Can You Put a Sedum Roof on a Flat Roof?

What a sedum roof is

Sedum is a family of succulent plants that store water in their leaves and cope with thin soil, sun and drought. A sedum roof is a layered system: a waterproof membrane, a protection layer, a drainage layer that holds a little water and lets the rest escape, a filter fabric, a shallow substrate and the plants on top. It is sometimes called an extensive green roof, to separate it from a deep rooftop garden.

The appeal is plain. It softens the look of an extension, it can help rainwater run off more slowly, and it gives insects and birds somewhere to forage. It also protects the membrane from sunlight and temperature swings, which can extend the life of the covering beneath.

The weight question

Weight is the first thing to settle. A bare membrane adds almost nothing to a roof. A saturated sedum system adds a good deal, since the substrate and mat hold water. A flat roof built to carry only its own weight and the usual snow load may not have the margin. The load comes from the finished system when fully wet, not from the dry product on the pallet.

Newer extensions with properly sized joists often cope. Older flat roofs, light timber porches and roofs with a worn deck may not. A structural check by an engineer or your building control body settles it. If the covering will end up heavier than the old one by a significant margin, approval is needed under Approved Document A. Our guide to Building Regulations for roofing work covers when that applies.

The waterproofing underneath

A sedum roof sits on a membrane, and the membrane must resist roots and last. A leak under a green roof is hard to trace and expensive to reach, since the plants must be lifted to get at it. So the sensible approach is to lay a new, full-quality waterproof layer at the same time as the green roof goes on, not to lay sedum over a tired old roof.

Some membranes have a root-resistant formulation, and others are protected by a separate barrier layer. GRP and EPDM are both used under green roofs when specified correctly, and the choice depends on the manufacturer's system. It is another reason to treat the whole build-up as a package. Our page on EPDM, GRP and felt explains the main coverings.

Slope, drainage and edges

Sedum is happiest on a modest slope. A roof that is dead level can hold water in the drainage layer and drown the plants, while a steep pitch can let the substrate slide. A flat roof with a proper fall, with the gradient explained under how much fall a deck needs, suits a sedum system well. Outlets need an inspection chamber or a gravel border, so that they remain clear of soil and roots.

An edge of gravel or paving round the perimeter keeps plants away from the trim and gives a safe path for maintenance. It also acts as a firebreak at the boundary, which some designs require.

Wind, fire and the neighbours

A planted roof must stay put in a gale. Mats and substrate are held down by their own weight, by edge restraints and sometimes by netting on exposed sites, and the perimeter strip of gravel stops the edge being scoured out. Dry sedum is not a fire risk in the way a thatched roof is, but dead leaf litter and cut grass left around outlets can be, and a gravel border is part of the design for that reason. If the roof overlooks a neighbour, tell them what is going on; a living roof can be a pleasant surprise for them, but scaffolding and deliveries are less welcome without warning.

Caring for it

Sedum is low-maintenance, not no-maintenance. It needs weeding for the first couple of years while it establishes, a check of outlets and gutters, and perhaps a feed. Dry spells in the first season need watering. Bare patches fill in over time, or can be patched with cuttings. Because the roof is no longer a simple weather surface, access needs a safe plan, and leaves and debris should still be cleared from drains.

Planning is usually not an issue for the plant layer itself, but a change that raises the roof visibly is worth mentioning to your council's planning team. We start by looking at your roof in a free survey, with photographs of the deck and the existing covering, then say honestly whether it can carry a green roof. If it can, the fixed written quote covers the whole system. If it cannot, we suggest an alternative. Read more on flat roofing in Basingstoke.