Every so often the topic waterhead testing and resulting ratings come up.
I happen to think that the way tent fabrics are usually measured isn't an accurate representation of the way the fabric behaves in real life , that is when the tent is pitched and it rains on it.
The usual way to measure the "waterhead" is to use a Suter tester or something like that.
So a piece of 12x12cm fabric is clamped on to the tester , water is pushed against it at an increased level of pressure.
When you get 3 drops of water passing through the fabric you have achieved the waterhead level of that fabric.
These are the problems I have with that :
For a start tents are not set up with the fabric at an even pressure and that difference is greater in some designs than others, therefore (in my view...) the exact same fabric can give you a different result from tent to tent because of this.
To better understand this, think of the "handkerchief over a glass of water" trick.
(Put an hankie loosely over a glass of water , turn the glass upside down it will drip, now put the fabric of the hankie on tension and it won't leak any longer)
Next is, how does that fabric behave when under pressure from contact with poles underneath it ?
(think of the famous finger against a cotton wall tent..)
But here is another one. Some materials "wet out" . So what happens after a few hours of rain ?
Could it be that a fabric that holds heavy rain for a short time does not do so after a few hours?
Finally , I never seen rain falling evenly (corner to corner without gaps) on a tent.
I suspect that having raindrops over a piece of fabric creates a different surface tension than the Suter type test does.
But of course I could be completely wrong...
(edited to insert photo)