A shed roof is the cheapest roofing job there is and the one most often done twice, because the covering that came with the shed was chosen to make the shed cheap. The deck is thin, the felt is light, and the edges are stapled. Doing it once properly costs very little more than doing it badly and lasts several times as long.
Check the deck before choosing a covering
Shed roofs are usually a thin sheet of oriented strand board or a few softwood boards, and once water has been getting in at the edges for a season or two the board delaminates at the corners and loses its grip on the fixings. Pressing along the eaves with a thumb finds it. Recovering a soft deck means the new covering follows the sag and the fasteners have nothing to hold, so the deck is either replaced or overlaid with new board before anything else happens.
What the coverings are worth
Standard shed felt is the cheapest and lasts a few years, less if it is stapled at the edges rather than nailed with clout nails under a batten. Heavy mineral felt lasts appreciably longer for a small extra cost. Rubber sheet in a single piece is the option that most often reaches twenty years on a small shed, because it has no joints, and it is now cheap enough to be sensible. Corrugated bitumen sheet suits a shed with a decent pitch and is quick. Plastic sheeting is a summer house solution and not a British winter one.
Where shed roofs actually leak
At the edges, every time. The felt is turned over the fascia and stapled, and the staples let go. The fix is a batten over the turned edge, nailed rather than stapled, with the felt lapped so water runs over the joint rather than into it. The second place is the ridge on an apex shed, where the top sheet should overlap both sides by a good margin and often does not. The third is where a lean-to shed meets a wall or a fence, which needs a flashing rather than a bead of sealant.
Questions people ask about shed roof repair
Can I recover a shed roof over the old felt?
On a sound deck with a flat well fixed old covering it is common and works. Over a wrinkled or lifted old felt it is not, because the new covering follows the shape and fails at the same ridges.
Do I need permission to re-roof a shed?
For an outbuilding within permitted development, re-covering the roof does not normally need approval. Changing the height or the structure can, and different rules apply in a conservation area.
What is the longest lasting shed roof covering?
A single piece rubber membrane on a sound new deck with proper edge trims, in most cases. It has no joints, and joints are where shed roofs fail.