Sizing and engineering · Rolling Shutter Guide
Why do wide rolling shutters sag, and does a steel or aluminum axle help?
The curtain hangs from an axle held only at its two ends, so the axle bends under the curtain's weight. A wider curtain is both heavier and a longer span, so sag grows with roughly the fourth power of the width. Roll-A-Shield estimates the sag on every shutter and moves up one box size when it reaches 1/2 in.
Key facts
- Roll-A-Shield's configurator estimates axle sag on every shutter with the standard beam formula, treating the curtain as an even load across its width. Roll-A-Shield data
- When the estimated sag reaches 1/2 in, the point where the roll risks rubbing inside the box, the box is stepped up one size; a smaller hand-picked box is refused. Roll-A-Shield data
- Axles are 40 mm on most strap and crank shutters and 60 or 70 mm on motorized, spring and single-wall shutters; a 55 mm motorized curtain moves to the 70 mm axle above 140 lb or 144 in wide. Roll-A-Shield data
- In the sag check, the 70 mm steel axle is the stiffest of the four tubes, and both thick-walled aluminum axles are stiffer than the thin-walled 60 mm steel axle. Roll-A-Shield data
- Curtains over 90 lb get a heavy-duty axle bracket, except 55 mm curtains, which already carry a heavy-duty bearing plate. Roll-A-Shield data
Applies to: Rolling shutters whose curtain winds on an axle in a box, any make; Roll-A-Shield 38 mm and 55 mm insulated and 63 mm single-wall shutters on 40, 60 and 70 mm steel or aluminum axles.
Every rolling shutter hangs its whole curtain from one axle, and that axle is held only at its two ends, in the end caps of the box. Between them it bends a little under the weight. On a narrow window the bend is too small to matter; on a wide patio or storefront shutter it can decide the box size and sometimes whether the opening should be one shutter or two. This page explains how Roll-A-Shield sizes for it.
Why does a wide shutter sag so much more than a narrow one?
Because width works against the axle twice: a wider curtain is heavier, and that weight hangs over a longer span. Roll-A-Shield's configurator treats the axle as a beam supported at both ends with the curtain spread evenly along it, and estimates the sag with the standard beam formula: sag = 5 × W × L³ / (384 × E × I), where W is the curtain weight, L the width, E the stiffness of the metal and I the stiffness of the tube's cross-section.
The curtain weight itself is width × height × the slat's sizing weight per square foot. Put the two together and, at a fixed height, the estimated sag grows with the fourth power of the width:
- 10% wider means about 46% more estimated sag.
- Twice as wide means about 16 times the estimated sag.
- A taller curtain at the same width adds sag in proportion to the extra weight: 25% taller, about 25% more.
The slat matters through its weight. The sizing weights are 0.92 lb per sq ft for the 38 mm insulated slat, 1.11 for the 55 mm insulated slat and 1.14 for the 63 mm single-wall slat. See how much a rolling shutter weighs for worked weights.
How much sag is too much?
Half an inch. When the estimated sag reaches 1/2 in, the configurator treats the axle as sag-critical, the point where the sagging roll risks rubbing inside the box, and moves the shutter up one box size from the normal sizing chart. The step only ever goes up: if someone picks a box by hand that is smaller than the sag check calls for, the configurator refuses it.
There is no single width where this happens. The point moves with the curtain width and height, the slat's weight and the axle size and material, so the configurator runs the check on each shutter as it is sized rather than using a fixed cut-off. The result is an estimate used to pick the box, not a measurement of sag on a finished shutter.
Which axle does my shutter get?
