Wet sandstone: why you wait for it to dry

The rule nobody explains

Every sandstone area has the same rule, and it usually arrives as a flat instruction rather than an explanation. Do not climb on it when it is wet.

The Southern Sandstone climbing code puts it in five words, "avoid climbing on wet rock", and adds that doing so can cause irreversible damage. The guidance for Fontainebleau, published in cooperation with the French forestry office, the ONF, says sandstone is extra fragile when wet and that wet holds can break or crumble. In Northumberland, Geology North describes Fell Sandstone as prone to rapid erosion after rain, and notes that climbing on it in that state speeds the process up.

Autumn is when this starts to matter again. The friction comes back and everyone wants to be outside, but the weather is far more mixed than it was in July. Plenty of dry Saturdays sit on top of a wet week.

Why wet sandstone breaks

Sandstone is exactly what it sounds like. Grains of sand, cemented together over a very long time. How well a hold survives depends on how firmly those grains are bound.

It is also porous. Water does not sit on the surface, it soaks in, and while the rock is saturated the grains hold together less well. An edge that has survived years of traffic can snap or crumble in one damp session. Footholds wear away faster, and the sand that comes off under a wet shoe is the problem getting smaller.

None of that damage grows back. A broken edge is gone for good, and it changes the climb for everyone who comes after you, including you next year.

Not all sandstone is equal

The BMC's advice on climbing in the wet groups sandstone and gritstone together as porous rock that is weakened when wet, and says limestone is affected to some extent. In practice it is a sliding scale.

Soft sandstone is where the rule is absolute. Southern Sandstone, on the Kent and Sussex border at crags such as Harrison's Rocks and High Rocks, is soft enough that the local code also limits climbing to top-rope, bouldering and soloing, and asks that moving ropes never run over the rock. Fontainebleau and the Fell Sandstone of Northumberland, Bowden Doors and Kyloe among it, belong in the same careful category.

Gritstone is a coarse sandstone and far harder-wearing, but it still soaks up water. Damp grit is also miserable to climb on, which is a warning in itself. Leave the thin edges and the flakes alone until it has dried.

Harder, non-porous rock is the way out. The BMC points wet-weekend climbers towards volcanic and igneous rock, granite and mountain crags among it, and towards steep limestone.

Dry on top is not dry

The mistake is rarely climbing in the rain. Nobody does that. It is climbing the morning after, when the surface looks fine and the inside of the boulder is still full of water.

There is no honest number of hours to wait. It depends on how much rain fell and for how long, the wind, the sun, and which way the rock faces. An exposed, breezy boulder after a short shower can come back quickly. A shaded one under trees after a wet week can stay damp for a long time. Local climbers and area guidance know their rock better than any rule of thumb, so ask them first.

At the rock itself, check four things.

• Colour. Wet sandstone is darker. Dark patches, or streaks running down from the top, mean water is still draining through.
• The ground. If the sand or soil at the base is wet, the bottom of the boulder is often still damp as well.
• The feel. Rock that feels cold and clammy under your fingers is holding water.
• Sand on your fingers. If grains come away when you pull on a hold, stop. That is the hold wearing away while you watch.

If in doubt, leave it. The problem will still be there next weekend, as long as nobody has pulled it apart in the meantime.

Look after it when it is dry

The same thinking holds on a good day, because dry sandstone is still soft.

• Clean your shoes. Sand on your soles is sandpaper on the footholds. The Fontainebleau guidance asks climbers to wipe their shoes on a cloth or a mat before they start, and a small doormat weighs almost nothing.
• Go easy on chalk. Both the Southern Sandstone code and the Fontainebleau guidance ask for less of it, and resin products are out altogether in Font.
• Soft brushes only. Harsh brushes and toothbrushes are banned under the Southern Sandstone code. There is more on brushing without harm in the brush guide and in tick marks, chipping and brushing.
• Walk off. On Southern Sandstone the code asks you to walk off rather than lower off or abseil, which keeps ropes off the rock.

The wider habits of being a good guest at the crag are in crag etiquette and access.

What to do with a wet weekend

A damp sandstone forecast does not have to mean a lost weekend.

Change rock. Go and find something that sheds water, using the BMC's wet-weather suggestions as a starting point. Chasing conditions covers how weather and aspect decide which crag is worth the drive.

Go inside with a plan. A board session, or a morning on the section you always walk past, is far more useful than an hour sitting under a dripping boulder hoping.

Go and look. A walk round a bouldering area when it is wet costs nothing. Read the problems, check the landings, and plan the day you will come back for.

The short version

Sandstone is porous and at its weakest when wet. Holds that break do not come back, so every sandstone area asks the same thing: stay off it until it has dried right through, not just on the surface. Look at the colour, the ground and the feel of the rock, and if you are not sure, go and climb something harder.

Sendit keeps your sessions, projects and outdoor days in one place, so by the end of the season you can see how it actually went, including the weekends you sensibly left the rock alone.