How to Read a Waterproof Rating on Mats, Blankets and Ponchos

Waterproof on a picnic mat or a poncho means three separate things, and a product can be good at one and poor at another. Buyers who ask for the three numbers get what they expect; buyers who ask only for waterproof often do not.

Hydrostatic head (HH)is the water pressure a fabric resists before it leaks, measured in millimetres. Around 800–1,500 mm handles damp grass and a light shower; 2,000 mm and above handles sustained rain. For a groundsheet or a poncho that will be sat on, 3,000 mm is a sensible floor.

Seamsare where most waterproof products fail. A fabric can be rated at 5,000 mm and still leak through stitch holes, so taped or sealed seams matter as much as the coating. Ask whether seams are taped, and whether the tape sits on the face or the back.

DWR(durable water repellent) is the surface finish that makes water bead and roll off. It is not waterproofing — it is what stops the face fabric wetting out and feeling cold and clammy. It wears off with abrasion and washing, which is why care instructions matter: a low-heat tumble or a wash-in treatment revives it.

We supply hydrostatic head test certificates with the relevant products and can run PFC-free DWR. Enquiries: linda@reliableoutdoors.com.