The applicator has quoted, and now wants a schedule you do not have
The fire strategy says the structural frame is ninety minutes. The steel drawings say nothing about fire at all, and the only trace of the subject in the whole package is one specification clause requiring intumescent coating to all structural steelwork to achieve the period in the strategy. The applicator priced that clause off the tonnage, because tonnage was the only quantity anybody gave him. Now he has the job, his booth is booked for the week after next, and he wants a schedule: every mark, its section factor, how many sides are exposed, and the thickness that follows. Nobody has produced one, and everybody assumed somebody else had.
The reason it does not exist is that it falls between two documents that never meet. The period comes from the fire strategy, which takes it from Approved Document B or BS 9999 in England, or from Table 601 of the International Building Code in US practice — all of which say the frame shall achieve ninety minutes and say nothing about how much material that takes. The thickness comes from a product assessment report, indexed not by building type or storey height but by one number describing the member: how much steel there is, against how much of its surface the fire can reach. Between them sits an arithmetic step that belongs to nobody's scope and takes an afternoon.
That step is what this page is, and it carries no thickness figure anywhere. A dry film thickness is a property of one named product's assessment, valid only over the periods, limiting temperatures and section factor range that product was tested across; a number lifted from a general article would be wrong against every product it was read into. What can be written down is the number you hand the applicator so his own table gives the right answer.
Why one period produces forty different thicknesses
Steel does not burn and it does not need to. It loses strength as it heats, gradually at first and then quickly through the range where a fire puts it, and somewhere around five to six hundred degrees a fully worked member no longer has the capacity to carry what is standing on it. Protection does not stop that happening; it postpones it. Everything on the outside of the steel is a thermal resistance, and everything inside it is a heat sink, and the period a member survives is the ratio between the two.
Which makes the governing quantity a shape property rather than a material one. Heat arrives through the surface the fire can touch and is absorbed by the mass behind it, so the rate a member heats at goes as the heated perimeter divided by the cross-sectional area. That ratio is the section factor. Eurocode notation writes it A/V, surface area per unit length over volume per unit length; older British practice and most product literature write it Hp/A. Both are the same division, and both are quoted in units of one over metres — which is the one place the arithmetic trips people, because a perimeter in millimetres over an area in square millimetres comes out per millimetre and has to be multiplied by a thousand to reach the figure a loading table is indexed by. Whichever way it is written it runs from about forty for the heaviest columns to well past three hundred for light bracing, and a bigger number always means a member that heats faster.
American practice inverts it. There the member is described by W/D, its weight in pounds per foot over its heated perimeter in inches: the same relationship the other way up, so a high W/D is a massive, slow-heating shape. Nothing is lost in translation except the direction of the arrow, and the hazard is reading one convention with the other's instinct. Whichever the literature uses, the consequence is what the tonnage-based quote missed entirely: the light beams in a frame need several times the film thickness of the heavy columns, and they are also the members carrying the most surface to cover. Both errors run the same way.
The area is already sitting on your weight sheet
The half of the section factor that looks like it needs research does not. Cross-sectional area and mass per metre are the same fact in two units, joined by the density of steel, so any mark that already carries a kilograms-per-metre figure carries its area too. Divide the mass per metre by 7850 and the answer is square metres; multiply the mass per metre by 127.4 instead and the answer is square millimetres, which is the unit section tables use. A member at 24.8 kg/m has an area of about 3,160 mm². A member at 198 kg/m has about 25,200 mm². In US units the same constant reads as an area in square inches of roughly the weight per foot divided by 3.4.
Where the designation does not state a mass — an imported angle, a cold-formed rail, a welded girder made this morning — that figure has to be produced from geometry first, which is a job with its own guide on this site rather than a paragraph here. What matters at this end is a quirk of the arithmetic that runs in your favour: a three-rectangle model of an I section omits the root fillets, so it returns an area slightly below the tabulated one, and a slightly small area gives a slightly large section factor, which takes you to a slightly thicker row. The simplification errs toward more protection rather than less. Use the tabulated area where a table exists all the same, because on a heavy column the fillets are not a rounding.
- Extend the mark schedule the tonnage came off with three columns: cross-sectional area, heated perimeter, section factor.
- Fill the area column from the mass per metre by multiplying by 127.4 for square millimetres, or read it off the section table where the designation appears in one.
- Settle the exposure before touching the perimeter column — three sides or four, profile or boxed.
- Group by section size, not by grid line; forty marks usually carry six or eight distinct section factors, and only the distinct ones are needed.
