Start with a spot that does half the work

A well-sited den will shed most of the rain before you have woven a single branch. Walk the wood first and look for ground on the sheltered side of a thicket, ideally under a canopy of mature oak or beech, but clear of any dead limbs overhead. In Britain the prevailing wind and rain arrive from the south-west, so a spot with a natural bank, hedge or tree trunk to your back gives you a ready-made windbreak. Slope the back of the den into that weather and open the front towards the calm side.

Avoid hollows where water pools after a downpour, the foot of steep banks where runoff gathers, and boggy patches that stay sodden for days. Check the ground itself, too: no burrows, no wasp nests, no dense nettle beds. And do check whose land you are on. Most woodland in the UK is privately owned, and while access on foot is often permitted, building structures is not automatically part of that. Ask the landowner or the estate office, and keep your den modest and temporary.

Choosing branches that will actually hold

You want fallen wood, not cut wood. Dead branches lying on the forest floor are perfect; standing timber is not yours to take, and sawing live limbs damages trees and looks dreadful. Choose limbs between thumb and wrist thickness, and test each one by bending it. Good den wood flexes and springs back. If it cracks dryly and snaps, it is too brittle to be a rib, though it will make fine kindling or a short peg.

Ideal pieces to collect:

• Forked branches — natural Y shapes make uprights that hold a ridge pole without any tying at all.
• Long, straight poles around 2 to 2.5 metres for the ridge and main ribs.
• Thin, whippy shoots of hazel, willow or birch for weaving the walls.
• Leafy brash and conifer branches for the outer thatch.

Test any branch by pressing your weight on it before it goes overhead — rotten wood hides well under bark. Skip anything with fungal brackets, soft spongy patches or a hollow knock. Avoid dead ash, which can be dangerously brittle, and never work directly beneath a leaning or hanging branch.

Lashing a frame that will not collapse

The quickest reliable structure is a lean-to. Find two uprights with natural forks, or lash your own, and push them firmly into the ground about a metre and a half apart. Drop the ridge pole into the forks. Then lean ribs against the ridge at roughly 45 degrees, spacing them 20 to 30 centimetres apart, with their bases pushed into the soil or wedged under a log.

For the lashings, a simple wrapping knot is all you need: loop your cord around both pieces, wrap it tightly eight or ten times, then finish with three or four tight half hitches. Garden twine, paracord or a ball of string works well. If you have no cord, notch the joint with a small saw and wedge the pieces together — a snug notch holds surprising weight. Check the whole frame by pushing down on the ridge; if it moves now, it will collapse in wind later.

Weaving the walls and laying the roof

Once the frame is solid, weave horizontal branches in and out of the ribs, starting wide at the ground and tapering as you climb. This is the job children adore, and it does not need to be neat — dense matters more than pretty. Leave a gap at the front for a doorway and, if you are feeling ambitious, build a low wall around the entrance to cut draughts.

Then thatch it. Layer material like roof tiles, working from the bottom upwards so each new layer overlaps the one below and water runs over the surface rather than under it. Leaf litter, bracken, moss, dry grass, conifer brash and handfuls of dead ferns all work. Aim for a finished thickness of 20 to 30 centimetres, and cap the ridge with a final roll of moss or a bundle of bracken tied along the top.

Making it genuinely weatherproof

A steep pitch is your best defence — 45 to 60 degrees sheds water far better than a shallow slope. Pack loose leaves around the base of the walls to stop rain driving in at ground level, and lay a thick bed of dry leaves or a folded tarpaulin inside so nobody sits on wet earth.

Then inspect from within. Sit in the den and look up. If you can see daylight through the roof, so can the rain, and that patch needs another layer. Leave a small vent near the ridge on the sheltered side so the den does not steam up, and clear a shallow drainage channel around the outside if the ground is heavy clay. Ten minutes of tidying after the first shower turns a leaky pile of sticks into a proper hideaway.

Keeping it safe and keeping them busy

Gloves and eye protection are worth the fuss when you are snapping branches, and it is worth glancing up each time you work, because dead limbs come down on their own schedule. Never let children climb the structure. Long trousers and a tick check at bath time are sensible habits, and everybody washes their hands before the sandwiches appear.

Give the children real jobs: gathering brash, weaving walls, testing the roof with a cup of water, laying the bedding. Bring a flask, a spare pair of socks and a bothy bag for the walk home. When you leave, scatter the materials back into the woodland — dead wood is a habitat, and a den that quietly disappears overnight is the sign of a job well done.