How to Compare Commercial Roofing Bids Line by Line

A property manager who asks for three commercial roofing bids and then lays the three totals side by side is doing the one thing almost guaranteed to buy the wrong roof. Commercial flat roof bids are not priced the same way, and two contractors can both write replace the roof while describing genuinely different assemblies: different membranes, different insulation, one tearing off to the deck and one recovering over what is already there. Circling the low number without reading what is under it is how a building ends up with the shortest-lived roof on the block and a warranty that never gets honored. The discipline that fixes it is simple to state and rarely followed: normalize the scope first, then compare the price. This guide is the line-by-line spec sheet that turns three unlike commercial bids into an honest, apples-to-apples decision.

01 / THE SHORT ANSWERMatch the assembly before you compare the price

You cannot compare two commercial roofing prices until you know they are buying the same roof assembly. A flat or low-slope reroof on a small to mid-size metro Detroit building runs $9,000 to $16,000 installed, and that spread is not noise. It reflects real differences in what goes on the deck: which membrane and how thick, how it is attached, how much insulation and to what R-value, whether the old roof comes off or gets recovered, how the edge metal and flashings are detailed, and who stands behind the finished system. A bid that sits thousands below the others has almost always subtracted something from that list, and the missing item does not vanish. It moves from the bid to a leak call two winters from now. For a larger or unusual building, no honest range covers it from the street, which is exactly what the free inspection is for.

So the whole method is two steps, in order. First, read every bid against a fixed spec sheet and mark what each one includes and excludes until all of them describe the identical assembly. Second, and only then, compare the prices, because now the numbers finally mean the same thing. Most owners skip step one and go straight to step two, which is precisely how the cheapest bidder wins jobs the best roofer should have won. The rest of this page is that spec sheet, plus the red flags that tell you a bid is incomplete before you ever call a reference.

02 / THE SPREADWhy three flat roof bids disagree

It helps to know where the gap actually comes from, because it is rarely one contractor being greedy and another generous. On a commercial flat roof the difference between a high bid and a low bid usually lives in a handful of specific, invisible-once-installed choices, and each one is a decision the contractor made about how to build the roof.

Different assemblies, described the same way

Two bids can both say "new single-ply roof" and mean very different work. One means a full tear-off to the deck, fresh insulation to current R-value, new edge metal, and a fully adhered membrane. The other means a mechanically attached membrane laid straight over the existing roof with no new insulation and reused edge metal. Both are technically a new roof, and the second is genuinely cheaper to perform, so its lower price is not a discount on the first job. It is the honest price of a lesser one. The trouble is only that both are printed in the same four words.

The details you cannot see from the parking lot

Membrane thickness, attachment method, insulation package, and cover board are all levers a contractor pulls to hit a target price. A thin membrane, a single thin layer of insulation, and reused metal will always beat a thicker membrane over a full insulation package with a cover board on price. The finished roofs look similar from the ground on the day the crew leaves, and they age very differently across fifteen Michigan freeze-thaw seasons. Because most of these choices disappear the moment the last sheet is welded down, they are the easiest place for a bid to save money quietly. Our overview of recover versus tear-off on commercial buildings unpacks the single biggest one.

Overhead that protects you, or does not

Part of the spread is the business behind the truck: proper liability and workers compensation coverage, a permit pulled and inspected, manufacturer certification to install the specified membrane, real fall protection, and a warranty that will be honored. A bid that undercuts everyone is sometimes cheaper because it skips exactly these things, and those savings come out of your protection, not the contractor's margin. Cheap can mean efficient, or it can mean uninsured and uncertified, and the spec sheet below is how you tell the two apart before you sign.

03 / THE SPEC SHEETThe line-by-line commercial spec sheet

Here is the core of the method. Go through every bid and confirm each line below is spelled out in writing. Where a bid is silent on an item, that silence is your answer: assume it is not included and ask. When every bid accounts for every line, and only then, you have real prices for the same roof.

