Title 24 Cool Roof Requirements
A low-slope nonresidential roof needs an aged solar reflectance of 0.63 and a thermal emittance of 0.75, or an SRI of 75, in climate zones 2, 4 and 6 through 15. Steep-slope roofs have a much lower bar, and not every climate zone is covered.
Two roof slopes, two very different bars
Low-slope roofs carry the demanding requirement: aged solar reflectance 0.63 with thermal emittance 0.75, or alternatively a Solar Reflectance Index of 75. Steep-slope roofs need aged solar reflectance of only 0.20 with the same 0.75 emittance, or an SRI of 16.
The gap is about how much sun the surface actually receives. A near-flat roof takes the midday load almost square-on, so its surface has by far the most work to do. A pitched roof takes it at an angle and for less of the day.
The climate zones are not all of California
The low-slope requirement applies in climate zones 2, 4 and 6 through 15. The steep-slope requirement applies in a much narrower set: zones 4 and 8 through 15.
Read the gaps and the logic is visible. Zone 1 on the far north coast and zone 16 in the mountains are outside both — in a heating-dominated climate a reflective roof rejects warmth you were paying to keep. Zones 3 and 5 sit outside the low-slope list for the same reason at milder strength. This is one of the places where the zone number decides whether a requirement exists at all, not merely how strict it is.
"Aged" is a rated value, not an estimate
The requirement is written against aged reflectance rather than the initial figure, because a roof gets dirty. Weathering and soiling take a meaningful bite out of a new membrane's performance in the first few years, and the code prices that in from the start.
You cannot compute this yourself. Any roofing product used for compliance must be rated and labelled by the Cool Roof Rating Council, which is the body the code designates. A product without a CRRC rating cannot be used to demonstrate compliance whatever its data sheet claims, and for 2025 SRI calculations follow ASTM E1980 (2019).
Reflectance and emittance are separate properties
Reflectance is how much solar radiation the surface throws back. Emittance is how readily the surface sheds the heat it did absorb. A roof can be good at one and poor at the other, which is why both numbers are specified — and why bare metal is a problem: it is reasonably reflective and a poor emitter, so it holds what it takes in.
Nothing changed from 2022
The prescriptive cool-roof values did not change between the 2022 and 2025 cycles. This is one of the few envelope areas where a specification carried over from a 2022 project is still current — worth knowing, since most of the rest of the 2025 code cannot be assumed forward that way.
Where it bites hardest
Re-roofing. New construction picks a compliant membrane at specification stage at little or no premium. An existing building re-roofing to a required colour, or matching an adjacent phase, may find the reflectance figure is the constraint — at which point the U-factor trade available on low-slope alterations becomes the route rather than a compromise.