Water Resistant vs Waterproof

These two words get used loosely, and the difference decides whether your things stay dry.

Water resistant

A water-resistant fabric has a DWR coating: a treatment on the outside that makes water bead and roll off. It handles a shower, a bit of spray, and the walk from a taxi to a door.

It is a coating, not a barrier. It wears off with use and dirt, it does not cover the seams or the zips, and under sustained rain the fabric eventually wets through. Most travel packs are water resistant, and that is usually the right choice, because a fully waterproof pack is heavier and harder to live with.

Waterproof, and the number after it

A waterproof fabric has a membrane behind it, and its rating is a hydrostatic head measured in millimetres: how tall a column of water the fabric holds before it leaks.

• 10,000 mm handles steady rain for a few hours. Fine for a commute or a

shower on a walk.

• 20,000 mm handles a full day of heavy rain, and rain driven by wind, and the

pressure of a pack strap pressing wet fabric against your shoulder.

The second number you will see is breathability, in grams per square metre per 24 hours. A jacket that keeps rain out but traps sweat will still leave you wet, just from the inside. Higher is better, and there is always a trade with warmth.

Seams are where it actually leaks

A waterproof fabric with unsealed seams is not a waterproof jacket. Look for fully taped seams and water-resistant zips. This is the single most common reason a cheap shell fails.

Fully waterproof

A roll-top dry bag with welded seams and no zip at all is in a different category: it can be submerged. That is why dry bags do not have zips, and why a zip is always the weak point.

What to do about a pack

Keep the pack water resistant and carry a pack cover for the days it really rains, or line the inside with a dry bag. Both weigh very little and are far cheaper than a waterproof pack that is heavier every other day of the year.