How to Repair a Built-Up Asphalt Flat Roof

Asphalt flat roofs are a common type of commercial and industrial roof. Over time, the elements take a toll and repairs become necessary. Fixing minor issues early can help prevent more costly repairs later. This guide will walk through the steps for repairing a built-up asphalt flat roof.

Inspect the Roof

The first step is a thorough inspection and identification of any problem areas. Look for:


Blisters form when moisture becomes trapped between layers. They appear as bumps or bubbles on the surface. When blisters burst, they expose the inner layers to water damage. It’s important to repair them quickly.


Cracks can form from thermal expansion and contraction of the roof deck or substrate movement. Left unaddressed, cracks allow water intrusion that leads to interior leaks. Inspect for cracks around roof penetrations, seams, and edges.

Ponding Water

Low spots where water collects speeds deterioration. Ponding water should evaporate within 48 hours. Standing water beyond this time indicates improper drainage that needs correction.

Exposed Felts

The reinforcing felts provide strength between asphalt coats but are not meant to remain exposed. Bare areas likely mean overlaying asphalt has worn away. These spots need recoating.


Alligatoring describes severe blistering with cracks resembling reptile skin. It signals advanced deterioration of roof felts. These sections often require complete replacement.


Leaks indicate water penetration through the roof. Inspect plumbing vent stacks, skylights, adjoining walls, and interior ceilings for any signs of leakage. Detect and repair the source right away to avoid structural damages.

Loose Materials

Make sure all roofing components like aggregate stones, flashing pieces, or insulation boards are securely fastened. Wind or foot traffic can displace loose materials over time.

Make Repairs

Once problem areas are identified, proper repairs can be made. Safety should always come first when doing roof work. Use fall protection, tie-offs, and toe boards when working at heights. Repair methods will vary depending on the issues discovered.

Recoat Bare Spots

If underlying felts are exposed, recoating with asphalt is often the solution. First remove any loose rocks or dirt from the surface. Heat the asphalt to 400°F to achieve proper viscosity for spreading. Use a squeegee to apply a uniform layer 1/8” to 1/4” thick across bare areas. Embed loose gravel over top while asphalt is still warm.

Patch Blisters

Carefully cut out the blistered portion. Scrape away any delaminated felts or saturated insulation. Allow the area to fully dry. Apply roofing cement, then overlap with new modified bitumen and smooth with a roller to re-adhere. Top with matching aggregate.

Repair Cracks

Clean out cracks wider than 1/4” with compressed air and prime. Fill narrow cracks with asphalt roofing cement. For large cracks, heat strips of modified bitumen to soften and place over the crack. Adhere with roofing cement and use a roller to smooth.

Install New Flashing

Where flashing has detached or become damaged, install new flashing. Cut it to overlap the roof edge or penetration by at least 4”. Set into asphalt roof cement and nail in place. Apply another coating of cement over top for a watertight seal.

Drainage Correction

Improper drainage leads to ponding water, accelerated wear, and leaks. Survey for any low spots. Raise these areas by installing a tapered insulation fill. Adhere with hot asphalt for positive drainage.

Replace Alligatored Sections

For severe alligatoring, full replacement is needed. Remove the damaged section down to roof deck. Cut replacement modified bitumen to size and set into hot asphalt. Adjoin all seams with cement. Top with matching aggregate.

Roof Overlay

For extensive deterioration, a roof overlay may be the best solution. This involves installing a new roof membrane over the existing one. This avoids the time and expense of a complete tear-off. Ensure proper drainage first, then adhere insulation, modified bitumen sheets, and aggregate stones.

Leak Repair

Start by locating the source of the leak. Clean roof area, dry thoroughly, and prime. For small punctures, apply roofing cement and a patch split to overlap 3” in all directions. For corner leaks, install new corner flashing secured with roofing nails and cement.

Resecure Materials

Check for any loose aggregate, insulation boards, sheet metal, or flashings. Clean and dry surfaces. Set materials back in place and adhere using hot asphalt or roofing cement per manufacturer guidelines.

Maintain the Roof

Roof repairs address current issues, while maintenance helps minimize future problems. Include roof maintenance as part of standard building operations.

Inspect Annually

Schedule professional roof inspections at least once per year. Technicians can spot early signs of damage and recommend repairs when problems are small. Annual inspections protect roofs and avoid major issues down the road.

Clear Debris

Remove accumulated debris like leaves, branches, dirt, trash from roof surface and drains. Clogged drains lead to ponding water which accelerates deterioration. Keep materials cleared for proper drainage.

Recoat Every 5-7 Years

Even well-maintained roofs need recoating every 5-7 years. This maintains water protection and extends the roof’s service life. Plan ahead and budget accordingly.

Check Flashings

Inspect perimeter edge flashings and around penetrations. Look for gaps, tears, loosening, or deterioration. Deficient flashings are a common source of leaks. Early remediation prevents costly water damage.

Note Changes

Inspect for any signs of new blistering, splitting seams, punctures, or exposed membrane. Early intervention keeps small issues from becoming major repairs.

With diligent maintenance and prompt repairs when needed, built-up asphalt roofs can deliver decades of reliable performance. Staying proactive preserves roofs in quality condition. Address minor problems before they grow into roof failures requiring full replacement.

Frequently Asked Questions

Here are answers to some common questions about repairing built-up asphalt flat roofs:

What are signs my roof needs repair?

Indications your roof needs repair include blisters, cracks, ponding water, exposed felts, alligatoring, leaks, and loose materials. Addressing issues promptly extends the roof’s life.

What if I have a leak inside?

Start by locating the source of the leak on the roof. Look for punctures, gaps in flashing, deteriorated membrane, etc. Make repairs to stop the leak, then address any interior water damage.

How long do built-up roofs last?

With proper maintenance, built-up asphalt roofs typically last 20-30 years. Recoating every 5-7 years extends lifespan. Prompt repairs help prevent premature failure.

When should I do a roof overlay?

Consider an overlay when 20% or more of the roof surface needs repair. Overlays are also good options for roofs nearing end of lifespan but still in repairable condition.

How often should roof inspections be done?

Commercial roofing experts recommend inspecting flat roofs at least twice per year, ideally in spring and fall. Annual inspections are a minimum. This helps catch issues early.

Can I make roof repairs myself?

Roof repair can be dangerous and is best left to qualified contractors. They have the skills, proper equipment, and follow safety standards for working at heights and with hot asphalt.

How much does built-up roof repair cost?

Costs vary based on the repair method, roof size, access difficulty, and local labor rates. Simple repairs like re-flashing may run $200+ while an overlay could cost $4-$8 per square foot or more.

What is the benefit of preventative maintenance?

Preventative maintenance like annual inspections, debris removal, and re-coating extends the roof’s lifespan. Performing regular care reduces long term costs compared to waiting for major failures and replacements.

How can I avoid ponding water on my roof?

Check for low spots during inspections. Raise areas through the use of tapered insulation fills. Keep roof drains clear of debris. Extend downspouts to move water well away from the building.

What safety gear should be used for roof work?

Proper safety equipment for roof repairs includes fall arrest harnesses and lanyards, toe boards, hearing protection against loud equipment, protective clothing, and eye shields.


Flat roofs are vulnerable to damage and leaks if not properly maintained. By completing thorough inspections and timely repairs, costly premature failures can be avoided. address small issues before they become major. When hiring a contractor, be sure they follow best practices for built-up roof repair. With proper maintenance, a quality asphalt roof can provide 20-30 years of reliable performance. Staying vigilant through regular inspections and prompt repairs keeps roofs in top condition.