Having written a comprehensive review of tent design, several people asked if I could write something similar about tent fabrics. This was a subject, at the outset, which I knew rather less about. However having spend eight years as a material scientist working with powders I have been pleasantly surprised how this skill set has transferred and been valuable when reading around the topic of tent fabrics. As with my original article my aim is to present the facts objectively and without bias. I am simply a hill walking & backpacking enthusiast of long standing with a background in chemical and material sciences but no professional or financial interest in the tent industry.
Definitions / Glossary
I am going to limit myself to the fabrics used with modern backpacking tents, if you want to know more about cotton canvas, this is not the article for you. Let’s start with some definitions to help you read the tent makers specifications and sift out the facts from the hype.
Denier – This is a comparative measure of the thickness of the fibres from which a fabric is made. It is the mass, in grams, of a 9000 m length of the fibre. The definition stems from the fact the 9000 m of single fibre of silk weighs one gram. If all else is equal, then the higher the denier the stronger the fabric. By definition, higher denier fabrics are heavier. But as we shall come to see, fibres made from different materials have different strengths.
Thread Count – This is the total number of threads (total of vertical and horizontal) within a square inch. Again if all else is equal, the higher the value, the stronger the fabric. Thread count is not universally quoted with tent fabrics.
Tensile strength – the force required to rip a sample when force is applied to opposing ends of the sample. This is a more useful measure than either Denier or Thread Count when comparing tent fabrics.
Specific strength – The tensile strength per unit weight of a material of the same dimensions and critically, thickness.
Ripstop – this is a fabric which periodically has a stronger fibre added into the weave with the aim that any tear be unable to propagate beyond that fibre.
Hydrostatic head (HH) – It is measured as a length, representing the maximum height of a vertical column of water that could be placed on top of the fabric before water started seeping through the weave. Whilst a HH of > 3000 mm may seem extreme at the outset, remember that HH is really a measure of pressure and the pressure on a tent fabric can be increased by wind-speed on the fly, or weight applied to the groundsheet.
Coating – a second material applied to the surface of fabric to change its material properties. In the case of tent fabric this is usually something applied to make it waterproof. Whilst historical fabrics such as canvas were intrinsically water resistant, modern fabrics rely on the coatings applied to make them waterproof, both polyester and nylon fabrics are not in themselves waterproof.
Today’s tents are generally made from one of three fabrics, polyester, nylon or DCF (Dyneema Composite Fabric). Each have their good and bad points. If you are looking for a lightweight backpacking tent in 2021 you will find that most are made from nylon with just a few made from DCF.
Polyester has a lower specific strength than nylon, so to be of the same strength it needs to be thicker and thus heavier. Whilst it is slowly degraded by UV light it is less susceptible than nylon. It’s highly significant advantage is that it does not stretch (significantly) when wetted. Traditionally cheaper tents has been made from polyester as the fabric itself is cheaper, but this comes at the cost of extra weight. The main reason it is cheaper is the scale on which it is made for other applications.
Because of it’s lower specific strength the lightest polyester fabric readily available is 20 denier. By contrast, 7-10 denier nylon is common.
Nylon has a higher specific strength than polyester, is more flexible / pliable and is thus easier to pack down to a small volume. However, it does stretch when wetted (Nylon 6,6 by 2.5%) and holds more water within its structure and thus gains more weight when wet than polyester. It is also slowly degraded by UV light, but in fairness today’s buyer would be more likely to change their tent because of an improvement in design than due to fabric degradation. Where UV damage is more of an issue is at high altitudes, above 5000 m, where UV light is a stronger component of the light spectrum. This can be mitigated by choosing a fabric colour which absorbs less UV light. Apparently green and brown fabrics should be preferred over red or orange.
DCF, Dyneema Composite Fabric or Cuban Fibre is not strictly a fabric because it is not woven. Now I’d headed the etymologists off at the pass let me tell you what it actually is. It is a non-woven mat of UHMWPE (a type of polyethylene) sandwiched between two layers of polyester film. Typical of all composite materials it marries the qualities of two materials , the UHMWPE is a very strong but flexible fibre and the polyester film is both waterproof and water impermeable. A very similar composite material called Ultra is now being used for some tents made by Terra Nova.
The specific strength of DCF / Ultra is far higher than polyester or nylon fabrics. So for comparable strength DCF can be significantly thinner and approx. 35% lighter for the same strength. It does not stretch when wetted as it does not absorb any water at all. Thus it both dries quickly and does not get significantly heavier when wet. It does not stretch under strain either so anchor points have to be stronger as they take all the force, it is not dissipated across the body of the fabric. DCF seams are commonly bonded rather than sewn, eliminating any need for seam sealing.