- Flag everything above about 250 for a second look, because that is where product tables start running out of rows.
- Hand over section factors and exposure, never a thickness — that is the applicator's to read from his own assessment report, and his certificate rests on it.
For a welded girder or any I section without a table entry, this gives the mass per metre and, in its breakdown, the cross-sectional area in square millimetres — the denominator of the section factor, with a perimeter in millimetres over it and a thousand on the end to bring it to per metre.
The I-beam's overall depth, measured outside-to-outside of the flanges.
The width of each flange.
The thickness of each flange.
The thickness of the web connecting the two flanges.
Weight per unit length
31.8 lb/ft
Simplified rectangular approximation omitting web-flange fillet area — for exact values, use the published AISC (or equivalent) shapes table for your specific designation.
- Cross-sectional area
- 9.33 in²
They open the calculator with your figures already in it
Steel I-Beam Weight-per-Length Calculator: 31.76 lb/ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 31.8 lb/ft — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Both flanges are built from the single width and the single thickness you enter, doubled, so the figure describes a doubly-symmetric shape; a fabricated girder with a wider or thicker bottom flange has to be worked out as two separate flange areas and added, not averaged into one entry.
- Nothing here asks how long the beam is. The answer is the mass of one metre of section, so the piece weight that sets the crane class and the needle sizing comes from multiplying it by the actual stick length, and it reads identically for a 1 m (3.3 ft) stub and a 12 m (39 ft) span.
- The section is taken as constant from end to end. Copes, notches, web penetrations and castellated cells remove steel from a real member, while end plates, stiffeners, cleats, bolts, weld metal, primer, galvanizing and an intumescent film all add it back, and none of the four dimension fields can see any of that.
- The entry fields stop at 60 mm of flange thickness, 40 mm of web and 1.2 m of depth, and anything beyond is brought back to the limit with a notice. The heaviest jumbo column sections and deep fabricated plate girders sit outside that range, so what comes back for them is the weight of a smaller section than the one on the drawing; at the light end a web under 3 mm is lifted to 3 mm and reads heavy.
- Depth, flange and web go in and only a cross-sectional area comes out — those same dimensions are never assembled into a second moment of area. So nothing on this page states what the beam can span, carry or deflect, and two sections with the same weight per metre can differ widely in stiffness depending on how much of the steel sits in the flanges rather than the web.
The same beam has four heated perimeters
The numerator is where the judgement lives, and it is not one number. A heated perimeter depends on how many sides the fire can reach and on whether the protection follows the shape or wraps a box around it, and those two questions have four combinations. A beam under a concrete slab is exposed on three sides, because the top of its upper flange is against something that will not let much heat through; the same beam under an exposed steel deck, or a column standing free in a lobby, is exposed on four. Independently of that, a sprayed or coated system follows every face of the profile, while a board encasement has the perimeter of the box, which is very much shorter.
The four-sided profile perimeter of an I section is the same figure as its painted area per metre, and its derivation — including why flange thickness cancels out of it entirely — is worked through in the steel takeoff guide next door rather than repeated here. What that guide flags in one clause is the whole business of this page. Take a box value into a paint quantity and you underbuy on every open section; take a box value into a fire schedule when the protection is being sprayed and you under-protect the frame, which is a different order of mistake.
Put a real pair through it and the spread stops being an argument. A light beam of the 305 by 102 serial size at 24.8 kg/m has an area near 3,160 mm², a depth of about 305 mm, a flange near 102 mm and a web near 5.8 mm. Its four-sided profile perimeter is a little over 1,005 mm; over 3,160 mm² that is 0.318 per millimetre, which is a section factor of about 318 per metre. Boxed on four sides it is 813 mm, or 257. Three-sided profile drops the top flange face: 903 mm and 286. Three-sided boxed: 712 mm and 225. One beam, four legitimate answers spanning nearly a hundred units, and the choice between them is made by the slab above it and by whoever picked between spray and board — neither of which anybody thinks of as a fire decision. Run the same arithmetic on a heavy column of the 305 by 305 serial size at 198 kg/m, profile perimeter about 1,900 mm against an area of 25,200 mm², and the section factor is 75. Same building, same period, same drawing, and one member is more than four times as hungry as the other.
