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Roof Curb Coordination Guide

How to review roof curbs for rooftop equipment, structure, flashing, insulation height, roof slope, drainage, and maintenance access.

EnvelopeMay 7, 2026

Roof curbs support rooftop units, exhaust fans, skylights, hatches, duct penetrations, and other roof-mounted items. They also interrupt the roof assembly, so they must coordinate with structure, flashing, insulation, drainage, and maintenance access.

A curb that fits the equipment can still fail if it is too low for insulation, lands between structural supports, blocks drainage, or conflicts with warranty details.

Coordination Checks

Review roof curbs on the architectural roof plan, structural framing plan, mechanical roof plan, equipment schedule, and roofing details. Each drawing carries part of the requirement.

  • Curb size, height, slope correction, and equipment orientation.
  • Structural support, opening framing, and concentrated loads.
  • Flashing height, counterflashing, membrane tie-in, and warranty requirements.
  • Insulation thickness, cricket layout, drainage path, and ponding risk.
  • Service clearance, fall protection, and safe walking routes.

Common Problems

Common roof curb misses include curbs ordered before final equipment selection, curb heights that do not clear tapered insulation, ducts that do not align with openings, and curbs placed too close to parapets or drains.

Helonic helps flag these conflicts by comparing the roof and MEP drawings that are often reviewed separately.

Frequently Asked Questions

Why must curb height account for insulation?
The curb has to rise above the finished insulation and membrane so flashing can terminate at the required height, commonly eight inches above the roof surface in NRCA details. Tapered insulation raises the effective roof height, so a curb sized to the deck can end up too short. A short curb leads to flashing that cannot lap correctly and can void the roof warranty.
What structural coordination does a roof curb need?
Curbs transfer concentrated equipment loads to the framing, so the opening framing and support have to align with the curb footprint. A curb that lands between joists or over an unframed opening needs added steel or dunnage. This is checked on the structural framing plan, not the mechanical roof plan.
Why order roof curbs after final equipment selection?
The curb has to match the selected unit footprint, duct openings, and orientation, which can change between basis of design and the purchased unit. Curbs ordered early often have duct openings that do not align. Confirming the equipment schedule before fabrication avoids field modification.
How do roof curbs affect drainage?
A curb interrupts the roof plane and can dam water if crickets and slope are not detailed around it. Ponding against a curb stresses the membrane and can void the warranty. The cricket layout and drainage path should be checked on the roof plan against the curb locations.
What access issues come with roof curbs?
Curbs placed too close to parapets, other units, or drains can block service clearance and safe walking routes. Fall protection and clearances for coil pulls or filter changes need room around the curb. These conflicts appear only when the roof plan and equipment clearances are reviewed together.
MG

Manas Gandhi

Co-founder & CTO, Helonic

Manas is the co-founder and CTO of Helonic, where he leads engineering and AI research for construction drawing analysis. He works directly with structural, MEP, civil, and fire protection engineers to translate the way they review drawings into AI systems that flag the issues that actually matter in the field. Before Helonic, he built machine learning pipelines for technical document understanding and has spent the last several years interviewing licensed design engineers and discipline leads to ground product decisions in real practice rather than industry assumptions.

Areas of focus
  • AI for technical document understanding
  • Cross-discipline coordination workflows
  • Code compliance automation (IBC, NEC, NFPA, IPC, IMC, ASCE)
  • Structural and MEP drawing review systems

How this page was researched: Roof curb and flashing review points were checked against NRCA roofing details and SMACNA rooftop equipment support practice, with insulation height and drainage cross-referenced to the roof membrane manufacturer warranty requirements. Examples reflect the curb conflicts Helonic most often flags when comparing roof plans, structural framing, and mechanical equipment schedules.

Last reviewed by Manas Gandhi · May 2026

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