A sagging gutter is usually the clips
I have watched a shed gutter fill, sag, and then pour down the wall past the water butt. The downpipe was still attached. The water never reached it. The run had pulled out of two clips in the middle and the fall had gone. I blamed the gutter. The gutter was a straight length of plastic. The clips had let go.
A gutter is a shallow channel with a fall toward an outlet. Clips hold the channel to the fascia or the shed board so that fall stays put. Plastic moves with heat. A wet run gets heavier. If the clips are too few, or they have opened, the middle drops. Standing water makes it heavier still. Then the next clip opens. That is the failure. It looks like a gutter problem. It is a clip problem.
This is the same family as the butt kit. A watering can still earns its hook next to a hose. The butt only earns its floor if water actually enters it. A loose diverter that drips down the wall is not collecting rain. A sagging gutter that dumps onto the path is the same leak, one metre higher. You get a wet wall, a stained board, and a butt you still fill from the kitchen tap.
I would start at the clips, not at the shop. Count them. A long run on two end clips will sag. I want a clip near each joint and enough between that the channel cannot belly. I push the gutter back up and see whether the clip still closes. A plastic clip that has gone brittle will not. A nail that has pulled from a rotten fascia will not hold a new clip either. Then the job is the board, not the gutter. I would not hang a new run on a soft fascia and call it fixed.
Fall matters more than colour. I want a consistent drop toward the outlet, not a dip in the middle and a rise at the stop end. I run a hose briefly and watch. Water that sits is a sag. Water that races past the outlet is a fall the wrong way, or an outlet blocked with wet leaves. The leaves are not the product. The outlet and the clips are. Clear the outlet. Do not invent a new gradient by twisting a length that is already oval.
I would also look at where the downpipe meets the butt. If the pipe has come out of the diverter because the gutter has dropped, clipping the gutter up may be enough. If the pipe is short and the butt has been shuffled, that is a different fiddle. Measure the drop with the gutter held in the clips, not with it hanging in your hand.
Skip a gutter kit sold with no length and no clip count. Skip the idea that a bead of sealant will hold a run that has no clips. Sealant is not a bracket. Skip joining three odd offcuts with no union and no clip at the join. That join will be the first to open. Skip climbing on a wet shed roof to force a clip. If I cannot reach the run from a firm step, I wait.
This shows in autumn, when the first real rain sits in a gutter you have not looked at since June, and again in winter when a charged run gets heavier. I will not invent a frost date. A clear, still night can freeze what is sitting in a sag, and then the ice does the levering. I now treat the first long wet week of autumn as a clip week, the same way I treat the first cold week as a fittings week in Hose fittings leak at the washer, not the hose.
A compost bay tight under a sagging run is a wet heap you did not plan. A compost bin is a box, not a bag of compost. I would rather move the bin than let the gutter feed it from the roof.
I do not need a new gutter because two clips have opened. I need the run to sit, fall, and empty into the pipe. The clips are the part that does the work. The length only looks like the job.
If I later add a buy link here, it will be a paid promo.