1. Membrane system, brand, and mil thickness

"Single-ply" is a category, not a product. A bid should name the membrane family (TPO, EPDM, PVC, or a modified bitumen system), the manufacturer, and the mil thickness of the sheet, because a 45-mil and a 60-mil membrane are different roofs at different prices. A bid that only says "rubber roof" or "TPO" with no manufacturer and no thickness is hiding the ball, and you cannot compare it to one that spells the product out.

2. Attachment method

How the membrane is held down drives both cost and wind performance: fully adhered, mechanically attached, or ballasted. Fully adhered systems cost more in labor and adhesive; mechanically attached use fewer materials but rely on fastener patterns and are noisier in wind; ballasted systems are limited by structure and slope. A bid should state the method, and if wind uplift matters on your building, it should tie the attachment to a design pressure, not just assert the roof will hold.

3. Insulation type, layers, and R-value

Above-deck insulation is a large, invisible line. A bid should name the type (polyiso is most common), the number of layers, whether a cover board is included, and the resulting R-value. This matters more than owners expect, because a commercial reroof often triggers current energy-code insulation minimums that quietly grow scope and cost, so a bid that reuses old insulation or adds a single thin layer is not just cheaper, it may be non-compliant. Any code R-value a contractor cites should trace to the adopted Michigan energy code, not to memory.

4. Tear-off versus recover

The single biggest scope difference is whether the old roof comes off. A full tear-off to the deck lets the crew see and correct the deck and gives a clean substrate; a recover lays the new system over the existing one and hides everything beneath it. Code generally allows a recover only over a single existing dry layer, and mandates tear-off when there are already two layers, wet insulation, or a damaged deck. Recover is cheaper on paper and can be the right call on a sound roof, but it must be a stated choice with the code conditions verified, not a silent cost cut. The conditions live in our recover versus tear-off guide.

5. Edge metal, flashings, and penetrations

The field of the roof is the easy part; the edges and details are where flat roofs leak and where bids diverge. A complete bid states new edge metal (a code-tested perimeter is not optional on commercial work), new base and counter flashings at walls and curbs, and how every penetration, drain, scupper, and rooftop unit is tied in. Reused edge metal and reused flashings are a common quiet saving and a common source of the next leak. Count the penetrations against your own roof and make sure the bid did too.

6. Wet-deck and unit pricing in writing

Nobody can see the full condition of the deck and insulation until tear-off, so wet or deteriorated sections are the most common legitimate change order. An honest bid states a per-square-foot unit price for replacing wet insulation and a per-sheet price for deck repair up front, so a surprise is a known unit cost rather than a blank check. A bid with no unit pricing is not cheaper; it is deferring the money conversation to the worst possible moment, when the crew is already open on your roof and the sky is the schedule.

04 / THE LOW BIDWhat the cheapest bid is really telling you

Once you run the spec sheet, the low bid usually explains itself, and it is worth naming the pattern plainly: a commercial bid thousands under the others is almost never the same job at a discount. It is a different, smaller job at its honest price. The savings are the tear-off you did not get, the insulation that was reused, the edge metal that stayed, or the certification and insurance the contractor does not carry. Every omission has a cost; it is simply scheduled for later instead of now. Here are the red flags that a bid is incomplete before you even compare totals.

• Vague "to code" language with no membrane brand, no mil thickness, and no stated R-value. "We will bring it up to code" is a sentence, not a specification.
• No core cut taken. A contractor who bids a flat roof without cutting a core sample to see the existing assembly is guessing at what is under the surface, and guesses become change orders.
• No unit price for wet deck or insulation. Without a per-unit number in writing, a soft-deck discovery becomes an open-ended bill mid-project.
• Warranty terms that never appear in writing. A spoken "twenty-year warranty" that is nowhere on the page is not a warranty, it is a sales line.
• Reused edge metal and flashings described as "existing to remain" on a roof old enough to need a new membrane.

A commercial bid is not cheaper because the contractor is generous. It is cheaper because it is buying less roof. The only question worth answering is whether you saw what was subtracted before you signed.

That does not mean the lowest number is always wrong. Sometimes a contractor really is more efficient, closer to the site, or hungry for winter work, and their bid is both cheaper and complete. The spec sheet is what lets you tell the two situations apart. If a low bid accounts for every line, named membrane and thickness, stated attachment, code-level insulation, a clear tear-off or recover decision, new edge metal, and unit pricing, it may be a genuinely good deal. If it is low because half the sheet is blank, it is not cheaper at all. It just moved the bill downstream.

