5.5 Roof Coatings & Restoration Systems

5.5 Roof Coatings & Restoration Systems

The published C-42/R-42 scope of work expressly includes applying "urethane foam, and roof insulation or coatings on or above the roof deck." Coating and restoration is a licensed CR-42 activity, and in Arizona it is an enormous share of the low-slope market: intense ultraviolet exposure, negligible rainfall and a state-wide interest in reflectivity make fluid-applied restoration economically attractive on roofs that still have sound insulation and a sound deck.

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1. When Restoration Is Legitimate — and When It Is Fraud

A coating renews a weathering surface. It does not fix wet insulation, failed fasteners, inadequate slope or a deteriorated deck. Before any restoration is proposed:

1. Survey for entrapped moisture using infrared thermography (ASTM C1153), capacitance or nuclear methods as described in Section 8.3, and cut cores to verify.
2. Remove and replace every wet area. Coating over saturated insulation seals the moisture in, drives blistering as the sun heats the assembly, and accelerates deck corrosion.
3. Confirm attachment. Coatings add negligible wind-uplift resistance; a mechanically attached membrane with backed-out fasteners is not a candidate.
4. Confirm drainage. Ponding water defeats most coating chemistries other than silicone, and it usually signals a slope defect the coating cannot cure.
5. Confirm remaining service life. A membrane that has lost its reinforcement to embrittlement will not be rescued by a surface film.

[!WARNING] Selling a coating over known wet insulation is precisely the kind of workmanship failure that produces an ROC complaint under A.A.C. R4-9-108 and A.R.S. § 32-1154(A)(2), with a two-year complaint window under A.R.S. § 32-1162 running from completion.

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2. Coating Chemistries

Substrate Compatibility

• BUR and gravel-surfaced: spud and vacuum loose gravel, or use a gravel-embedding base coat; asphalt emulsion or aluminum-fibered systems are traditional, acrylic and silicone over a bleed-blocking primer are modern.
• Modified bitumen: excellent candidate; check for surface bleed-through and prime accordingly.
• EPDM: always requires a primer; unprimed acrylic over cured EPDM will delaminate.
• TPO and PVC: require a manufacturer-specified primer; PVC also presents plasticizer migration risk, so use only systems tested against PVC.
• Metal: treat rust to bare or stabilized metal, prime, and stripe-coat every seam and fastener with reinforcing polyester fabric before field coating.
• SPF: the intended and best substrate; recoating is part of the SPF maintenance cycle described in Section 5.3.
• Silicone: only silicone bonds to silicone. A previously siliconized roof forecloses every other chemistry without full removal.

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3. Thickness and Coverage Mathematics

Coatings are sold by the gallon and specified in mils (thousandths of an inch). One gallon spread one mil thick covers a fixed, calculable area:

$1 \text{ gallon} = 231 \text{ in}^3 \quad\Rightarrow\quad \frac{231\text{ in}^3}{0.001\text{ in}} = 231{,}000\text{ in}^2 = \mathbf{1{,}604\text{ ft}^2 \text{ at 1 mil wet}}$

From that single constant, everything else follows:

$\text{Wet film thickness (WFT)} = \frac{\text{Dry film thickness (DFT)}}{\text{Volume solids}}$

$\text{Gallons per square} = \frac{100 \times \text{WFT}}{1{,}604}$

Worked Example 1 — Silicone at 96% solids, 20 mils DFT specified:

$\text{WFT} = \frac{20}{0.96} = 20.8\text{ mils} \qquad \text{Gal/square} = \frac{100 \times 20.8}{1{,}604} = \mathbf{1.30\text{ gal per square}}$

A 60-square roof therefore needs 78 gallons, plus waste and stripe-coat material.

