How to Compare Flat Roof Bids When Hiring a Subcontractor

(Contractor Edition — Written for Indiana GCs and Builders)

Estimated Reading Time: 13–15 Minutes

Summary

Flat roof bids are notoriously difficult to compare because no two roofing subcontractors submit proposals the same way. Some include detailed scope breakdowns; others submit single-page quotes. Some list membrane thickness and insulation values; others leave out crucial information entirely. This creates confusion, hidden costs, and situations where the lowest bid can become the most expensive mistake a general contractor can make.

This guide teaches Indiana general contractors exactly how to evaluate and compare flat roof bids: line-by-line scope analysis, membrane differences, insulation levels, drainage design, flashing standards, warranty quality, crew ability, hidden fees, and long-term value. By using a standardized comparison system, contractors can finally compare bids fairly — and select the right subcontractor with confidence.

Introduction: Why Flat Roof Bids Are Almost Impossible to Compare Without a System

Commercial flat roofing proposals vary more than bids from almost any other construction trade. One subcontractor may provide a 10-page detailed scope breakdown with full specifications, while another submits four sentences: “TPO roof, materials included, warranty provided.” On the surface, both sound like roofing proposals — but in reality, the scope, material quality, and installation standards may be entirely different.

The most dangerous mistake a general contractor can make is assuming all roofing bids are comparable. They aren’t.

Indiana contractors deal with harsh winters, freeze–thaw cycles, heavy snow loads, and strict code requirements. Flat roofs in this region must meet specific standards for insulation, drainage, flashing, fastening, and warranty eligibility. If two bids don’t address these details equally, they cannot be compared fairly.

The purpose of this guide is simple:
To give GCs a system for comparing roofing bids that cuts through the noise, eliminates surprises, and protects budgets and timelines.

Section 1: Start by Comparing the Scope of Work Line-by-Line

A bid is only as good as the scope it’s based on. If the scopes vary, comparing price is meaningless.

This is where most general contractors get burned. One bid includes full tear-off, tapered insulation, reinforced flashing, and drains — while another only includes a membrane overlay and calls it “the same job.”

To make a fair comparison, you must standardize the scope across all bids.

What the Scope MUST Include

Before you evaluate pricing, verify that every subcontractor has specified:

• Roof system type (TPO, PVC, EPDM, coating, or hybrid)
  
  
  

• Membrane thickness (45, 60, 80+ mil)
  
  
  

• Reinforced vs. non-reinforced membrane
  
  
  

• Fastening method (mechanically fastened, fully adhered, induction welded)
  
  
  

• Flashing systems and termination methods
  
  
  

• Penetration and curb standards
  
  
  

• Insulation type (ISO, EPS, XPS)
  
  
  

• Total R-value required by Indiana code
  
  
  

• Tapered insulation layout (critical for drainage)
  
  
  

• Drainage design and slope plan
  
  
  

• Demo or tear-off details
  
  
  

• Deck repair pricing
  
  
  

• Debris removal and dumpster plan
  
  
  

• Crickets behind HVAC units
  
  
  

• Edge metals and parapet details
  
  
  

• Safety requirements and roof access plan
  
  
  

If even one of these items is missing, the bid cannot be evaluated accurately.

Why This Matters

Contractors who don’t include these details almost always cut corners.
Their price looks good — until the leaks start.

If a roofer refuses to provide a detailed scope, you are not comparing bids… you are comparing guesses.

Section 2: Evaluate Membrane Specifications (Where Cheap Bids Hide Their Cuts)

The membrane is the most visible part of a roof — but price differences often come from what isn’t visible.

Cheap bids often cut membrane costs without telling you:

• Thinner membrane instead of 60–80 mil
  
  
  

• No reinforcement scrim
  
  
  

• Unbranded materials
  
  
  

• Secondary manufacturers without real warranties
  
  
  

Specs to Compare

To properly compare membranes, look at:

• Brand (Carlisle, Firestone, GAF, etc.)
  
  
  

• Thickness (45, 60, 80 mil)
  
  
  

• Color (white TPO/PVC vs. black EPDM)
  
  
  

• Weld vs. adhesive application
  
  
  

• Scrim reinforcement
  
  
  

• Energy efficiency ratings
  
  
  

• Fire ratings
  
  
  

• Wind uplift ratings
  
  
  

A reputable sub lists these clearly.
A cheap sub avoids specifying them altogether.

Red Flags

• Membrane thickness not listed
  
  
  

• Brand missing or unknown
  
  
  

• Generic “white TPO” without details
  
  
  

• No reinforced edges
  
  
  

• No weld temperature specifications
  
  
  

• No mention of warranty eligibility
  
  
  

If the membrane section is vague, the installation will be too.

Section 3: Compare Insulation Types and R-Values (The Largest Cost Difference)

Insulation is often the single biggest line item in a commercial flat roofing bid — and the area where poor subcontractors cut corners the most.

Cut 1 inch of ISO?
You just lowered R-value.
Cut tapered insulation?
You just created ponding water.

These cuts create failure conditions that don’t become visible until after the job is finished — long after the roofer is paid.

Insulation Options

Typical insulation choices include:

• Polyiso (ISO) – Most common, highest R-value per inch
  
  
  

• EPS – Lower R-value, cheaper
  
  
  

• XPS – Moisture-resistant, often used in specialty applications
  
  
  

• Hybrid assemblies – Combined insulation for performance
  
  
  

What to Examine

You must compare:

• Total R-value (Indiana code is strict on this)
  
  
  

• Polyiso thickness
  
  
  

• Tapered insulation layout
  
  
  

• Slope direction and drainage path
  
  
  

• Fastening schedule
  
  
  

• Perimeter and corner fastening enhancements
  
  
  

• Crickets behind HVAC units
  
  
  

If insulation is missing or under-specified, the bid should be thrown out immediately.

