Commercial Roofing Considerations for New and Existing Buildings

A commercial roof protects everything under it: inventory, equipment, tenants, and the building structure itself. Yet roofing decisions often get made late in design or only after a leak forces the issue. That timing costs owners money, schedule, and years of roof life.

Across East Tennessee, we see the same pattern on new builds and older buildings alike. Offices in Knoxville, medical buildings in Maryville and Alcoa, retail centers in Blount County, and hospitality properties in Townsend and Sevierville all face the same roofing questions. Which system fits the building? Can the old roof stay, or must it come off? What does the energy code require? How do you keep the roof performing for its full service life?

This guide answers those questions. It covers roofing considerations for new commercial construction and for existing buildings, plus the best practices that protect your investment over time.

Quick Answer: The Commercial Roofing Considerations That Matter Most

Whether the building is new or existing, these factors drive a good commercial roofing decision:

1. Roof slope and drainage: Water must leave the roof fast. Ponding water shortens roof life.
2. Roof system type: TPO, PVC, EPDM, modified bitumen, built-up roofing, or metal. Each fits different buildings.
3. Structural capacity: The deck and framing must carry the roof, rooftop equipment, snow, and any future solar.
4. Wind uplift resistance: Attachment and edge details must meet code for the site's wind exposure.
5. Insulation and energy code: Roof replacements and new roofs must meet current insulation requirements.
6. Penetrations and rooftop equipment: Curbs, pipes, and HVAC units cause most leaks. Plan them early.
7. Recover vs. tear-off: Building codes limit when a new roof can go over an old one.
8. Warranty and maintenance: A warranty only holds value if the owner maintains the roof as required.

1. Roofing Considerations for New Commercial Construction

On a new building, the roof is a design decision, not a product pick. The best results come when the general contractor, architect, structural engineer, mechanical engineer, and roofing contractor plan the roof together during design.

Design Drainage Into the Structure

Most commercial buildings use low-slope roofs. "Low slope" does not mean flat. Membrane roofs need real slope to move water to drains, scuppers, or gutters. Under the International Building Code, most single-ply and built-up systems need a minimum slope of 1/4 inch per foot.

Slope can come from the structure itself or from tapered insulation. Key drainage planning steps include:

• Primary and secondary drainage: Codes require overflow drains or scuppers in case primary drains clog.
• Crickets and saddles: These small sloped areas push water around curbs and toward drains.
• Drain placement at low points: Drains belong where the roof deflects, not just where they fit the plan.
• Gutter and downspout sizing: East Tennessee storms drop heavy rain fast. Size for real rainfall intensity.

Water that sits on a roof for more than 48 hours after rain is a warning sign. Most membrane manufacturers treat long-term ponding as a maintenance or design problem, and it can affect warranty coverage.

Match the Roof System to the Building

No single roof system fits every building. The right choice depends on building use, budget, roof traffic, chemical exposure, and expected service life. GAF, one of the largest roofing manufacturers in North America, publishes these typical lifespan ranges:

• TPO (thermoplastic polyolefin): 20 to 30 years. A white, heat-welded single-ply membrane. Popular for offices, retail, and warehouses.
• PVC (polyvinyl chloride): 15 to 30 years. A heat-welded membrane that resists grease and chemicals. Common on restaurants and food-service buildings.
• EPDM (synthetic rubber): 10 to 15 years per GAF's estimate. A flexible membrane with a long track record on low-slope roofs.
• Modified bitumen: 10 to 30 years for SBS and 10 to 20 years for APP. A multi-ply asphalt system with strong puncture resistance.
• Built-up roofing (BUR): 15 to 20 years. Layers of asphalt and felts, often with a gravel surface.
• Metal roofing: 40 to 60 years or more. Standing seam metal suits pitched roofs, event spaces, and buildings that need a long service life.

Real lifespan depends on installation quality, drainage, climate, and maintenance. A well-installed, well-maintained roof often outlasts a premium roof that gets neglected.

