The main metric used to measure the waterproof capacity of tents is hydrostatic head (HH). HH ratings are applied to products during lab testing in which an open-ended test tube is placed on top of the fabric and filled with water to apply water pressure to the surface of the fabric. The water volume (and, thus, pressure) is then steadily increased and the point at which the water passes through the fabric determines the fabric’s HH.
Hydrostatic head ratings are given in millimeters and commonly found on product specifications as follows: 10,000 HH or 10,000mm HH. Simply put, the higher the HH, the more waterproof a product’s fabric is.
Hydrostatic head in practice
In most countries, a HH rating of at least 1,000mm HH is required for any tent to be marketed as “waterproof.” In reality, however, tents with a 1k HH rating are capable of withstanding only the very lightest of short-lived showers.
The hydrostatic head rating of the rain flies on most three-season tents ranges from around 1,500mm to 2,500mm. Tents at the lower end of this scale can be prone to leakage after extended exposure to moderate rainfall but those around and above the 2k-mark are usually capable of withstanding a combination of heavy rain and strong winds and, as such, are suitable for all but the most extreme conditions.
Tents with a HH rating in excess of 2,500mm fall into the proverbial “bombproof” category, meaning they are able to withstand heavy downpours, gale-force winds, and the added pressure of objects or bodies pressed against the fabric from the inside, something that can cause leakage in models with a lower HH rating. This HH rating is most commonly found in the fabric used on expedition and mountaineering tents.
In most cases, the floor used in any camping tent will have a much higher HH rating than the rainfly. This is because more pressure is applied on the flooring by the tent’s occupants and their gear, which increases the likelihood of groundwater seeping through the fabric. Consequently, most tent floors have HH ratings of over 2,000mm, with three-season models usually falling into the 3,000-4,000mm range and four-season models often scaling as high as 8,000mm.
While HH ratings give us an indication of how waterproof a tent’s fabric is, the fabric alone is not the only factor contributing to a tent’s overall waterproofness. Three other factors to look out for are the use or absence of a DWR coating, the seams, and the zippers.
In addition to the PU coating or membrane that forms a tent’s main resistance against water, many rain flies (and some tent inners) are treated with a DWR (durable water-resistant) coating. In short, this coating causes water to bead up on the surface of the fabric and then roll off, thus reducing the chance of saturation.
The seams on a tent are one of its greatest points of potential weakness owing to the fact that they join two pieces of separate fabric and are pocked with hundreds of tiny holes where the fabric has been stitched together. As such, the most waterproof varieties of tent usually have seams that are double-stitched (to secure the bind of the two sheets of fabric) sealed (treated with a layer of hydrophobic glue or sealant to prevent leaks), or taped (lined with waterproof tape during manufacturing).
Another factor contributing to how well a tent will resist the elements is the protection provided around the zippers. A good tent will always have storm flaps—strips of fabric that fold over the zipper and fastened with velcro to prevent rain from coming through the teeth of the zipper.