The complete guide
How much magnetic strip do you need
The part that makes the system work
A magnetic window screen holds on because there is a strip of magnetic material stuck around the window frame for it to grip. No screws, no drilling, no holes.
This is that strip, sold on its own by the metre, for when you are building a screen from parts, repairing a run that has lifted, or extending an existing screen onto another window.
Working out how much
Perimeter, not area. Twice the width plus twice the height.
A 100 cm by 150 cm window comes to 5 metres. Order a little over that, because the corners and one mis-cut eat more than people expect, and running out three sides into a four-sided job is a frustrating way to end an afternoon.
Fitting it properly
This is where it either works for years or lifts in a fortnight, and the difference is almost entirely surface preparation.
Wipe the frame face down and let it dry completely. Dust, grease or damp under the adhesive is the single most common reason a strip comes away later, and the adhesive only gets one attempt at bonding.
Peel the backing as you work rather than all at once. Press the strip down along its whole length, not just at the ends. A strip tacked at the ends looks fitted and is not, and it will lift in the middle first.
Do each side as its own length rather than bending one continuous run around the corners. It is easier to control and the corners end up neater.
Why not just use screws
Because of what happens when you take it off.
Magnetic strip lifts away without pulling the frame finish with it. That's what puts these screens into rented flats, where a drill is out of the question, and it is why people are happy to run them seasonally: the screen and the strip both come off at the end of summer and the window looks untouched.
When a strip fails
Replace the run, not the lot. Cut a fresh length, clean the frame properly, and press it down.
It is worth knowing that magnetic strip does not lose its magnetism through use. If a screen has started sagging away from the frame, the problem is the adhesive bond or something on the face of the strip, not the magnet. Grit is the usual culprit: anything sitting on the face holds the screen a fraction proud, and a fraction along a whole edge is a gap.