Worked Example 2 — Acrylic at 55% solids, same 20 mils DFT:

$\text{WFT} = \frac{20}{0.55} = 36.4\text{ mils} \qquad \text{Gal/square} = \frac{100 \times 36.4}{1{,}604} = \mathbf{2.27\text{ gal per square}}$

The same specified dry thickness costs 75% more acrylic material and must be applied in at least two coats, because 36 wet mils cannot be laid down in one pass without sagging and mud-cracking. This is why high-solids silicone frequently wins on installed cost even at a higher price per gallon — and why a bid quoted in gallons rather than dry mils is meaningless.

Field verification uses a wet film thickness gauge (a notched comb) pressed into the fresh coating at a documented frequency, backed by material reconciliation: gallons actually consumed ÷ squares actually covered must reconcile to the specified rate. Manufacturers void warranties on under-application, and under-application is invisible once the coating dries.

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4. Surface Preparation

1. Clean. Power wash at roughly 3,000 to 4,000 psi with a manufacturer-approved cleaner; rinse thoroughly so no detergent residue remains, and let the roof dry completely.
2. Repair. Correct every open seam, split, blister, fishmouth and failed flashing before coating. A coating is not a repair.
3. Reinforce. Embed polyester reinforcing fabric in a wet base coat over all seams, penetrations, drains, transitions and repaired areas, then top-coat. This detail work is where restoration systems succeed or fail.
4. Prime where the substrate requires it — EPDM, TPO, PVC, bleeding asphalt, rusted metal.
5. Adhesion test. Apply test patches and pull them after cure (ASTM D3359 tape adhesion, or ASTM D4541 pull-off for quantitative work) before committing to a full roof.

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5. Reflectivity, Energy Codes and Cool Roofs

White coatings deliver an initial solar reflectance around 0.80 to 0.87 and thermal emittance near 0.85 to 0.90, which is why coatings are the least expensive route to a cool roof on an existing building. Two cautions belong in every proposal:

• Rate on three-year aged values, not initial values. The Cool Roof Rating Council publishes both. Silicone in particular loses substantial reflectance to dirt pickup in its first year.
• Cool roofs shift the dew point. Lowering the membrane temperature in a humid interior — a grocery, a natatorium, a commercial kitchen — can move condensation into the assembly. In arid Arizona this is usually benign, but it is not automatically so, and Section 6.2's reverse-vapor-drive analysis applies.

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6. Code Status, Warranties and Documentation

For one substrate the code answers the question directly: IBC Section 1511.3.1(4) and IRC Section R908.3.1(4) expressly permit "the application of a new protective coating over an existing spray polyurethane foam roofing system" without tear-off of existing roof coverings. That is a coating-specific allowance written into the model code.

For every other substrate, a fluid-applied coating is normally treated as maintenance rather than as a roof recover, so the recover limits in IBC Section 1511.3.1.1 and IRC Section R908.3.1.1 — including the two-covering bar — generally do not reach it. A full fluid-applied restoration membrane with reinforcing fabric is sometimes classified as a recover assembly instead. Confirm the classification with the authority having jurisdiction before pulling or skipping a permit; getting it wrong is a code-departure ground under A.R.S. § 32-1154(A)(2).

Manufacturer restoration warranties (commonly 10, 15 or 20 years) require a contractor certified by that manufacturer, a pre-installation core-and-moisture survey, documented mil thickness with reconciled material usage, and a final field inspection. They are usually renewable — recoating at the end of the term extends coverage, which is the central economic argument for restoration over replacement.

Restoration Decision Checklist

• Moisture survey performed and all wet insulation removed and replaced
• Deck, fasteners and attachment verified sound
• Positive drainage confirmed, or silicone specified for ponding areas
• Substrate identified and the correct primer selected (never assume EPDM, TPO or PVC needs none)
• Dry mils specified, converted to wet mils and gallons per square, and verified in the field
• Seams, penetrations and drains reinforced with embedded fabric
• Aged, not initial, reflectance values used in any energy claim
• Recover-versus-maintenance classification confirmed with the building official