Corrugated Metal Decking Span Deflection Checker

This is a line load across one metre of deck width, not a pressure — multiply the design area load by 1 m of width, so a 3 kPa load on the deck becomes the 3000 N/m default shown here. THE DENOMINATOR STAYS METRIC WHEN THE SPAN DOES NOT. This field carries no unit dimension, so it is read as N/m whichever measurement system the page is set to, while the span below it does follow the metric/imperial switch. A US deck load table gives the same quantity in lbf per linear foot of width, and the WHOLE unit converts, not only the denominator: 1 lbf is 4.4482 N and 1 foot is 0.3048 m, so 1 lbf per foot is 14.5939 N/m. Multiply an lbf-per-foot figure by 14.5939 before entering it — the 3000 N/m default is 205.6 lbf per foot. Skip that step and nothing here stops you: 205.6 sits inside this field's 100 to 30,000 range, so it is taken as 205.6 N/m with no notice of any kind, and because deflection is directly proportional to the load the answer comes back 14.59 times too small. A 6 m span carrying 5000 N/m on an I of 20 ×10⁶ mm⁴/m deflects 21.09 mm and fails the L/360 limit of 16.67 mm; the same load typed as 342.6 gives 1.45 mm and passes. Only loads under about 1,460 N/m give a figure below 100, and those are pulled up to the minimum instead — that path at least prints a notice saying so, this one prints nothing.

Clear span between the purlins or joists it sits on. Decking is normally continuous over three or more supports, which makes it noticeably stiffer than the same sheet spanning between two — so the span condition entered has to match how the sheet is really laid. The END bay, where the sheet has a support on one side only, is the worst case on the roof and is the one worth checking.

This value is profile- and gauge-specific and cannot be derived from beam theory alone — look it up for your exact deck profile and gauge. Both halves of this unit are metric — ×10⁶ mm⁴ per METER of width — and the field has no unit selector, so it stays exactly as typed while the span above switches to imperial. SDI and the US deck makers publish I in in⁴ per foot of width, and that is a different number: 1 in⁴ per foot of width is 1.3656 ×10⁶ mm⁴ per m, because the per-foot denominator converts as well as the in⁴ (416,231 mm⁴ per in⁴, divided by 0.3048 m per foot). Multiply an in⁴-per-foot figure by 1.3656 before entering it — NOT by the 0.4162 used for a plain in⁴ section property, which converts the numerator only and lands 3.28 times low. The default 20 here is 14.65 in in⁴-per-foot terms, and 14.65 is inside the 0.1 to 500 this field accepts, so typing it is neither refused nor clamped: it is taken as written, and because deflection goes as 1/I the sag reads 36.6% high — 0.791 mm becomes 1.080 mm at the default 3000 N/m over a 3 m span — with the comparison against the limit then made on the inflated figure. That error runs in the conservative direction, so it will not turn a failing deck into a passing one, but it will condemn a deck that is fine.

On a low-slope deck this limit often exists to stop PONDING rather than to protect a finish. Water collecting in a sagging bay adds load, the added load increases the sag, and the deeper sag collects more water — a feedback that does not settle at a new equilibrium if the roof is flat enough. That is why deck limits can look tight for a structure carrying nothing brittle.