05 / THE BACKINGWarranty type and the exclusions that void it

Warranty is the part of a commercial bid owners compare least and regret most, because two roofs that look identical on the day the crew leaves can carry completely different protection. Commercial warranties come in tiers, and a fair comparison keeps them straight.

Material, workmanship, and NDL

A manufacturer material warranty covers defects in the membrane itself and often requires certified installation and a full accessory package to be valid. A system or NDL (no dollar limit) warranty is broader, covering both material and the manufacturer-inspected installation up to the cost of repair, and it usually requires a certified contractor and a registered roof. The contractor workmanship warranty covers the installation, the far more common source of a leak, and it is worth exactly what the company behind it is worth. Make every bid state which tier it is offering, in years, in writing, and confirm the contractor holds the certification the manufacturer warranty requires. Our warranty guide decodes the tiers in full.

The exclusions that quietly void coverage

The fine print is where warranties come apart. Standing water is the classic one: many membrane warranties exclude damage from ponding, so a roof with a drainage problem can carry a long warranty that pays nothing where it matters. Unauthorized repairs, damage from other trades working on rooftop equipment, and a lapse in required maintenance can all void coverage too. Read what each warranty does not cover as carefully as its term length, and pair it with a documented maintenance habit so the coverage survives. The other half of a warranty being honored is the contractor still being reachable next spring: a local, insured, certified roofer is a different promise than a crew that vanishes after the checks clear.

06 / THE MATRIXBuilding your own bid matrix

You do not need special software to do this well. Write the spec lines down the left side of one sheet, put your bidders across the top, and fill in what each bid says for every line. The exercise does the work for you: the gaps light up, and the questions to ask each contractor write themselves. When a box is blank, that is the call to make before you compare a single dollar figure.

COMPARE THIS LINE	WHAT A COMPLETE BID SAYS
Membrane	Family, manufacturer, and mil thickness named
Attachment	Fully adhered, mechanically attached, or ballasted
Insulation	Type, layers, cover board, and R-value stated
Tear-off vs recover	A stated choice with code conditions verified
Edge metal and flashing	New, not "existing to remain"
Wet deck	Per-unit price in writing for replacement
Warranty	Tier, term in years, and certification confirmed
Exclusions	Ponding and maintenance terms read and understood

Once the matrix is full, normalize before you judge. If one contractor specifies a thicker membrane or a fuller insulation package than the others, ask the others what that upgrade would cost so you are pricing the same assembly. If one is missing new edge metal, ask them to add it and requote. The goal is bids describing the identical roof, at which point the lowest complete price genuinely is the best value and the decision gets easy. To sanity-check whether a normalized total lands where a metro Detroit flat roof should, our cost calculator and the published pricing canon both run the same ranges, and the contractor selection checklist covers vetting the company behind the number.

07 / THE REAL NUMBERGetting a fully-scoped bid, free

The method above makes the bids you already hold honest with each other, but it still starts from numbers someone else wrote. The cleanest way to anchor the whole exercise is to add one more complete, itemized bid to the pile, written by a roofer who took a core cut and walked your actual roof rather than guessing from the lot. Our inspection is $0 with no obligation: we read the membrane and its condition, cut a core to see the assembly and check for wet insulation, count the penetrations, and examine the edges, flashings, and drainage, then put a whole-job price in writing with every spec line spelled out. That gives you a fully-scoped reference bid to hold the others against.

If you want a ballpark before anyone visits, the instant estimator on our homepage uses the same published ranges as this page, and you can browse the rest of our commercial roofing guides to price the individual pieces first. For owners weighing a possible insurance angle after a storm, our storm and insurance claims page explains what a trustworthy contractor can and cannot promise, and the full roofing services hub ties the commercial work to the rest of what we do across Macomb County. When the bids finally line up on the same assembly, the choice stops being a gamble on the lowest total and becomes a straightforward decision between contractors who each priced the identical, correct roof.