On Roll-A-Shield shutters the axle follows the slat, the operator and the curtain weight:
| Shutter | Axle |
|---|---|
| Strap, crank or wand, 38 mm insulated | 40 mm |
| Strap, crank or wand, 63 mm single-wall | 60 mm |
| Motorized, 38 mm insulated | 60 mm |
| Motorized, 55 mm insulated | 60 mm up to 140 lb and 144 in wide, otherwise 70 mm |
| Motorized, 63 mm single-wall | 60 mm under 100 lb, 70 mm at 100 lb and over |
| Battery (OZRoll) motor | 60 mm steel |
| Spring-loaded | picked from the curtain height and weight |
A 60 mm axle measures about 2.36 in across its flats and a 70 mm axle about 2.76 in. The motor has to match the tube, so the axle size also limits which motors fit; see what size motor a shutter needs.
Heavy curtains also get stronger end support: curtains over 90 lb get a heavy-duty bracket at the axle end, except 55 mm curtains, which already ship with a heavy-duty bearing plate.
Steel or aluminum: which axle sags less?
It depends on the tube's wall thickness as much as the metal. Steel is about 2.8 times as stiff as aluminum, but the configurator's sag check models Roll-A-Shield's aluminum axles with much thicker walls than the steel ones.
In the configurator's sag check that puts the four tubes in this order, stiffest first:
- 70 mm steel
- 70 mm aluminum
- 60 mm aluminum
- 60 mm steel
So in that check a 60 mm steel axle is estimated to sag more than a 60 mm aluminum one carrying the same curtain, and the 70 mm steel axle sags least. By default the configurator specifies aluminum, and steel for spring-loaded shutters and for motorized shutters over 120 in wide overall; the factory checks the final choice at build.
What can be done about sag on a very wide opening?
Most of it is handled automatically when the shutter is sized; the choices left to you are the slat and whether to split the opening.
- A larger box. The configurator steps the box up when the sag check calls for it. You can ask for a larger box than required, but not a smaller one, and the extra box height adds to the overall size. Box sizes and how far they stick out are on what size box do I need.
- A bigger or stiffer axle. The 70 mm axle comes in automatically on heavy 55 mm and 63 mm curtains, as in the table above.
- The right slat for a long span. Roll-A-Shield's own field guidance is that a 38 mm curtain around 144 in wide and 96 in tall is not recommended where the shutter will stay closed for long periods; the 55 mm slat is preferred there. Why your slat choices narrow covers the slat limits.
- Two shutters instead of one. Two half-width shutters each carry half the weight over half the span, so each one's estimated sag is about 1/16 as much. See covering a 16-20 ft opening.
The maximum widths per slat are on how wide and tall a rolling shutter can be. For a specific opening, request a quote or call (480) 921-0200; the box and axle come out of the same sizing run.
Frequently asked questions
Is a steel axle stronger than an aluminum one?
Not automatically. Steel is the stiffer metal, but wall thickness matters as much: the factory's sag check models the aluminum axles with much thicker walls, so it rates the 60 mm aluminum axle stiffer than the 60 mm steel one, and the 70 mm steel axle stiffest of all.
Can I ask for a bigger box to be safe?
Yes, within the box range for your slat. You can choose a larger box, but never one smaller than the size the sizing chart and sag check call for, and a larger box adds to the overall height, so check that it fits the wall.
Does splitting a wide opening into two shutters help?
A great deal. Two shutters of half the width each carry half the weight over half the span, so each one's estimated sag is about 1/16 of the single wide shutter's. See the page on covering a 16-20 ft opening for how a split with a post works.
Which axle does my shutter have?
On Roll-A-Shield shutters, strap and crank units use a 40 mm axle (60 mm on 63 mm single-wall), and motorized and spring units use 60 or 70 mm, chosen from the slat, the curtain weight and, for 55 mm, the width. Look at the axle end inside the box, or call (480) 921-0200 with your shutter's details.
Parts and next step
Need a part or a hand?
Replacement motors, remotes, straps, slats and hardware ship from our Tempe, Arizona factory. Phoenix-area owners can book a service visit; everywhere else, our technicians help by phone. Call (480) 921-0200; the factory answers technical questions for any rolling shutter, not only ours.