When a blanket that was fine last summer starts letting water through in a line, the cause is almost always geometric rather than material.

The same fold, every time

A blanket is put away the way it came out of the packaging: usually in half, in half again, in half again. That puts the whole life of the product on four crease lines.

Films are flexible but not infinitely so. Repeated tight folding work-hardens the material along those lines, and cold weather makes it far worse.

What the failure looks like

Hold the blanket up to a light and look along the folds. The film goes pale, then crazed, then split.

Because it is a line rather than a patch, the leak appears right across the middle, which is the most-used part of the blanket.

Rolling instead

A roll distributes the curvature over the whole width instead of concentrating it, so nothing is creased at all.

This is the real reason a carry strap is worth paying for: not convenience, but that it makes rolling the natural way to put the blanket away.

Specify the backing for flex

Films differ enormously in how they take repeated flexing, and the specification for it is separate from the water rating.

Ask for both. A blanket can be genuinely waterproof and still fail in two seasons if the backing was chosen only on the first number.

And keep the seams out of the middle

On large blankets, every stitch through a laminate is a hole in the waterproof layer. A plain seam across the centre is a line of them in the wettest place.

Lapped and sealed seams cost a little more and remove an entire failure mode.