before shingles over 350 pounds a square go down on “two or more layers of standard-weight shingles or one layer of heavyweight shingles,” and says of its own 430-pound Grand Manor and 350-pound Carriage House that over another heavy roof “the combined weight of the roofs, more than likely, will be too much weight for most frame buildings.” What nobody can tell you from the ground is what your own rafters and spans carry, and SPS 321.02(1)(a) leaves that with whoever builds the roof.
How do I know how many layers are on my roof right now?
The rake is where layers show, the sloped edge running up a gable end. Look at the cut edge of the shingles there: one layer reads as a single shingle thickness over the drip edge, the metal strip along the edge, and two layers as a stepped double edge. The eave, the gutter edge, is harder, because the starter strip, the trimmed course that goes down under the first row of shingles, passes for a second layer. Lifting the bottom row to look for a full granulated shingle underneath settles it, but do that gently and from a ladder, or skip it: the count belongs in the written estimate, and the estimator reads it at the rake and in the attic. InterNACHI notes that “many asphalt shingle roofs have a second drip edge installed with a second roof covering,” so a second drip edge under the top one is its tell. The attic gives a third, reasoning rather than rule: roof-over nails have to reach through both layers into the deck, so the tips standing through thin sheathing run longer. The count matters on the invoice, because a two-layer roof is two layers to haul.
What a tear-off adds to the price
The roof-over’s case is the money, so here is what a tear-off adds, without a percentage, because no published source puts one on a house.
Removal labor is the first: stripping shingles, underlayment, and old flashing to the wood before the new roof starts. Hauling is the second: a dumpster in the driveway, and the 25 squares coming off that ranch weigh two and a half to nearly three tons before the nails and felt. Disposal is the third: Seven Mile Creek on Olson Drive is the area landfill, and it quotes its own rate for a shingle load. Less of that load stays in the ground than used to, since ground shingles go into hot-mix asphalt for roads, and the DNR reports the shingle share of landfilled construction debris fell from 29.5% in 2009 to 10% in 2020. The fourth is the cost a roof-over never spends: the decking allowance, which exists because a tear-off looks at the wood.
The roof replacement page prices a simple single-story roof in a basic architectural shingle at $9,000 to $12,000, with removal, hauling, disposal, and the decking allowance already inside that number. How many sheets of decking need replacing is one of the drivers that page names for where a roof lands in the range, which is why the number is a range and why nobody prices your roof from the driveway. The roof-over’s saving is those four lines, once, at the front of a roof that then carries every row of the table above for the rest of its life.
How much life that costs is the figure nobody publishes. The 18 to 25 years an asphalt roof reaches here is a tear-off number, and InterNACHI says shingles laid over old shingles “will not last as long” without ever saying how much less. The honest answer is fewer than 18 to 25 years, and no one can tell you how many fewer. If you want the roof-over anyway, we are not the ones to build it; what we hand you is a tear-off number, and the comparison you are making is that against the other roofer’s, with the rows above as the difference.