Two force models, and they hand the problem to each other
A curtain rod is a simply supported beam: it rests on two or more points and carries load between them. The bending moment is greatest at mid-span, which is why the failure looks like a smile. And the deflection grows roughly with the fourth power of the distance between supports — so going from 2 m to 1 m between brackets does not halve the sag, it cuts it by something closer to sixteen. That single relationship explains why adding a centre bracket is almost always a better answer than buying a thicker rod.
A bracket, meanwhile, is a cantilever — a crowbar. The curtain's weight hangs a certain distance out from the wall, and that distance is a lever arm. A 5 kg curtain on a bracket projecting 10 cm puts roughly 0.5 kgf·m of torque into the top screw, pulling it straight out of the wall. The further the rod sits from the wall, the harder that screw is being levered, and the more the anchor has to resist a pull, not a push.
And here is why the two are one story: once the rod sags, its centre of gravity moves outward, which increases the pull-out force on the end brackets. So sag is the warning and pull-out is the result. A rod that has bowed for a season is a rod whose brackets are already working harder than they were designed to.
Three faults, in the order you should suspect them
- Sagging in the middle. Span against diameter. The practical ceiling is about 122 cm between supports for a 25 mm steel rod, and 61 to 76 cm for heavy drapes. Above roughly 150 cm, add a centre bracket; above 240 cm it is not optional; above 360 cm you need two or more. Diameter matters too, and non-linearly: bending stiffness climbs with roughly the fourth power of the outside diameter, so a step from 25 to 28 mm buys more than it sounds like. Wood and aluminium are both softer than steel at the same diameter, so drop a tier for either.
- A bracket pulling out of the wall. Torque against anchorage. Projection, wall material, anchor type and over-tightening all feed into it. Do not over-tighten: crushing the plaster around the screw or stripping a plastic plug can cost up to 40 per cent of the capacity, which is the opposite of what people do when they want something to hold. And size the system for about twice the static weight, because pulling a curtain closed every day is a dynamic load, not a hanging one.
- A rod that is crooked. Usually assembly rather than strength: the two end brackets at different heights, or a centre bracket whose projection does not match the ends, which twists the rod into an arc and looks exactly like a bent rod. The check costs nothing — stand at one end, drop your eye to the level of the rod and sight along it. A straight line is obvious; a deviation is more obvious.
The two assembly traps
Never put a bracket on a telescoping joint. These rods have a smaller inner tube sliding inside a larger outer tube, and a bracket clamped at that joint crushes the inner tube — after which the rod will not extend or retract properly again. Keep brackets at least 5 cm clear of any joint. And make the centre bracket's projection match the ends: if it sits further out or closer in, the rod is pulled into a curve and every later measurement is measuring the curve rather than the wall.