Are you wondering what the mm means when it comes to waterproofness? How about gsm?

There are a lot of confusing “technical” terms used in modern waterproofing technologies, but don’t panic – our waterproofing guide will explain mm, gsm and cover all of the essential information.

What do the numbers mean?

Generally speaking, the higher the mm rating, the more waterproof the fabric is. The downside of this though is that the fabric is also less breathable. For example a plastic sheet will have a very high mm rating, but if you lie on it then you will sweat as no water will be allowed to pass through the sheet.

In order to test waterproofness, a test called the Static Column Test is used. A tube, 1 inch in diameter, sits vertically on a piece of material. The tube is then filled with water and when it starts to leak through, the height in mm is the waterproof rating used. This tests the pressure of the water required to leak through the fabric.

The gsm rating of fabric is how breathable the fabric is. It means how many grams of water can pass from the inside to the outside of  a square metre of fabric over a 24 hour period. If some fabric is 2,000 gsm then it means that 2,000 grams of water can go through a square metre of the fabric over 24 hours.

Read more about this here.

Features to look out for

When picking out a waterproof coat or jacket there are some key features to keep an eye out for. These features will make either make your waterproof coat one of the best bits of kit you own, or compromise the core function of a waterproof jacket: keeping you dry and comfortable.

  • Taped/sealed seams – this means the stitching is protected against water leaking through the needle holes and along the thread
  • Hood – it should be adjustable to fit around your head to provide comfort and warmth
  • Waterproof zips – similar to the taped or sealed seams, this prevents water getting through the teeth of the zip into the coat
  • Adjustable cuffs – make the cuffs fit your wrists to keep in warmth and keep water out
  • High collar – protects against the wind and rain and keeps in warmth
  • Chin guard – a comfortable piece of material to protect against the zip rubbing on your chin
  • Pockets – deep, warm and zipped are best
  • Length – it should be long enough that it doesn’t move up your back (especially with a bag on) and short enough that it doesn’t get in your way

What is best for me?

All in all, the best waterproof material will depend on what you want to do with it. For example, a ground sheet should be highly waterproof to prevent any water getting inside your tent but you wouldn’t want to lie directly on top of the ground sheet.

If it’s a coat you’re looking for then, again, it depends on what you want it for. If you’re looking for a coat to stick on when you’re taking a trip to the park then you wouldn’t need a massively expensive high mm and gsm rated coat. Something a little more comfortable, warm and lightweight would probably be ideal. If you’re planning a trip walking in the mountains then you will want a higher mm and gsm rated coat that fits well.

Looking for a tent or tarpaulin? Most tents are rated in mm and 1,500mm is widely agreed to be the minimum for summer camping in the UK with 3,000mm should be able to withstand most conditions in all seasons in the UK.

Hopefully you now understand a little more about how waterproofness is measured and what you should be looking for when buying anything waterproof.

If this has helped you, or there is something you think should be added to this guide then please comment below.

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