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A leaking summerhouse roof rarely announces itself politely. You usually notice a damp patch on the inside of the boarding, a soft spot underfoot, or a stain spreading across the ceiling after a windy night. The temptation is to climb up with a roll of felt and start nailing. Resist it. Water is a patient traveller, and the hole you can see is often nowhere near the hole that is letting it in.

Start by tracing the water, not the damage

Water running across the underside of felt or boarding will track along rafters, drip onto a joist, and land somewhere completely unrelated to the failure. Before you buy anything, spend half an hour working out the route.

• Go inside during or just after heavy rain and watch where the drips appear. Mark the spot on the ceiling or a rafter with a pencil.
• Check whether the leak only shows in certain wind directions. A leak that appears only in a south-westerly usually points to a flashing or an edge, not a hole in the middle of the roof.
• Look for staining patterns on the underside of boards. Dark streaks fan out from the true entry point, so follow them back uphill.
• If the summerhouse has power, isolate it at the consumer unit before you start poking about near wet timber and fittings.

Once you have a rough idea of the area, you can go up with a purpose rather than a general hope.

Survey the roof properly, from a safe position

Most garden summerhouses have a shallow pitch, often between 5 and 15 degrees, and many have a single slope with a low front edge. That low pitch is exactly why small defects cause big problems: water sits on the surface longer and finds its way under laps far more readily than on a steep house roof.

Use a proper platform or a stepladder on firm, level ground, and never lean a ladder against the felt itself. Take a torch, a scraper, and a notebook. Work methodically along these points:

• Laps and joins: felt should overlap by at least 50mm horizontally and 75mm vertically on a shallow roof. Lift the edges gently with a scraper and look for grit, moss, or old adhesive that has broken the seal.
• Nail heads: every clout nail is a potential entry point once the felt around it has perished. Look for rust stains and dimpled or torn felt.
• Ridges and hips: these take the most wind-driven rain and are the most common failure on a pitched summerhouse roof.
• Flashings: where the roof meets a wall, a chimney breast, or an adjacent structure, the mortar or mastic may have cracked away. Check the whole length, not just the obvious gap.
• Edges and fascias: felt that has lifted or split at the drip edge lets water straight onto the top of the boarding.
• Gutters and outlets: if the summerhouse has a gutter, a blockage will send water back under the felt at the eaves. Clear it before you blame the roof.

Prod any suspect timber with a screwdriver. If it sinks in easily, the board beneath is rotten and the repair is bigger than a patch.

Dry the structure before you seal anything

This is the step most people skip, and it is the reason a repair fails within a season. Trapped moisture will rot the boards from the inside and lift new felt adhesive from below.

Strip away any loose or saturated material, then let the exposed timbers dry for several days in decent weather. A small fan heater on a low setting, placed safely and never left unattended, will speed things along. Aim for timber that feels dry to the touch and shows no dark, damp patches. If you are repairing in autumn or winter, consider covering the area with a temporary tarpaulin and waiting for a dry spell rather than sealing in damp.

Check the rafters and any boarding you intend to keep. Replace anything that crumbles, delaminates, or has gone soft. On most summerhouses, 12mm exterior plywood or OSB3 is the standard deck, and both are cheap enough to replace rather than patch.

Make the repair in the right order

Work from the bottom of the roof upwards and from the highest point downwards on the laps, so every new layer sheds water over the one below.

• Small splits and nail holes: clean the area, let it dry, then apply a self-adhesive flashing tape, pressing it down firmly and smoothing out air bubbles. Overlap the tape at least 50mm past the damage on all sides.
• Larger tears: cut a patch of matching felt, generously oversized, and bed it in felt adhesive. Weight it with bricks overnight if the weather is cool.
• Failed flashings: rake out cracked mortar or old mastic, then apply a lead replacement flashing strip or a quality self-adhesive flashing tape dressed into the angle. Prime dusty surfaces first.
• Lifted edges: re-bed with adhesive and re-fix with clout nails, then cover the nails with a strip of flashing tape.
• Worn-out felt overall: if the surface is brittle, cracked across large areas, or has lost its mineral coating, patch repairs are false economy. Re-cover the whole roof, laying new felt over the old where the deck is sound, or strip and replace if it is not.

Finish with a check around the whole perimeter, run a hose gently over the repaired area for ten minutes, and watch the inside for drips. Then clear the gutters, trim back any overhanging branches, and note the date of the repair so you can spot the next one early.