| Case | Heated perimeter | When it applies |
|---|---|---|
| Four-sided, profile | Four flange widths, plus twice the depth, less twice the web thickness | A free-standing column, or any member sprayed or coated all round with fire on every face |
| Three-sided, profile | The same, less one flange width | A beam coated to its profile with a concrete slab bearing on the top flange, which the fire cannot reach past |
| Four-sided, boxed | Twice the sum of the depth and the flange width | A column encased in board or in a boxed casing, where the fire sees the box and not the shape inside it |
| Three-sided, boxed | Twice the depth, plus one flange width | A beam boarded on three faces up to the underside of the slab — the lowest of the four, and the cheapest to protect |
| Hollow section, any exposure | Twice the sum of the two outside dimensions, or pi times the diameter | The bore is sealed and never heated, so the outline is the only perimeter; on a rectangular section a boxed encasement repeats that figure, on a circular one the box is four diameters |
| Angle, flat and open bracing | Twice the sum of the two legs, or the full outline of the flat | Exposed all round in almost every case, which is why these are the worst members in the package |
Hollow sections, where the section factor is very nearly one over the wall
A closed section behaves differently from everything else on the schedule, because the inside of it is not in the fire. Unless the tube has been deliberately vented its bore is a sealed void, so the heated perimeter is the outside outline and nothing more: twice the sum of the two outside dimensions for a square or rectangular section, pi times the diameter for a round one. Boxed and profile values are the same figure, since for a rectangular hollow section they are the same shape. That alone makes an HSS the easiest member in the frame to schedule.
It also collapses into a rule of thumb worth carrying. For a thin-walled tube the cross-sectional area is very close to the perimeter times the wall thickness, so the section factor is close to the reciprocal of the wall: a six millimetre wall lands near 170 per metre, ten near 100, twelve and a half near 80, and the outside dimensions barely enter it. A 200 by 100 by 6 works out at about 174 by the full calculation against 167 from the reciprocal, and the gap is the four corners the thin-wall shortcut counts twice; the real corner radii move it back the other way by two or three. On site that means a hollow bracing member can be brought inside the loading table by ordering a heavier wall rather than a bigger tube — a conversation to have before the steel is rolled, not after the applicator has priced it. Where the tube is to be concrete-filled none of it applies: a filled section is a composite member with its own route in BS EN 1994-1-2, and a filled and reinforced column can sometimes achieve the period with no applied protection at all.
The breakdown line gives the cross-sectional area in square millimetres. Divide twice the sum of the outside width and height by it, multiply by a thousand, and you have the section factor in the per-metre units the loading table is indexed by — a 200 by 100 by 6 lands near 174.
The tube's outer width.
The tube's outer height.
The tube's nominal wall thickness.
Weight per unit length
12.76 lb/ft
- Cross-sectional area
- 3.75 in²
They open the calculator with your figures already in it
HSS (Square/Rectangular Tube) Weight Calculator: 12.76 lb/ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 12.76 lb/ft — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The 7,850 kg/m³ (490 pcf) built into this is carbon steel and nothing else. The same outline in austenitic stainless is nearer 8,000 kg/m³ (499 pcf) and in aluminium about 2,700, so an aluminium section reads close to three times heavier here than it weighs. That gap matters most where the figure is feeding a lift plan, a temporary works check or a freight cost rather than a steel schedule.
- Weight is not a substitution rule. Two sections at the same mass per unit length can behave nothing alike — a deep thin-walled tube and a small thick-walled one of equal mass have very different second moments of area, section moduli and radii of gyration, and it is those, not the mass, that set deflection and buckling. Swapping in a section because it weighs about the same changes how the frame moves.
The members that run off the end of the table
Every intumescent product has a section factor above which its assessment report simply has no row. Where that ceiling sits varies by product, period and limiting temperature, but the members that meet it are always the same: light angle bracing, flats, thin cold-formed rails, small open sections generally. An angle exposed all round has a heated perimeter of twice the sum of its legs, and an area of those legs less the heel, multiplied by the thickness — divide one by the other and the legs very nearly cancel, leaving approximately two over the thickness. Twice as bad as a hollow section of the same wall, and dependent on almost nothing except how thin the metal is.
The numbers are unforgiving. A 100 by 100 by 10 equal angle has an area of 1,900 mm² and a perimeter of 400 mm, giving a section factor of 211 per metre. An 80 by 80 by 6 has 924 mm² and 320 mm, giving 346. A 60 by 60 by 5 is worse again. Those are ordinary bracing members, they appear on every braced bay in the country, and on a ninety or a hundred and twenty minute frame there is a real chance that no thin-film intumescent in the catalogue has an entry for them.
