Two beams on the same floor, both requiring 90 minutes of fire protection, can need noticeably different intumescent thickness. The reason is section factor, and understanding it is the difference between a fireproofing budget that holds and one that moves.
What section factor measures
Section factor, written Hp/A and expressed in units of one per metre, is the heated perimeter of a steel section divided by its cross sectional area. It describes how quickly a member heats up in a fire. A light section with a large exposed surface relative to its mass has a high section factor and heats rapidly. A heavy column with the same perimeter but far more steel has a low section factor and heats slowly.
The practical consequence: heavy sections often need surprisingly little protection, while slender secondary steel needs the most. Thickness is not proportional to member importance; it is driven by thermal mass.
How it enters the quotation
Manufacturers publish loading tables mapping section factor and required fire resistance period to a dry film thickness for each product. An estimator takes your member schedule, reads the section factor for each profile, applies the fire rating, looks up the thickness and multiplies by the surface area. That produces a genuine material take off.
Without a member schedule, a supplier can only apply an average thickness to a tonnage figure, and averages are where surprises live. If your steel is mostly light secondary members, an average based quote will fall short. This is why our guide to UAE fire rating periods stresses getting the schedule right at enquiry stage.
Exposure configuration matters
The heated perimeter depends on how many faces are exposed. A beam supporting a concrete slab is protected on its top flange, giving three sided exposure and a lower section factor than the same beam exposed on four sides. Box protection versus profile following protection also changes the calculated perimeter. Loading tables list these cases separately.
Documentation for Civil Defence
The approval submission should show the member schedule, section factors, specified thickness per member and the test evidence supporting those loadings, from assessment to EN 13381-8 or testing to BS 476 for cellulosic exposure. Application records of achieved dry film thickness then demonstrate the built condition matches the approved design.
MENA Industrial Supplies supplies Interchar intumescent coatings with loading tables and certification support. Send your member schedule for a same day take off.