Awning Fabric Waterproof Rating - What Does It Mean?
When you fit an awning to your vehicle you expect it to be able to keep the rain off, and obviously that means it has to be waterproof. What does “waterproof” really mean though? The fact is nothing is completely waterproof – force water against it hard enough and it’ll get through. That’s why when you watch films about submarines you’ll notice the big dial has a red bit.
Obviously your awning isn’t going to be diving to 300 metres, so does that mean it’s guaranteed to be fine? Not quite. It’s almost certainly made from canvas with a waterproof coating on it, so it’s pretty good at keeping the wet stuff out, but there’s a limit to how much pressure it can stand before some starts seeping through. The water pressure a fabric can withstand is called the hydrostatic head, which is measured in millimetres, and it’s often marked on awnings and other waterproof gear.
What hydrostatic head means is the depth of water you can put on top of something before it leaks. Anything with a hydrostatic head of less than 1,000mm is showerproof, not seriously weather resistant, and it goes up from there. Obviously that doesn’t mean a showerproof jacket won’t leak until it’s a metre under water; rain can have a pretty high pressure when it hits because it’s moving fast, and high winds or bigger raindrops will increase it even more. Heavy summer rain can generate a hydrostatic head of nearly 1,500mm, so that’s the minimum you need for an awning. It’s also the maximum you really need to look for because if the weather’s bad enough to generate more pressure than that it isn’t an awning you want; it’s a proper tent. All-season tents are usually rated to 2,000mm and expedition ones can be 3,000mm and more. The highest ratings are usually found on groundsheets, because if you walk on one that’s lying on wet ground you’re creating a lot of force that squeezes water upwards. Look for 5,000mm here.
The reason we recommend canvas as an awning material is that it usually has a much higher hydrostatic head than a modern breathable fabric. Gore-Tex and the likes are designed to let water vapour out, and that means they have tiny pores. As the pressure goes up water can be forced through these. Breathable fabrics can have quite high ratings, but it tends to go down quickly with a bit of wear. Canvas will stay sealed a lot longer.
If the awning you’re looking at has a hydrostatic head listed, anything over 1,500mm will do you fine. Don’t be tempted to go below that even if the awning has other features that you like, because in anything more than a light shower it’s going to leak. It doesn’t matter how great it is in every other way if it doesn’t keep the weather off.