There are four honest ways out and none is applying more coating than the assessment covers. Re-specify the bracing in a heavier thickness purely for the fire case, which is a small steel cost. Substitute a hollow section, which halves the section factor for the same wall. Box the bay in board, so the quantity becomes a girth times a length rather than a coating thickness. Or take the member out of the protection scope, if it can be shown it is not required to hold the frame up at the fire limit state — an argument that exists in BS EN 1993-1-2 and is made by a fire engineer with a calculation, never by an estimator with an opinion. What is not available is quietly putting a thin-film product on a member its assessment does not reach: an intumescent works by expanding into a char many times its applied thickness, and outside the range it was tested over, nobody can say what that char does.
Two legs and a thickness. The breakdown gives the cross-sectional area in square millimetres, and twice the sum of the legs is the heated perimeter; divide, multiply by a thousand, and that is the per-metre figure. Run the bracing angle you have and the one thickness up from it, and see which side of the table's ceiling each lands on.
The length of the first leg, measured along the outside of the angle.
The length of the second leg, measured along the outside of the angle.
The uniform thickness of both legs.
Weight per unit length
10.1 lb/ft
Geometry only, and deliberately an underestimate: the heel fillet and radiused toes are excluded, so a rolled angle weighs slightly more than this — proportionally more on light sections. Use the published shapes table when the exact catalogue weight matters.
- Cross-sectional area
- 2.97 in²
They open the calculator with your figures already in it
Steel Angle (L-Shape) Weight Calculator: 10.1 lb/ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 10.1 lb/ft — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Assumes carbon steel at 7850 kg/m³ (490 pcf), and there is no material field to say otherwise. The same L100x100x10 outline is a shade heavier in stainless and about 5.1 kg/m (3.4 lb/ft) in aluminium against 14.9 (10.0 lb/ft) — roughly a third. Weigh aluminium angle with this and every lifting, handling and fixing decision downstream is out by a factor of three.
- Galvanising puts mass back on, and it lands hardest on the light sections. A hot-dip coating runs around 600 g per square metre of surface, and an angle carries a great deal of surface for its weight: about 2 % extra on a 100x100x10, roughly double that on a 40x40x4. On a load bought and craned by weight, that is the opposite correction to the fillets and worth applying alongside them.
- The underestimate holds for a HOT-ROLLED angle and reverses for a formed one. An angle folded from plate on a press brake has no heel fillet at all, only a bend radius that takes metal out of the corner, so this figure runs marginally over rather than under. It matters most when rolled and formed sections are being compared on price by weight.
Reading a loading table without misreading it
The document the thickness comes from is the product's assessment report, and it is a grid rather than a figure. Section factor runs down one axis in steps; the fire resistance period runs across the other, with a set of columns for limiting temperature inside each period. The cell is a dry film thickness. Beams and columns have separate grids, because the exposure differs and a column carries axial load through the whole event, and the report states whether it covers three-sided exposure, four-sided or both.
Limiting temperature is the column most often filled in by habit. It is not a property of steel but of this member at the load it is actually carrying in the fire situation: the temperature at which it stops being adequate. BS EN 1993-1-2 derives it from the degree of utilisation, so a lightly loaded member tolerates more heat and needs less protection than a fully worked one of identical section. The withdrawn BS 5950-8 tabulated the same relationship against load ratio, which is why British product tables are still published with columns at 550 and 620 degrees. US practice embeds it in the test rather than choosing it: ASTM E119 and UL 263 set steel temperature acceptance criteria for the unloaded route, around a thousand degrees Fahrenheit average on columns and eleven hundred on beams with a higher cap at any single point, while a loaded test simply requires the member to carry its load for the period. If the specification names a limiting temperature, use it; if it does not, ask, because taking the more generous column without authority is the cheapest way there is to under-protect a frame.
The grid has edges and they matter. Below its lowest tabulated thickness a product is not certified, so a very heavy column can end up with a minimum film thicker than the arithmetic wanted. Above its highest section factor there is no entry at all, which is where the bracing angles walk in. Interpolation between rows is allowed only as far as the report says, and extrapolation past either end is not allowed. Read the scope notes before reading a cell out of the grid: they carry the substrate conditions, the primers assessed, the thickness limits per coat and whether the figures are for shop or site application.
