Because rain falls on a flysheet and a body presses on a floor, and pressure is what the rating measures. A hydrostatic head is the height of a water column a coated fabric resists, so the fabric that has weight on it needs the larger number.
The two loads are not comparable
Rain arriving on a fly is a light, intermittent load spread over a large area. Water on the ground under a tent is a standing load, and the moment a knee, a hip or an elbow comes down on it, that water is being pushed against the fabric with a large part of your body weight behind it over a few square centimetres.
That is exactly the situation a hydrostatic head test models: pressure applied to a fabric by a column of water. So makers publish a higher figure for the groundsheet than for the flysheet, and it is a design decision rather than an inconsistency.
Two numbers, and this site wants both
A tent has two rated fabrics. A listing that publishes one and does not label it has told you half the story, and the missing half is normally the one that decides a night on a saturated pitch.
The same split shows in the denier. The floor is usually the heavier fabric — because it is also the face against grit, roots and stones — and the fly is the light one. Where a maker publishes both hydrostatic heads and both deniers, you can compare two tents properly. Where it publishes one number and no label, treat that as a gap and say so.
What actually gets a floor wet
Three things, and only one of them is the fabric rating.
Abrasion. A floor pitched repeatedly on gravel wears where it is walked on, and a worn coating leaks regardless of what it was rated at when new.
Standing water. A pitch in a hollow puts the floor under a genuine head of water for hours. That is the case the rating is for, and it is also the case a better campsite avoids entirely.
Water channelled underneath. This is the avoidable one and it usually comes from a footprint cut too large. A groundsheet protector that sticks out past the flysheet collects rain off the fly and delivers it under the tent, which is the exact opposite of the job. A footprint must be slightly smaller than the tent floor; if you cut your own from sheeting, take the floor dimensions from the tent specification and cut 3 to 5 cm inside the edge all the way round.
What a footprint does and does not do
It adds a sacrificial layer that takes the abrasion, which is what makes a tent floor last, and model-specific ones carry ring-and-pin or webbing loops that share the tent’s peg points so that fly and footprint go down together.
It does not change the figure the maker publishes for the tent floor. Footprints carry their own hydrostatic head and their own denier, and this family runs from ultralight film to heavy coated polyester — so the sheet under the tent can easily be rated lower than the tent.
Reading the pair
Collect four figures: hydrostatic head and denier for the fly, hydrostatic head and denier for the floor. Add the seam treatment, because taped seams are what turn a rated fabric into a waterproof shelter, and a needle hole is a hole whatever the coating does.
Then look at the pitch itself, which is free. A floor that never sits in standing water is never asked the question the rating answers.
What people ask before buying
Can I raise the floor rating with a footprint?
A footprint adds a layer under the tent and protects the floor fabric from abrasion, which is most of what shortens a groundsheet's life. It does not change the figure published for the tent floor itself, and it has to be cut slightly smaller than the floor or it channels rain underneath.
What if the maker publishes only one figure?
Then half the specification is missing, and it is worth saying so rather than assuming. Where a single unlabelled number appears, it is usually the flysheet's, since that is the one makers lead with in marketing.
Last reviewed 6 September 2026