California Title 24 Cool Roof Reroofing Guide | 2025

By the NC Roofing Solution team

California’s cool-roof requirement is often reduced to a slogan that is not accurate: that every reroof in the state needs a cool roof, or that only the hottest climate zones are covered. Neither statement gives a homeowner enough information to plan a compliant project.

For residential reroofing, the answer depends on four questions: Is the work a reroof or a repair? How much of the roof area is being replaced? Is that roof area steep-slope or low-slope? Which California climate zone applies to the property? Exceptions and the selected compliance path can also change the result.

This guide explains the 2025 California Energy Code rule for alterations. Its primary source is Section 150.2(b)1H and the reroof examples in Chapter 9 of the California Energy Commission’s 2025 Single-Family Residential Compliance Manual.

The Short Answer

The prescriptive cool-roof requirement for a residential reroof is triggered when 50 percent or more of the roof area is replaced and the roof is in a covered climate zone. Exactly 50 percent is included; one-half of the roof area meets the threshold.

The climate-zone rules are:

• Steep-slope roofs, 2:12 or greater: Climate Zones 4 and 8-15.
• Low-slope roofs, under 2:12: Climate Zones 4 and 6-15.

This is not a statewide prescriptive requirement. Properties in climate zones outside the applicable list do not trigger this particular cool-roof requirement merely because 50 percent or more of the roof is replaced. Other Energy Code provisions, building-code requirements, permit conditions, or local rules may still apply.

Why a Reroof Is an Alteration, Not a Repair

The Energy Code separates additions, alterations, and repairs. That classification matters because the Energy Code does not apply to a repair in the same way it applies to an alteration. Calling a reroof a repair does not make the alteration rules disappear.

An alteration is a change to an existing building’s systems or envelope that is not an addition. The CEC manual expressly lists replacing or installing a new finish surface on an existing roof, and replacing part or all of a roof assembly, as alterations. Replacing an existing roof covering with a new one is therefore a reroof alteration.

A repair is reconstruction or renewal performed to maintain an existing building component. A localized roofing repair may fall into that category when it maintains the existing roof rather than replacing roof surface area as a reroofing project. The actual permitted scope controls; the label on an estimate does not.

The manual addresses this distinction directly in Example 9-14. Replacing an existing wood-shake roof with asphalt shingles is not treated as a repair. Because the roofing component is being replaced and the Energy Code contains requirements for it, the work is an alteration. Once work is classified as a reroof alteration, the area, slope, climate zone, exceptions, and compliance path must be evaluated.

The 50 Percent Trigger Includes Exactly Half

Section 150.2(b)1H uses a clear threshold: cool-roof requirements are triggered when 50 percent or more of the roof area is being replaced. A scope covering exactly one-half of the roof meets the threshold.

That wording has two practical consequences. First, a contractor should determine the area included in the reroof scope rather than describe the job loosely as a partial replacement. Second, splitting the roof into phases does not change what the permit documents say is being replaced under the project. The Building Division reviews the submitted scope and determines how the code applies.

If the project replaces less than 50 percent of the roof area, this specific prescriptive cool-roof trigger is not reached. That does not turn a reroof into a repair and does not remove other applicable permit or code requirements. It answers only the area-threshold question under the cool-roof alteration rule.

Roof Slope and Climate Zone Determine Where the Rule Applies

After the project reaches the area threshold, identify the slope of the roof area being replaced and the property’s official climate zone.

Steep-Slope Roofs: 2:12 or Greater

A roof with a slope of 2 units of vertical rise for every 12 units of horizontal run, or greater, is steep-slope for this rule. Under the prescriptive alteration path, the cool-roof requirement applies in Climate Zone 4 and Climate Zones 8-15.

Low-Slope Roofs: Under 2:12

A roof below 2:12 is low-slope. Under the prescriptive alteration path, the cool-roof requirement applies in Climate Zone 4 and Climate Zones 6-15.

These lists are different. Climate Zones 6 and 7, for example, are included for a low-slope reroof but not for a steep-slope reroof. That is why a property’s climate zone alone does not answer the question; slope must be checked too.

Roofs With More Than One Slope

Some homes combine a steep main roof with a low-slope addition, porch, or garage section. The manual’s Example 9-13 says that each area must meet the criteria that apply to its own slope when the prescriptive approach is used. A mixed roof should not be assigned one result based only on the larger section. The contractor must identify which replacement areas are low-slope and which are steep-slope, then apply the correct climate-zone rule to each.

Why the Wrong Climate-Zone Range Keeps Appearing

The “CZ 10-15” range comes from Table 9-1, which covers prescriptive additions, not reroof alterations.

