Recently purchased Ripstop fly nylon from Owf. Had a 1 oz urethane coating. Lynn of Owf has been very helpful in trying to answer my query re hydrostatic head equivalent of this coating but it appears she does not have this information. Does anyone know? eg is this a 2000mm product or whatever. Thanks in advance.

The 2000 mm is supposed to be the height of a column of liquid needed to create enough pressure for the penetration of water through the fabric.

Unfortunately, I don't recall if the reference liquid is water or mercury.

If you look at tent specifications, they quote numbers for rainfly performance as 1000 mm, 1200 mm and sometimes 2000 mm as a measure of water entry pressure.

So, relatively speaking, the fabric you are looking at would be considered quite waterproof.

The Kelty Gunnison 2 tent lists the floor material as 1800 mm PU coated nylon.

The 2,000 mm rating is for a water column, which evidently equals 147 mm of mercury.

For yucks I tried switching the numbers, which have 2,000 mm of mercury equaling 27,190 mm of water, or about 90 feet(!)

Interestingly, the conversion tool I used shows 2,000 mm of water only equaling 2.8 psi, which doesn't sound nearly as impressive.

1 oz coating on 70d ripstop would be a heavy coating and very waterproof. Typical of higher end
tent floor and fly material such as would be offered
in 4 season tents. Some fly or tarp material can go
down as .25 oz of urethane.

Thanks for your help, it sounds as if a 1 oz coating is pretty substantial so I have gone ahead c the project.