Every screen door sags at the same corner. Here is why, and why tightening it does not last.

Why it always sags at that one corner

A screen door is a rectangle of thin aluminum held square only by its four corner joints, with nothing at all resisting movement along the diagonal. That is the whole problem in one sentence.

Every time somebody pulls it, pushes it, leans on it or lets the closer slam it, a little diagonal load goes into that rectangle. The corner joints, which are the only thing keeping it square, give up a fraction of a degree at a time. Slowly the rectangle becomes a parallelogram: the hinge side stays where the hinges hold it and the free side droops. The corner that drops is the bottom one diagonally opposite the hinges, and it drops onto the threshold. That is your drag.

It is not a hinge problem, though the hinges get blamed. It is not the door being cheap, though some are. It is geometry, and geometry does not care how carefully you close the door.

Why tightening the corner screws does not hold

Because it treats the symptom. Pulling the corner screws snug drags the frame roughly back to square for a few weeks, and then the same daily loads do the same thing again — only now the screws are turning in aluminum that has already been chewed once, so they hold even less. The third time round the holes are stripped and the corner will not come square at all.

Every seasonal “door adjustment” on a screen door that is racking is a visit that will be needed again.

What actually fixes it

Restore the diagonal, then remove the reason it was being loaded.

• Remake the corners properly. Corner keys or gussets that grip the inside of the extrusion, rather than screws relying on thin aluminum that has already failed once.
• Give it a diagonal. A brace or a tensioned cable with a turnbuckle across the door, from the top hinge corner to the bottom latch corner. That is the member the door never had, and it is what stops a rectangle becoming a parallelogram.
• Hang it plumb. If the hinge stile is out, the door is fighting gravity from the moment it is fitted. Shim it genuinely plumb rather than adjusting the strike to suit a leaning door.
• Set the closer. Two adjustments on most closers: sweep speed and latch speed. Set so the door closes firmly and latches without banging. A door that slams is being racked several times a day by the thing that was installed to protect it.
• Check the threshold. A dragging door sometimes is not sagging at all — the deck has heaved or settled and the door is fine. Worth ten seconds with a level before anybody starts taking a door apart.

When a door is past fixing

Aluminum that has been racked, re-tightened and re-drilled several times over eight years is work-hardened, chewed at the corners and often bowed along the stile. A new door hung plumb, braced and with the closer set properly costs less than three more visits and it will actually stay square. We will tell you which side of that line yours is on.

The barrier rule, which is not optional

Where your enclosure is acting as the pool barrier, the door is part of a safety system, not a convenience. It must be self-closing and self-latching, and the latch has to be positioned where a small child cannot reach it. A door that no longer latches on its own has stopped performing a life-safety function, whatever else it is doing. We check this every time we touch a door, and if yours does not latch we will say so whether or not it was what you called us about.