That difference is easy to miss because additions and alterations appear in the same chapter of the compliance manual. Table 9-1 is titled “Envelope Roof/Ceiling Requirements for Prescriptive Additions.” It addresses new roof work associated with added conditioned floor area. A reroof on an existing residence is an alteration and is governed by the alteration provisions, including Section 150.2(b)1H.

Using the additions table for a reroof drops climate zones that the alteration rule actually includes. For an alteration, the correct ranges are Climate Zones 4 and 8-15 for steep-slope roofs, and Climate Zones 4 and 6-15 for low-slope roofs. The project category must be identified before any climate-zone range is applied.

Prescriptive Exceptions for Steep-Slope Reroofs

Even when a steep-slope reroof reaches the area threshold and lies in a covered climate zone, Section 150.2(b)1H provides prescriptive exceptions. The cool-roof prescriptive requirement is waived if the building meets at least one of these conditions:

• The building has at least R-38 ceiling insulation.
• The building has an attic radiant barrier meeting Section 150.1(c)2.
• The building has no ducts in the attic and is in Climate Zone 2, 4, 9, 10, 12, or 14.
• The building has R-2 or greater insulation above or below the roof deck.

These are alternatives, not a checklist requiring all four. Each is a factual building condition that should be documented correctly. The no-duct exception is itself limited to the listed climate zones; it should not be carried into every zone. An attic inspection, plans, insulation documentation, or other permit information may be needed to establish the exception used.

For low-slope reroofs, the manual identifies two separate prescriptive exceptions: one for meeting the aged solar reflectance requirement and another for meeting the roof-deck insulation R-value in Table 150.2-E. The steep-slope exceptions above should not be assumed to replace that separate low-slope analysis.

The Performance Approach Is an Alternative

Prescriptive compliance is not the only path. A residential alteration may instead demonstrate compliance through the performance approach in every case. Under that approach, approved compliance software evaluates the building and the proposed alteration as a whole against the Energy Code’s energy budget.

This is not an informal waiver. It is a documented code-compliance method, and applicable mandatory measures still apply. The performance approach may allow trade-offs with other qualifying energy features, but it normally requires project-specific modeling and compliance forms. The contractor and energy consultant, when one is involved, should identify the selected path before materials are ordered and the permit package is finalized.

Bay Area Service Cities: Verified Climate Zones

The table below covers only the cities served by NC Roofing Solution whose climate zones have been verified for this guide. “Both apply” means that, once the 50 percent area trigger is reached, the climate zone is covered for both steep-slope and low-slope roofs under the prescriptive alteration rule. Exceptions or performance compliance may still change the required assembly.

City	Climate zone	Steep-slope result	Low-slope result
Concord	12	Applies	Applies
Pleasant Hill	12	Applies	Applies
Lafayette	12	Applies	Applies
Orinda	12	Applies	Applies
Danville	12	Applies	Applies
San Mateo	3	Does not apply	Does not apply
Daly City	3	Does not apply	Does not apply
San Bruno	3	Does not apply	Does not apply
San Rafael	2	Does not apply	Does not apply

For example, a qualifying reroof in Concord is in Climate Zone 12, so both slope categories fall within their respective climate-zone lists. A comparable project in San Mateo is in Climate Zone 3, which is outside both lists. That difference is why the rule cannot accurately be described as applying everywhere in California.

Which Code Cycle Applies?

The 2025 Energy Code applies to permit applications submitted on or after January 1, 2026. The permit-application date matters; the year in which a homeowner first discusses or receives an estimate for the project does not by itself select the applicable code cycle.

Energy Code cycles change, and enforcement agencies review the facts of individual projects. NC Roofing Solution confirms the current requirement with the local Building Division on every permitted job. That confirmation should cover the applicable code edition, climate zone, reroof area, roof slope, compliance path, and any claimed exception.

What a Homeowner Should Ask Before Reroofing

You do not need to interpret the entire Energy Code yourself. Before approving a reroof scope, ask the contractor three direct questions:

1. Which climate zone is the property in? Ask for the zone used in the permit documents, not a guess based on a nearby city.
2. Does the permitted scope replace 50 percent or more of the roof area? Ask how the replaced area was determined, especially if the roof has several sections.
3. Which compliance path is being used? The answer should identify prescriptive compliance, including any documented exception, or the performance approach with the required energy documentation.

Also ask how mixed slopes are being treated. A low-slope section can have a different result from a steep-slope section on the same home. These questions should be resolved during planning, before the roofing specification and permit documents are final.

Confirm the Requirement for Your Reroof

NC Roofing Solution evaluates the roof area, slope, climate zone, and available compliance path as part of planning a permitted reroof. To discuss your property and confirm what applies to the proposed scope, contact NC Roofing Solution. We are a licensed California roofing contractor, CSLB #1111166, Classification C-39.