Plan for Structural Loads

The roof structure must carry more than the roof itself. Engineers account for:

• Dead loads: The roof assembly, insulation, deck, and any ballast.
• Rooftop equipment: HVAC units, exhaust fans, and their curbs and supports.
• Snow and ice loads: East Tennessee sees occasional heavy snow, especially at higher elevations near the Smokies.
• Ponding loads: Water that collects in a deflected area adds weight and causes more deflection.
• Future loads: Solar panels, added equipment, or a rooftop deck. Planning for these now avoids costly reinforcement later.

Engineer for Wind Uplift

Wind does not push a roof down. It pulls the roof up, especially at corners and edges. Roof systems must be attached to resist the wind pressures set by ASCE 7, the national standard for structural loads that the building code references.

Best practices include:

• Fastening patterns that increase at corners and perimeters, where uplift is strongest.
• Roof-edge metal tested to the ANSI/SPRI ES-1 standard, which the International Building Code references for edge securement.
• Systems tested and rated by recognized bodies such as FM Approvals or UL when the owner's insurer requires it.

Many roof failures start at the edge. Once wind lifts the perimeter metal, the membrane behind it can peel back fast.

Meet the Current Energy Code

Roof insulation is one of the largest energy decisions on a commercial building. According to the U.S. Department of Energy's Building Energy Codes Program, Tennessee's state commercial energy code is the 2021 International Energy Conservation Code (IECC) and ASHRAE 90.1-2019, effective April 17, 2025. Some local jurisdictions adopt codes on their own schedule, so confirm requirements with the local building department.

Energy code compliance affects:

• Insulation thickness: Required R-values drive roof assembly height, which affects parapets, curbs, and door thresholds.
• Continuous insulation: Insulation above the deck, installed in staggered layers, reduces thermal bridging and condensation risk.
• Roof color and reflectance: Light-colored "cool roof" membranes reduce heat gain and cooling demand in hot months.

Building these requirements into early design avoids surprises during permitting. Our guide to navigating local zoning and permits in East TN covers how plan review fits into the schedule.

Control Condensation Inside the Assembly

Warm, humid interior air can condense inside a roof assembly when it meets a cold surface. Over time, that hidden moisture wets insulation, rusts steel decks, and rots wood decks. Buildings with high interior humidity, such as kitchens, pools, and some medical spaces, carry higher risk.

A design team controls this risk with the right vapor retarder, insulation layout, and ventilation for the building's use.

Coordinate Penetrations and Rooftop Equipment Early

Most roof leaks start at penetrations, not in the open field of the membrane. Every pipe, vent, curb, and conduit is a potential leak point. On new construction, the general contractor coordinates trades so that:

• Mechanical units sit on properly sized and flashed curbs.
• Penetrations cluster where possible, with enough space between them for proper flashing.
• Conduit and gas lines run on supports, not directly on the membrane.
• Walkway pads protect high-traffic routes to equipment.
• Equipment is set before the final membrane work, so roofers flash it once and correctly.

This kind of trade sequencing is central to a proven construction process. It keeps crews from damaging finished roof work.

2. Roofing Considerations for Existing Commercial Buildings

Existing buildings bring different challenges. The roof may be aging, the building may be occupied, and hidden conditions may sit under the current membrane. The goal is to choose repair, restoration, recover, or replacement based on facts, not guesswork.

Start With a Professional Roof Assessment

A good decision starts with a full condition assessment. A thorough evaluation typically includes:

• A visual inspection of the membrane, seams, flashings, edges, and drains.
• Core cuts to identify the number of existing roof layers and the insulation type.
• Moisture surveys, such as infrared thermography or electrical capacitance scans, to find wet insulation.
• A deck check for rust, rot, or deflection.
• A review of interior leak history, ceiling stains, and past repairs.

Wet insulation is the key finding. It loses insulating value, adds weight, and can damage the deck. Its extent often decides between repair and full replacement.

