One of the questions we're often asked by people looking at tents is how to compare the materials used in competing tents?
Often the choice is not clear cut especially given that different manufacturers favour different material types, coatings and different density weaves (eg. 75d).
Tents these days are constructed of a number of materials, the most common of which are Polyester and Nylon (canvas is also used, but rarely in smaller tents used for hiking due to it's weight).
Polyester is not as strong as Nylon for the same weight of fabric, but on the other hand it is more resistant to degradation from ultraviolet light than Nylon. Polyester does not absorb water as Nylon does, but it is more waterproof and less breathable than Nylon. Tent Materials can also treated with coatings such as PolyUrethane (PU), these generally make the fabrics less breathable and increase its waterproofness by, in simple terms, filling in the gaps between the threads in the fabric. Fabrics can be Siliconised which is a Special UV resistance treatment that protects the fabric from degradation from the sun.
Ripstop is a special type of fabric that has extra reinforcement threads to prevent tears from spreading, this gives much more durability for only a small increase in weight. Ripstop is fairly standard on flysheets of quality tents so you should expect at least the fly to have this feature.
Pictured: Ripstop pattern - these can often be hexagonal, rhomboid or square (depending on manufacturer)
Nylon is commonly used for tent floors (due to lack of UV and superior strength to Polyester) and in fly sheets where light weight is essential. You should ensure that where it is used for flysheets, it is polyurethane treated and preferably siliconised. Nylon is more expensive than polyester to make, but a Siliconised Nylon Ripstop PolyUrethane Coated material is the best performing material for its weight in terms of durability and waterproofing. Keep in mind these treatments reduce breathability, so having vents in the tent is desirable.
Polyester is more common as a flysheet and inner material than Nylon due to its cheaper price and also superior resistance to UV light. For Flysheet materials, an added benefit of polyester is that untreated it will sag less when wet (as it does not absorb water as nylon does).
Fabric weights are rated by Denier which is a measure of weight per length of fabric. Its generally used as just the letter 'd' beside a number when referring to fabric (eg. 70d = 70 Denier), you can think of this basically as the weight of the fabric. Higher denier numbers for the same fabric generally mean higher water resistance and strength, but that also means higher weight.
Water Head rating is simply a measure of the waterproof-ness of a fabric It actually represents pressure and is the height (in millimetres) of a column of water that is required to leak through the material. Often when you see a fabric name it will have a number in the thousands that represents its water head, for example Vango's "HD nylon 5000 groundsheet" has a water head of 5000.
Water Head is a useful number in comparing two fabrics from two different tents for water resistance.Floors range from 4000-10000 of water head whereas Flysheet's range from about 1000-4000 of water head. For Floors: 7000 and above water head is reserved for hand core expedition tents, most treking and hiking tents are 4000 and above.For Flysheets: 5000 and above water head is reserved for hand core expedition tents, most treking and hiking tents are 1500 and above.
In summary Polyester is a better value material for flysheet construction, except where every gram counts. Where light weight is essential, appropriately coated and treated Nylon is the best material there is available for its weight. Whether Nylon or Polyester, flysheet fabrics should always be ripstop. Nylon is almost always used for inner and floor materials due to its strength.
The easiest way to compare materials used in tent construction for waterproof performance is to use the water head rating.