Commercial Roof Assemblies and Compatible Substrates

Read the Roof from the Deck Up

Compatibility Has Several Meanings

A product may appear to adhere and still be unsuitable. Professional compatibility includes:

• Chemical compatibility: The coating, primer, sealants, existing membrane, mastics, and previous coatings must not soften, swell, stain, inhibit cure, or damage one another.
• Adhesive compatibility: The specified system must establish and retain adequate bond after cleaning, preparation, priming, curing, weathering, and exposure.
• Mechanical compatibility: The cured system must tolerate the substrate's movement, texture, seams, fasteners, joints, and expected traffic.
• Moisture compatibility: The assembly must not trap unacceptable moisture or create blistering, corrosion, biological growth, or freeze-thaw damage.
• Code and assembly compatibility: The proposed system must be supported for the deck, insulation, attachment, roof covering, fire classification, wind design, and other required approvals.
• Warranty compatibility: Every component and repair material must be accepted by the organization issuing the warranty.

Metal Roofs

Metal roofs may include structural standing-seam panels, exposed-fastener panels, corrugated panels, metal roof decks used as direct substrates, and factory-coated or previously field-coated metal.

Identify the metal, factory finish, corrosion condition, panel profile, seam design, fasteners, clips, movement, penetrations, and previous repair materials.

Primary Concerns

• Loose or missing fasteners, deteriorated washers, backed-out screws, and enlarged holes
• Corrosion at laps, cut edges, fasteners, gutters, ponded locations, and dissimilar-metal contact
• Oil, grease, chalked factory finish, mill treatment, biological contamination, and loose coating
• Panel movement and stress at horizontal seams, end laps, ridge conditions, and transitions
• Adhesion to galvanized, Galvalume-type, painted, weathered, and previously coated surfaces

Rust conversion, rust-inhibitive primer, seam reinforcement, fastener treatment, and field coating must be selected as one compatible system. Coating does not restore metal lost to corrosion or correct inadequate structural attachment.

Single-Ply Membranes: TPO, PVC, and EPDM

Do not identify a single-ply membrane by color alone. Confirm the membrane type through records, markings, manufacturer information, controlled examination, or qualified testing.

TPO, PVC, and EPDM differ in surface chemistry, additives, aging, seam construction, cleaning requirements, and primer needs.

Confirm that the coating manufacturer approves the exact membrane type and condition. Conduct representative adhesion tests at the field membrane, repairs, seams, and other materially different surfaces.

Built-Up Roofing and Modified Bitumen

Asphaltic roofs may be smooth-surfaced, granule-surfaced, coated, flooded with asphalt, or covered with aggregate. Modified bitumen may be APP- or SBS-modified and installed in several ways.

Surface condition and age affect bleeding, oil migration, adhesion, cracking, blistering, and coating appearance.

Primary Concerns

• Wet insulation, blisters, splits, ridges, open laps, fishmouths, and displaced surfacing
• Loose granules, exposed reinforcement, brittle areas, soft asphalt, and incompatible mastics
• Asphalt bleed-through or staining that may require an approved bleed-blocking primer
• Surface irregularity that increases actual material consumption
• Drainage and recurring ponding that may exceed the selected system's limitations

Do not bury active blisters or unstable surfacing. Follow the roof and coating manufacturers' written requirements for aging, preparation, repairs, primers, and test patches.

Sprayed Polyurethane Foam Roofs

SPF depends on its protective coating to shield the foam from ultraviolet exposure and weather. Evaluate coating erosion, cracks, pinholes, exposed foam, bird damage, hail damage, traffic damage, poorly shaped drainage, repairs, and adhesion between the existing coating and foam.

Dark, friable, powdery, wet, or deteriorated foam must be removed to acceptable material and rebuilt with a manufacturer-approved repair procedure. Simply spraying coating over degraded foam can lock in a weak plane.

Verify whether the project requires recoating only, foam repair, scarification and replacement, or broader reconstruction.

Concrete Roof Decks and Cementitious Substrates

Concrete may serve as the structural deck, a direct substrate for a liquid-applied membrane, or a component below insulation and another roof covering.

Direct application demands evaluation of cure, moisture, surface strength, laitance, curing compounds, sealers, form-release agents, contaminants, cracks, joints, honeycombing, slope, and drainage.

• Weak laitance and unsound concrete must be removed rather than sealed beneath the membrane.
• Cracks must be classified as stable, moving, structural, or nonstructural before detailing.
• Expansion and structural joints require designed joint treatment; they are not ordinary cracks.
• Surface moisture and moisture within the slab require project-appropriate investigation.
• Pull-off testing on concrete may be specified to evaluate the prepared substrate and installed system.

Cover Boards and New-Construction Substrates

Gypsum-based, cementitious, wood-fiber, high-density polyisocyanurate, plywood, oriented-strand-board, and other panels do not behave identically.

Board joints, fasteners, facer adhesion, moisture sensitivity, surface porosity, dimensional movement, and required reinforcement must be addressed by the approved system design.

A board that is acceptable beneath a sheet membrane is not automatically approved for direct application of a liquid membrane. Confirm the exact board manufacturer, product, thickness, attachment, joint treatment, primer, membrane construction, fire classification, and wind-uplift assembly.

Existing Coatings

Identify the existing coating chemistry and condition whenever possible. Examine adhesion, chalking, embrittlement, checking, cracking, erosion, blistering, dirt pickup, biological growth, and previous repairs. Different areas may contain different generations of material.

Aged silicone is especially difficult for many non-silicone materials to bond to without specifically approved preparation and primer. Never assume that pressure washing alone makes unlike chemistries compatible.

A Practical Compatibility Process

1. Collect drawings, specifications, warranties, repair records, core information, and manufacturer records.
2. Confirm the deck, insulation, cover board, roof covering, attachment, surfacing, and previous coatings.
3. Map changes in materials, repairs, wet areas, distress, contamination, ponding, and details.
4. Compare the existing assembly with the proposed manufacturer's published eligible substrates and limitations.
5. Define cleaning, preparation, repairs, primers, reinforcement, and membrane construction in writing.
6. Install representative test patches using the complete proposed preparation and primer process.
7. Obtain written resolution of exceptions before pricing or production application.
8. Document substrate acceptance before covering the work.