Rain and guttering on a pergola

An open timber pergola lets rain straight through. Once you add a solid, polycarbonate or louvred roof, every drop that lands on it has to go somewhere. Most problems with a roofed pergola in a British winter come down to where that water ends up.

Where the water goes on each roof type

• Fixed solid or polycarbonate. The roof is laid to a slight fall. Water runs to the low edge, into a gutter if there is one, and off the end. With no gutter it falls in a line along that edge.
• Louvred. With the blades closed, water runs along channels in the blades into a gutter built into the frame. Many aluminium louvred kits then send it down inside one or more posts and out near the foot. With the blades open, rain falls through.
• Retractable canopy. Fabric canopies usually need a slope, and some makers say to retract them in heavy rain. Water can pool if the fabric sags.

The roof types guide compares the roofs in more detail.

Read the maker's drainage detail

Before you buy a roofed pergola, check the drawing and instructions for:

• Which way the roof falls, and whether you can choose.
• Which posts carry water, and where the outlet comes out: the side of the post, the base, or a spout.
• Whether the outlet can be connected to a pipe or drain, or only discharges at the foot.
• How the gutter is cleared. Leaves and debris block small gutters and post outlets.

We do not give drainage rates or roof areas here. Those depend on the model, and the maker's figures are the only ones to use.

The outlet still needs somewhere to go

A gutter does not make water disappear. It collects rain from the whole roof and lets it out in one place. In a heavy shower that can be a lot of water arriving at the foot of one post. If the ground there cannot take it, it can:

• pond on the patio and run back towards the house
• wash out soil or gravel round a pad or ground screw
• soak a timber post base, which is where rot starts
• run under a neighbour's fence.

Plan the discharge point as part of the job. Common answers include a water butt, a gully or existing drain if one is suitable, or a soakaway. Connecting to a drain, or building a soakaway, can raise building regulations and drainage questions. Ask building control or a competent drainage installer before you connect to anything.

Patio falls and the base

A patio is normally laid with a slight fall so water runs away from the house. Put the pergola on it and check that the post outlets discharge downhill, away from the wall. If the roof falls towards the house, the gutter and outlet need to cope with that. The base and fixings guide covers pads, slabs and screws.

Wall-mounted roofs

Where a roof meets the house, the join needs a seal or flashing, and the gutter must not empty against the wall. Water running down the house wall, or soaking the ground at the damp-proof course, is the classic problem.

Snow, debris and blocked gutters

A blocked gutter turns a louvred roof into a shallow tray. Water then spills over the edge or sits on the blades. Clear gutters and post outlets in autumn and after storms. Snow and ice can block outlets too. The wind and snow guide covers what makers say about snow on the roof.

Size and fall

A larger roof collects more water. If you are choosing between sizes, see what size pergola, and check that a larger model has enough outlets for its roof.

Does a louvred pergola keep the rain out?

With the blades closed it keeps most rain off the table. Wind-driven rain can still come in from the sides, and some water may drip at the blade ends or gutter joints. Side screens help with the first problem.

Can I connect a pergola downpipe to my house drain?

Possibly, but do not assume it. Whether it is allowed and how it should be done depends on the drain. Ask building control or a drainage installer first.

Why is water coming out at the bottom of my pergola post?

On many louvred pergolas that is the design. The posts act as downpipes. Make sure that water has somewhere to go.