Sprinklered vs. Unsprinklered Allowable Area for a Mixed-Occupancy Addition

The examiner must first determine whether the code permits the occupancies to be treated separately or requires the building to be evaluated as a single occupancy.

1. Establish the construction type and whether it is sprinklered. Type IIB is a noncombustible construction type with limited fire-resistance. Under the model code, allowable area per story for each occupancy group is increased when the building is equipped with an approved automatic sprinkler system. The applicant is correct that a sprinkler increase applies, but incorrect that it eliminates the need for frontage and open-space analysis.

2. Check the separation between occupancies. A 1-hour fire barrier between the A-2 and B areas is generally sufficient to allow each occupancy to be evaluated independently for allowable area, provided the barrier meets the required rating and is continuous. If the separation were less than required, the more restrictive occupancy would govern the entire building.

3. Compute allowable area for each portion. For the A-2 portion, the tabulated allowable area for Type IIB is relatively low; the sprinkler increase and any frontage increase must be applied. For the B portion, the tabulated area is higher. The examiner compares the actual area of each portion against its own allowable area, not the total building area against a single number.

4. Apply the frontage increase. The frontage increase is based on the building's perimeter that fronts on a public way or open space of sufficient width, expressed as a percentage of the total perimeter. With 60 feet on a public way and 40 feet on qualifying open space, the applicant may be entitled to a frontage increase, but only if the open space meets the minimum width and access requirements. The examiner should verify the open-space width and that the public way is truly a public way.

5. Conclusion. The examiner should not accept the blanket claim that sprinklering resolves the area question. The correct approach is to evaluate each occupancy separately, apply the sprinkler increase, verify and apply any frontage increase, and confirm that each portion's actual area does not exceed its allowable area. If either portion exceeds its allowable area, the applicant must reduce area, increase separation, or upgrade construction type.

A reduction in the separation rating changes the analytical basis of the review.

• Re-evaluate the occupancy separation. If the code requires a 1-hour separation for the combination of A-2 and B occupancies, a 30-minute barrier is noncompliant as submitted. The examiner should issue a comment requiring the barrier to be restored to the required rating or the design revised.
• Consider the single-occupancy alternative. If the applicant chooses to remove the separation entirely or reduce it below the threshold for independent evaluation, the building must be treated as a single occupancy. Under the model code, the allowable area for a mixed-occupancy building evaluated as a single occupancy is based on the most restrictive occupancy, and the sum of the ratios of actual area to allowable area for each occupancy must not exceed 1.0. This often produces a more restrictive result than independent evaluation.
• Document the decision. The examiner should record which method was used, the tabulated areas relied upon, and the basis for accepting or rejecting the separation. This protects both the applicant and the jurisdiction if the decision is appealed.
• Do not rely on the sprinkler system to cure a separation deficiency. Sprinklers may increase allowable area, but they do not substitute for a required fire-resistance-rated separation between occupancies unless the code expressly permits it.