The American route reaches the same place through a different document. The thickness comes from a design number in the UL Fire Resistance Directory, tested as an assembly rather than as a member, and each listing names the shape it was tested on. Because a project rarely uses that exact shape, the directory publishes a substitution relationship for sprayed fire-resistive materials on beams: the new thickness equals the tested thickness times the tested W/D plus 0.6, over the new W/D plus 0.6, with floors on both the ratio and the thickness that the individual design states. AISC Design Guide 19 sets out how that machinery is used, along with the column relationships, which are product-specific constants rather than one general formula. Different arithmetic, the same idea underneath: a thickness belongs to a member, not to a building.
From a thickness on the schedule to drums on the delivery note
Once every mark has a dry film thickness, the quantity is a volume rather than a coverage rate, and it is a different calculation from the one that buys primer. Film thickness times area gives dry volume; dividing by the volume solids fraction gives the wet volume that has to go on to leave it behind. A hundred square metres at 1.5 mm dry, from a product at seventy percent volume solids, is about 214 litres wet before a drop is lost to overspray or left in the pot. Turning that into containers through a loss factor is worked through in the steel takeoff guide and not repeated here; what belongs to fire protection is what comes before it, which is that the thickness is not yours to choose.
Watch the units, because this is where the two halves of the trade genuinely speak different languages. European data sheets quote dry film thickness in microns and site work in millimetres; American literature and most gauges read in mils. A mil is 25.4 microns, so a 1.5 mm requirement is 1,500 microns and about 59 mils, and the difference between reading 59 and reading 1,500 into the same box is a factor of twenty-five. For a ninety-minute period on a high section factor member the required film is routinely too thick to lay in one pass, which makes it a programme item as much as a quantity: multiple coats, an overcoat interval between them, and a cure period before the section can be handled or loaded out.
Then allow for the steel getting hit. A coated section that has been slung, stacked, transported and bolted arrives on site with damage at every point it touched something, and connection zones are usually held back and coated in place after erection in any case. Site make-good is a real quantity of material and of access, and it is the part most often left out of a shop-applied price. It is also where the finished thickness most often fails a check, because a brushed repair over a bolted cleat is the hardest place on the frame to build a uniform film.
What a coated column is, from the steel outward
- Decorative sealer — the colour coat, and the only layer chosen for how it looks — it still has to be one the intumescent's assessment permits over it
- Intumescent basecoat — the only layer whose thickness changes from mark to mark, read from a loading table against this member's section factor and limiting temperature Intumescent Fireproof Coating Volume Calculator
- Compatible primer — one of the primers the intumescent was assessed over, at or below the film thickness that assessment allows — a generic equivalent is not a substitution Structural Steel Primer Coverage Calculator
- Steel section — the heat sink the whole build exists to slow down; its mass per metre and its outline are the two numbers the thickness above it is derived from Steel I-Beam Weight-per-Length Calculator
Surface area, the dry film thickness the loading table gave you, and the volume solids off the data sheet. Enter the thickness in mils — 25.4 microns to the mil — and run it once per section factor group, because one frame has several.
The total steel surface area to be coated.
The cured coating thickness required for the specified fire-resistance rating.
The percentage of the wet coating that remains as solid film after cure.
Intumescent coating volume needed
9.57 gal
The dry film thickness required for a specific fire-resistance rating depends on the steel section's size factor (Hp/A) and is set by the manufacturer's UL-listed DFT tables for your exact rating and steel profile — this calculator only converts a DFT you already know into material volume, it does not determine the required DFT itself.
- Required wet film thickness
- 35.71 mils
They open the calculator with your figures already in it
Intumescent Fireproof Coating Volume Calculator: 9.57 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The solids figure has to be solids BY VOLUME, and data sheets print solids by weight right beside it. On a heavily filled intumescent the weight figure runs well above the volume figure — a product quoted at 78% by weight can be near 65% by volume — and entering the higher one understates the wet material by that same margin, on a coating whose whole job is to reach a listed dry thickness.
- Theoretical volume, with nothing lost between the pot and the steel. Spraying open sections — web, both faces of every flange, edges — throws a large share of the material straight past the member, and applicators deliberately overapply on top of that because gauge readings have to meet a MINIMUM everywhere rather than an average. Both push the quantity actually ordered well above this number.
- Covers the intumescent film alone. The system that carries the fire test is a specified primer, the intumescent, and usually a sealer or decorative topcoat, each with its own coverage rate and its own compatibility restrictions — a rated build-up is not the intumescent on its own, and a topcoat chosen for colour rather than from the listing can stop the film expanding.
