There is a particular kind of rain in the British hills that seems to come at you sideways, find the gap between your hood and your collar, and settle in for the afternoon. You can plan for it, or you can spend the evening wringing out a sleeping bag you have no way to dry. The difference between the two comes down to a packing system rather than a kit list, and it weighs almost nothing.

Think in three layers, not one big bag

Most rucksacks are described as waterproof, and most of them are lying. A pack's own fabric will shrug off a shower, but the seams, zip and lid are weak points, and a full day of driving rain will always find them. Treat your rucksack as a shell, not as a container, and pack in three layers.

The outer layer is the pack itself, plus a lightweight pack liner if you have one. The middle layer is a set of dry bags, each with a job. The inner layer is the arrangement of everything inside them, organised so that the things you need most are the things that stay driest.

That last point matters. If your spare fleece lives in the top of the pack and comes out every time you stop for a snack, it will get wet no matter how good your liner is. Decide what you will touch during the day and what you will not touch until camp.

The dry bag hierarchy

Not all dry bags are equal, and neither is what goes in them. Rank your kit by how badly it fails when soaked.

• Sleeping bag and spare insulation: the one bag that must not leak. Use a proper roll-top dry bag or a compactor bag, twisted shut at the top and folded over. Down loses nearly all its loft when wet; even synthetic takes an age to dry in a bothy with a damp fire.
• Spare layers and socks: a separate, smaller dry bag. Keep a dry baselayer and a dry pair of socks sealed until you are in the tent or bothy, and resist the temptation to change into them at a lunch stop.
• Stove, fuel and matches or lighter: a small dry bag of its own. A gas canister is fine wet, but lighters and matches are not, and damp tinder will end a brew stop before it begins.
• Electronics and head torch: one more small bag. Cold and wet is a bad combination for batteries.
• Food: a simple lightweight bag. It does not need to be perfect, just not swimming.

Everything else, including the tent, can ride in the main body of the pack. If your tent has a decent fly, a little dampness on the outside of the inner is survivable.

Packing order that works in horizontal rain

Line the rucksack with your liner first, then build from the bottom.

Sleeping bag and spare layers go at the very bottom, where they are hardest to reach and least likely to be pulled out by mistake. On top of that, the tent or bivvy. Then food, then the stove bag. On top, the things you actually want during the day: waterproofs that are not currently on your body, hat, gloves, map, and a flask or water bottle.

Keep your waterproof jacket and trousers at the top of the pack rather than buried, because you will want them on and off several times. If you are walking in them, that is fine, but pack a spare dry pair of gloves in a small bag. You will thank yourself at the col.

Anything strapped to the outside is a gamble. A wet map case flapping on a shoulder strap is one thing, but do not strap your sleeping mat or a spare layer to the outside in a downpour unless it is inside a dry bag.

Bothies: the wet-weather referee

Bothies are a gift in foul weather, and a few habits make them work better for everyone.

Leave wet kit in the porch or the first metre inside the door, not spread across the sleeping platform. Hang soaked waterproofs on a line or a hook rather than draping them over someone's sleeping bag. Do not dry boots on the stove or in the fire. Carry out everything you carried in, including any rubbish, and leave the bothy better than you found it.

A bothy can be cold and damp even in summer, so treat it as a dry shelter rather than a warm one. Your sleeping bag liner and a spare dry layer still earn their place.

Upland tarns and the morning after

Camping by a tarn is one of the great pleasures of the British hills, and it comes with its own packing demands. Ground beside standing water can be boggy, and valley bottoms collect cold air and mist, so pitch a little above the water and away from the inflow if you can.

Keep your pack inside the tent porch rather than outside it. A tarn-side pitch can be dewy even when it does not rain, and dew soaks a pack slowly and thoroughly overnight. Before you sleep, put tomorrow's dry layers inside the foot of your sleeping bag or under your knees so they stay warm and dry.

In the morning, pack the sleeping bag first, before you do anything else, and get it sealed while your hands are still dry. Everything gets harder once you are wet, cold and trying to make a brew.

None of this is complicated, and none of it is heavy. A liner, three or four small dry bags and a clear routine will get you through a British hill day that never quite stops raining, and out the other side with dry kit and a decent story.