The opening decides everything behind it

Ask for the clear opening before anything else. Four metres wide by four metres high is 16 m² of curtain, and at a 77 mm slat pitch that is 52 slats. The drum, the springs, the guides and the headroom all follow from those numbers. The motor does not, because it is chosen last and only has to overcome what the spring leaves behind.

Most shutter enquiries arrive the other way round: a width, a colour and a price request. The width is usually the structural opening the builder left, which is not the same thing, and the difference is where the first expensive mistake is made.

What half a millimetre weighs

A flat square metre of 0.5 mm steel is 3.93 kg. A double-skin slat uses about 1.3 times the developed width of the opening it covers, so a square metre of finished curtain is 5.1 kg, and 16 m² is 82 kg hanging on the drum. At 0.4 mm the same door is 65 kg; at 0.6 mm it is 98 kg.

That 33 kg spread is the whole argument about gauge. It is not carried by the motor, which is sized for friction and wind, but by the spring, and the spring is what the man at the bottom rail feels every morning. A door specified light because the coil was cheaper is a door that will not stay where it is left.

The spring holds a torque, not a weight

A shutter is not lifted, it is unwound. The curtain hangs on a drum of about 150 mm diameter, so 82 kg acting at a radius of 75 mm is 60 N·m at the drum, divided between two springs at 30 N·m each. Balanced, the door stays put when released and moves under a hand force of 30 to 50 N. Unbalanced, the same curtain needs 800 N to lift and comes down on its own.

This is why a shutter is bought by spring rather than by slat. The spring is also the part that separates a cheap door from a good one, and the part that fails first, usually on a Friday afternoon with a truck waiting.

Cycles, not years

Springs are rated in cycles. Ten thousand is the ordinary commercial figure, which at two movements a day is 5,000 days, close to fourteen years. At twenty movements a day, an ordinary loading bay, the same spring is finished in 500 days. Anything with traffic should be specified at 25,000 cycles or more, and springs replaced in pairs, because a new spring beside a tired one simply moves the load onto the older wire.

The spring is a consumable with a service life, not a part of the building. A door bought for fifteen years and opened twenty times a day will need two sets of springs in that time, and the second set is cheaper than the damage an unbalanced curtain does to the guides.

Wind on a 16 m² sail

0.6 kN/m² is a normal design pressure for a closed shutter in a built-up area, and over 16 m² that is 9.6 kN, roughly a tonne pushing on the curtain. Half of it lands on each guide, so 4.8 kN a side has to travel through the guide, the anchorage and the lintel. A windlock, the hook that engages the guide as the curtain bows, is what keeps the slats from walking out; above about 3 m of height it is not optional.

The anchorage is where the load is actually lost. A guide fixed into a 100 mm brick skin with six anchors holds nothing, however good the shutter. The same guide into a lintel or a steel frame holds the door in the weather it was specified for.

What the slat is formed from

Slats come off a roll former fed with narrow slit strip. The strip is DX51D galvanized strip with zinc from Z40 to Z275 and a tensile range of 200 to 550 N/mm², or Z275 coil slit down, at 0.12 to 2.0 mm with thickness held to 0.01 mm. That tolerance is not paperwork. The interlock is a sliding fit, and three hundredths of a millimetre decides whether 52 slats run freely or bind at the tenth.

Grade matters less than coating. DX54D lists roller shutter door among its own applications and forms cleanly; SECCN2 and SECCN5 name metal doors and garage doors on theirs; TDC51D pre-painted coil, 0.12-2 mm and any RAL colour, is what most street-facing shutters are made from. For a painted interior door, DC01 from 0.12 to 3 mm or ST12 at 0.5 to 1.0 mm does the job, and electro-galvanized SECC is the cheaper substrate where the door never sees weather.

Guides, edges and the bottom rail

The guide is a folded section, usually 2 mm, cut from Q235 plate, sheared, punched and bent, then hot-dip galvanized after fabrication so the slots and the anchor holes are coated. A pre-galvanized guide with punched slots has bare steel in every slot, which is exactly where the paint wears and the rust starts.

Every slat has two cut ends, and on a pre-painted door those ends are bare. They sit inside the guide, so the first rust on a shutter usually runs down the guide rather than appearing in the field of the door. The bottom rail takes the other damage: 3 mm with a rubber astragal on an industrial door, thinner on a domestic one, and always the part the fork lift finds.

Fire shutters are a different product

A fire shutter looks identical and is specified differently. It closes on a fusible link at 74 °C or on a signal from the alarm panel, drops under gravity against a governor, and is tested on a schedule rather than inspected by eye. The slats carry a certificate, the guides are heavier, and the drop speed is set so the bottom rail does not become a guillotine. Ordering a security shutter and expecting it to pass a fire inspection is a common and expensive mistake.

The two requirements also fight. A fire shutter has to close when nobody is there, which means no anti-lift device can hold it open, and a windlocked security door on the same opening is a separate leaf, not a modification.

The measurement that decides the order

There are two openings on every job. The clear opening is what the truck drives through. The structural opening is what the builder leaves, and it is about 100 mm wider on each side for the guides and 350 to 400 mm higher for the roll. Take the structural opening off site in millimetres before the coil is slit, because a shutter cannot be made wider at the dock.

The order then reads: clear opening, structural opening, slat profile, gauge and coating, spring cycles, wind class, finish. Given the first two and those five, a supplier can price the door in an hour. Given a width alone, he will still quote, and it will be the wrong door.