Repair, Restore, Recover, or Replace

Owners usually have four paths:

• Repair: Fixes isolated problems on a roof that is otherwise sound. Best for young roofs with localized damage.
• Restore: Applies a fluid-applied coating over a sound roof to extend service life. Works only when the existing roof and insulation are mostly dry.
• Recover: Installs a new roof over the existing roof. Saves tear-off time but has strict code limits.
• Replace (tear-off): Removes the roof down to the deck and installs a new system. Lets the team inspect and fix the deck and bring insulation up to current code.

Know the Code Limits on Roof Recovers

The International Building Code limits when a new roof can go over an existing one. Under the code's reroofing provisions, all existing roof layers must come off when:

• The existing roof or insulation is water-soaked or too deteriorated to serve as a base.
• The existing roof covering is wood shake, slate, clay, cement, or asbestos-cement tile.
• The building already has two or more roof coverings.

Even when a recover is allowed, it adds weight. An engineer should confirm the structure can carry the added load, especially on older buildings.

Check the Deck and the Structure

Tear-off is the only chance to see the deck for years. Steel decks can rust at drains and low spots. Wood decks can rot where leaks persisted. Concrete decks can crack or spall.

Older buildings may also predate current wind, snow, and seismic requirements. A structural review protects the owner before adding insulation, equipment, or solar to an older roof.

Plan Around an Occupied Building

Reroofing an occupied building takes careful planning. A contractor experienced in commercial remodeling and renovations plans for:

• Phasing: Tear off only what crews can dry-in the same day.
• Weather windows: Watch forecasts closely and keep temporary protection ready.
• Tenant communication: Share schedules, noise windows, and odor notices in advance.
• Air intakes: Shut down or protect HVAC intakes during adhesive or asphalt work.
• Site safety: Protect entrances, sidewalks, and parking areas from falling debris.

Address Roof Changes During Additions and Renovations

Building commercial additions often means tying a new roof into an old one. These tie-ins are common leak points. Best practices include separating the two roofs with an expansion joint where structures move independently, and matching or replacing the old membrane near the connection.

Interior work matters too. New HVAC equipment, kitchen exhaust, or plumbing vents during tenant build-outs all create new roof penetrations. Each one needs a qualified roofer to flash it, or the existing roof warranty may be void.

3. Commercial Roof Warranties: What Owners Should Know

Not all roof warranties are equal. Owners should understand the two main types:

• Contractor workmanship warranty: Covers installation errors for a set period. Its value depends on the contractor staying in business.
• Manufacturer warranty: Ranges from material-only coverage to a "no dollar limit" (NDL) system warranty that covers both materials and labor.

Before signing, ask these questions:

• Does the warranty cover materials only, or materials and labor?
• Is coverage prorated over time?
• Does it cover wind speeds above a stated limit?
• What maintenance and inspections does the owner have to perform and document?
• Who may legally cut into or repair the roof without voiding coverage?
• Does the warranty transfer if the building sells?

4. Best Practices for Commercial Roof Maintenance

Maintenance is the cheapest way to extend roof life. Industry guidance, including from the National Roofing Contractors Association (NRCA) and major manufacturers, recommends inspecting commercial roofs at least twice a year and after major storms.

A Practical Maintenance Schedule

1. Spring inspection: Check for winter damage, cracked flashings, and loose edge metal.
2. Fall inspection: Clear leaves and debris before winter. Confirm drains and gutters flow freely.
3. After severe weather: Inspect after hail, high winds, or heavy snow. Document damage with photos for insurance.
4. After rooftop work: Inspect after any HVAC, electrical, or plumbing service on the roof.

What a Good Inspection Covers

• Drains, scuppers, gutters, and downspouts
• Seams, laps, and membrane surface condition
• Flashings at curbs, walls, and penetrations
• Edge metal, copings, and parapet caps
• Ponding areas and signs of deck deflection
• Sealant condition at pitch pans and terminations
• Damage from foot traffic or dropped tools

Keep a Roof File

Every commercial building should have a roof file with drawings, warranty documents, inspection reports, repair records, and photos. A complete file speeds insurance claims, protects warranty coverage, and helps future contractors understand the roof.