So is DCF / Ultra the fabric of our dreams? That depends on your application and your budget. It is considerably more expensive that Si-nylon (200-300%) and whilst it has a very high tear strength, because of it’s nature and thickness (or the lack thereof) it is far more easily punctured than the other options. It also has a lower abrasion resistance because it is a film rather than a fabric. It can be very easily field repaired, but that is likely to be required more often than you might hope.
Fabric coating options
Both polyester and nylon need to be coated to make them waterproof, DCF does not. The same coating options exist for both.
Polyurethane (PU) coating
This is usually applied to the inside of the fabric and bonds to it’s surface. A significant benefit is that seam tape can be stuck to the PU and thus the seams can be readily sealed. PU hydrolyses with time (reacts with water to break down) and this will eventually cause it to delaminate and fail. That said my Si coated / PU lined Solar 2 flysheet lasted 15 years before it started to fail.
Silicone coatings soak into the weave of the fabric and actually increase it’s strength. This is true for both polyester and nylon. They are far more chemically stable than PU and thus have a greater lifetime. The only downside is that because of their ‘slippery’ nature they cannot accept seam tape*. Brush-on seam sealant is available but is usually expected to be applied by the buyer rather than the manufacturer. Some tent manufactures overcome the seam issue by designing the seam positions to reduce the water loading onto the seam. Hilleberg uses specially cooled needles for flysheet seam sewing which leads to holes small enough not to allow water to pass through them.
*Thanks to recent development work at Vaude a way to apply seam tape to Si-Nylon has been developed, but is currently (Feb-20) unique to them. In parallel with this they have also started offering bonded rather than stitched seams on a couple of their flysheets which they describe as seamless. A bonded seam does not, in theory require secondary sealing it should be fundamentally water tight.
Silicone coatings are more expensive than PU as you might imagine, and designing out the seam issue comes also at an added cost. However, if your budget allows, silicone is the superior coating because it adds strength, allowing for lighter base fabrics and has a longer service life.
Some manufacturers apply PU to the inside of the flysheet and silicone to the outside. I’ve read extensively and can see only one reason why this might be considered technically superior. That is that it allows for factory seam taping. I cannot help but think that the primary driver is cost.
The groundsheet of a tent needs to be more robust that the fly and this is achieved by both higher denier fabric and the application of a greater number of layers of waterproof coating. The groundsheet does not need to have a 5-10 meter hydrostatic head for the purpose of keeping the water out, but this comes as a bonus once you’ve made your floor robust enough for it’s duty.
The inner tent fabric is something that is little discussed, but experience has taught me that it is advantageous for this to have some form of water repellent coating (DWR). A fabric that wets less easily will dry more quickly (because the water transport coefficient of a material is proportional to contact angle since you asked!) and this can be very handy if you have a damp inner tent because you have packed it next to the condensation left on your fly. I favour ‘all in one’ pitching / striking and find it takes ≤ 20 min for my inner tent to become totally dry post a re-pitch. Having reviewed the websites of a wide range of manufacturers I only found one which claims this feature. Others may offer it, but they certainly don’t shout about it. Perhaps spraying your inner tent with Nikwax Direct would be worthy of consideration?
So what is the best flysheet option? That depends on your planned application and your budget. When choosing a tent I would be guided primarily by design, Unless you are buying a ripped off design made in the Far East you should expect a well designed tent to have an appropriately specified materials. A strong fabric with poorly re-enforced guying points which are badly positioned will not make the most of even the best fabric. Once you have chosen the design that fits your application, then your choice of manufacturer is likely to be primarily influenced by your budget and weight target. It is an eternal truth that less is more! Some manufacturers offer different ranges of the same designs with cheaper heavier models and more expensive light-weight versions.
 If you are fastidiously careful, want to travel very light and have very deep pockets then you might well consider DCF / Ultra as your tent fabric.
 If you favour peace of mind and longevity then silicone coated nylon is probably for you.
 I hope that if you are buying on a budget that this article will allow you to see the strengths & weaknesses of cheaper materials and ensure that your chosen supplier has compensated appropriately. 70D polyester in place of 40D nylon for example. Remember that the lower price is a strength if it is what makes your adventure possible.
Finally, the research behind this article suggests that there is a gap in the market for someone to develop a stronger low denier polyester fabric. There are a couple of niche manufacturers who offer 20D Si-polyester based flysheets (TrekkerTent, The Tent Lab) which may offer you the sweet-spot between weight, strength and a nice taut pitch.