Section 4: Evaluate Drainage Plans (The Most Overlooked Part of Any Bid)

Drainage is the difference between a 20-year roof and a 5-year one — yet it’s the most neglected section in cheap bids.

Bad drainage = ponding
Ponding = membrane failure
Membrane failure = expensive callbacks

Key Drainage Details to Compare

• Slope compliance (¼” per foot minimum)
  
  
  

• Tapered insulation included or missing
  
  
  

• Scupper size and placement
  
  
  

• Internal drain layout
  
  
  

• Tie-ins to existing drainage systems
  
  
  

• Crickets at parapet walls
  
  
  

• Crickets behind rooftop AC units
  
  
  

• Gutter integration
  
  
  

If a roofer says, “Your drainage will be fine,” without providing a plan… it won’t be fine.

Section 5: Compare Flashing, Edging, and Penetration Standards

Flashing — not membrane — causes the vast majority of leaks.
This is where roofing bids should be the MOST detailed.

But most cheap bids skip these details entirely.

Critical Flashing Items to Compare

• Minimum flashing height (must be 8–12 inches)
  
  
  

• Reinforced corners
  
  
  

• Inside and outside corner details
  
  
  

• Curb flashing thickness
  
  
  

• Vent and pipe boot types
  
  
  

• Edge metal type (snap-on, gravel stop, drip edge)
  
  
  

• Parapet termination methods
  
  
  

• Counterflashing details
  
  
  

• Termination bars (required for many systems)
  
  
  

Vague flashing = guaranteed leaks.

Red Flags

• “Flashings included” with no details
  
  
  

• “Caulking” listed as a primary sealing method
  
  
  

• No details on edge metal
  
  
  

• Cheap EPDM boots used on TPO/PVC roofs
  
  
  

• Missing termination bar details
  
  
  

If flashing isn’t fully detailed, expect failures.

Section 6: Verify Warranty Differences (NDL vs. Labor-Only vs. No Warranty)

Two bids may say “20-year warranty,” but they can be completely different products.

Types of Warranties to Compare

1. Manufacturer NDL (No-Dollar-Limit) Warranty

• Covers entire roof system
  
  
  

• Requires certified installer
  
  
  

• Requires inspections
  
  
  

• Best and safest warranty
  
  
  

2. Manufacturer Material Warranty

• Covers membrane only
  
  
  

• Labor and flashings not included
  
  
  

• Does not protect contractor fully
  
  
  

3. Subcontractor Labor Warranty

• Covers installation defects
  
  
  

• No manufacturer involvement
  
  
  

• Often voids if roofer goes out of business
  
  
  

Red Flags

• Roofer offers NDL but is not factory-certified
  
  
  

• Roofer refuses to allow manufacturer inspections
  
  
  

• Warranty not documented in writing
  
  
  

• Generic warranty statements like “20-year roof”
  
  
  

Warranties are not equal — and some are essentially worthless.

Section 7: Evaluate Crew Quality, Safety, and Project Management

Price means nothing if the crew is untrained.

Quality Signals to Look For

• Dedicated crew (not day labor)
  
  
  

• On-site foreman with years of experience
  
  
  

• Safety plan included
  
  
  

• Daily project reporting
  
  
  

• Photo documentation
  
  
  

• Weather delay procedures
  
  
  

• Detailed timeline with milestones
  
  
  

Why This Matters

A roofing system is only as good as the crew that installs it.
Incorrect welding, skipped fasteners, poor flashing, or bad hand-offs can ruin even the best materials.

Section 8: Identify Hidden Costs Inside the Bid

This is where low bids become high bids.

Common Hidden Cost Traps

• Dump fees not included
  
  
  

• Crane time not listed
  
  
  

• Deck repair “as needed” with no price
  
  
  

• Moisture testing not included
  
  
  

• Freight surcharges
  
  
  

• Mobilization fees
  
  
  

• Winter roofing conditions upcharges
  
  
  

• No pricing for penetration additions
  
  
  

• Permit fees excluded
  
  
  

• Safety compliance costs added later
  
  
  

A bid that looks $6,000 cheaper can easily become $12,000 more expensive once the change orders start rolling in.

Section 9: Pricing Isn’t Everything — How to Choose the Best Bid

Once you’ve evaluated all the above categories, price becomes only one factor.

How to Decide on the Best Bid

• Compare apples-to-apples scopes
  
  
  

• Choose the best value, not the lowest number
  
  
  

• Prioritize certifications and warranties
  
  
  

• Give weight to communication, detail, and professionalism
  
  
  

• Consider long-term performance and future repairs
  
  
  

• Favor subcontractors who provide clarity instead of vague language
  
  
  

Cheap roofing leads to headaches, callbacks, and reputation damage.
Smart contractors choose the bid that protects them long-term.

Conclusion: A Systematized Approach Prevents Expensive Mistakes

Flat roofing bids can be confusing and inconsistent — but once you know what to look for, the decision becomes clear. By comparing membrane specs, insulation, drainage, flashing, warranty quality, crew capability, and hidden costs, you gain control of the bidding process and eliminate uncertainty.

A strong roof protects the building.
A strong subcontractor protects your business.

Use this system for every flat roof bid you review — and you’ll avoid the costly surprises that trap less experienced contractors.