5. Commercial Roofing Planning Checklist

Use this checklist before you start a new build, a reroof, or a major renovation:

1. Design and drainage:
   • Does the roof have a minimum slope of 1/4 inch per foot toward drains?
   • Are overflow drains or scuppers designed in?
2. System selection:
   • Does the roof system match the building's use, traffic, and chemical exposure?
   • Is the expected service life clear for each option?
3. Structure and wind:
   • Has an engineer confirmed the deck and framing can carry all loads, including future solar?
   • Are attachment and edge details designed for site wind speeds?
4. Code and energy:
   • Does the insulation meet the current energy code for the jurisdiction?
   • Do recover plans meet the building code's reroofing limits?
5. Coordination:
   • Are rooftop equipment and penetrations set before final membrane work?
   • Do all trades know who may cut into the roof?
6. Long-term care:
   • Is the warranty type clear, with documented maintenance requirements?
   • Are twice-yearly inspections on the calendar?

Roofing scope also affects early budgets. Building it into a detailed pre-construction estimating process helps avoid change orders later.

Protect Your Building From the Top Down

A commercial roof works best when owners treat it as a building system, not a product. On new construction, that means designing drainage, structure, insulation, and penetrations together from day one. On existing buildings, it means starting with a real assessment and following the building code's reroofing rules. On every building, it means steady maintenance and clear records.

Richardson Construction has built across East Tennessee since 2004. We coordinate roofing as part of our light commercial construction services, from new builds to additions and renovations. For dedicated commercial roof replacement, repair, and storm work, our partners at RC Roofing serve Blount County, Knox County, and the surrounding area.

Planning a new commercial building, an addition, or a reroof in Townsend, Maryville, Alcoa, or Knoxville? Contact Richardson Construction or call 865-268-9727 to talk through your project.

Frequently Asked Questions

What is the best roof for a commercial building?

The best commercial roof depends on the building. TPO is a common choice for offices, retail, and warehouses because it is reflective and heat-welded. PVC suits restaurants because it resists grease. Standing seam metal suits pitched roofs and buildings that need a long service life. A roofing professional should match the system to the building's use, slope, and budget.

How long does a commercial roof last?

Lifespan varies by system and care. GAF estimates 20 to 30 years for TPO, 15 to 30 years for PVC, 10 to 30 years for modified bitumen, 15 to 20 years for built-up roofing, and 40 to 60 or more years for metal. Drainage, installation quality, and regular maintenance often matter as much as the material.

Can a new commercial roof go over the old one?

Sometimes. The International Building Code allows a roof recover only when the existing roof is dry and sound. A full tear-off is required when the existing roof is water-soaked or deteriorated, when it is slate, tile, or wood shake, or when the building already has two or more roof coverings. An engineer should also confirm the structure can carry the added weight.

How often should a commercial roof be inspected?

Industry guidance recommends inspecting a commercial roof at least twice a year, typically in spring and fall. Owners should also schedule inspections after hail, high winds, heavy snow, and any rooftop equipment work. Regular inspections catch small problems early and help protect warranty coverage.

Does a commercial reroof need to meet the current energy code?

Generally, yes. When a commercial roof is replaced, energy codes typically require the new roof assembly to meet current insulation requirements. Tennessee's state commercial energy code is the 2021 IECC and ASHRAE 90.1-2019, though local jurisdictions may differ. Confirm the requirements with your local building department during planning.

What causes most commercial roof leaks?

Most commercial roof leaks start at penetrations, flashings, and roof edges, not in the open membrane. Pipes, HVAC curbs, drains, and wall transitions are the most common trouble spots. Ponding water, clogged drains, and damage from foot traffic also shorten roof life.