The primer underneath decides whether the certificate means anything
An intumescent is assessed as a system, and the primer is inside it. The assessment report names the primers the product was tested over, specifically or by generic type, and states a maximum film thickness for them — because a thick, soft or incompatible undercoat is exactly what fails when the coating starts expanding and needs something to stay stuck to. Steel arriving from the fabricator in a shop primer chosen purely for corrosion protection in transit, to a scheme selected against ISO 12944, is the commonest way a frame turns up unprotectable: nothing is wrong with the primer, it is simply not one the fire product's assessment covers, and the choice is then blasting it off or finding a different intumescent.
Galvanised steel is the sharper version of the same problem. A zinc surface to ISO 1461 or ASTM A123 is a poor substrate for most intumescents and normally needs a specific tie coat, or is excluded from the assessment altogether — not something to discover after the secondary steel is back from the galvaniser. The topcoat has the same status at the other end of the build: a decorative sealer is part of the assessed system, at the thickness the report permits, because a hard finish over a reactive coating can restrain the char it is supposed to protect. Agree all three layers with one supplier, in writing, before the first section is blasted. How the litres for the primer coat are worked out is the takeoff guide's territory next door; what belongs here is only which primer is allowed under the film.
When the frame in front of you is not all steel
Increasingly it is not, and the contrast changes where the money sits. A timber member protects itself by burning slowly: the outer material chars at a notional rate, the char insulates what is behind it, and the reduced cross-section method in BS EN 1995-1-2 — or Chapter 16 of the National Design Specification in US practice — checks whether what survives can still carry the load. Steel has no equivalent; it needs the jacket. So a mixed frame pays for the period twice in two currencies: millimetres of extra timber section bought at the sawmill, and a coating line that arrives at the very end of the job.
The place the two meet is the connection, and that is the detail to look for on a mass timber project with steel in it. A plate or a bolt buried in timber is protected by the timber over it, and the standards set how much cover that takes; the same plate exposed on the face of the member is unprotected steel with a section factor far worse than any beam on your schedule, because a plate is thin. An exposed steel connection in a timber frame either gets protection of its own or gets recessed and plugged, and which of those it is belongs at shop drawing stage rather than to the day the coater is standing under it.
What has to exist before the scaffold comes down
The finished protection is worth only what can be shown about it, and the record is assembled during the work rather than reconstructed afterwards. Wet film readings taken with a comb as the coat goes on are the applicator's process control; the dry film thickness measured after cure is what the certificate rests on, and how many readings, where, and how they are averaged come from his inspection and test plan and from whatever third-party certification scheme he holds. For sprayed cementitious protection the equivalent measurements are thickness and density to ASTM E605 and bond to ASTM E736, with AWCI Technical Manual 12-A setting out the field practice. None of those documents let a reading be taken where the coating happens to be thickest.
File it against the mark, not against the building. What you want at handover is one schedule showing, per mark: the section factor calculated and the exposure assumed, the limiting temperature and the period, the product and its assessment report reference, the specified thickness and the readings achieved, and the primer underneath. It answers every question the approving authority, the insurer and the next refurbishment will ask. Its absence is what turns a competent job into an argument nobody can win six years later, when the only evidence left is a coating whose thickness cannot be measured without cutting into it.
What the applicator needs from you, mark by mark
Six answers turn a fire resistance period into a thickness. None of them is on the steel drawings, all of them are recoverable from a mark schedule in an afternoon, and the thickness itself is the only one that is not yours to decide.
- The period, and which element it applies to — Off the fire strategy, which takes it from Approved Document B, BS 9999 or IBC Table 601. Beams and columns in one frame can carry different periods, and transfer structures often carry more.
- Cross-sectional area for every mark — Mass per metre times 127.4 gives square millimetres, because area and mass per metre are one fact in two units. In US units it is the weight per foot over about 3.4, in square inches.
- Three-sided or four-sided exposure — A beam with a concrete slab bearing on its top flange is three-sided; a free-standing column and a beam under an exposed steel deck are four. It is the slab, not the beam, that decides.
- Profile or boxed — Sprayed and coated protection follows the outline; board encasement wraps a rectangle. On one 305 by 102 beam those two give section factors of 318 and 257, and the choice is made by whoever picked the system.
- The limiting temperature — Derived from the member's degree of utilisation under BS EN 1993-1-2, not a constant. If the specification names one, use it; if it does not, ask rather than taking the more generous column.
- Which marks are off the top of the table — Angle bracing sits near two over its thickness — 346 per metre for an 80 by 80 by 6. Heavier metal, a hollow section, a boarded box or an engineer's exemption, decided before the steel is ordered.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.