Green roofs on commercial buildings

1What sits on top of the waterproofing

A sedum roof is built up in layers. The waterproof membrane stays at the bottom. Above it go a root barrier, so plant roots cannot work into the joints, then a drainage layer that holds a little water and lets the rest escape, a filter fabric, a thin growing medium, and the planting itself. Sedum, a low succulent, is the usual choice because it copes with thin soil and dry spells.

The roof is not a lawn on a shed. Each layer has a job, and leaving one out is how green roofs end up leaking or waterlogged. The membrane below must be one the system supplier accepts, and the supplier will usually want to see how it is joined, as a failed joint under a finished green roof is costly to reach.

This is why many green roofs go on with a new membrane rather than over an old one. If you want to understand the lower layers first, our guide to commercial flat roof refurbishment explains what is under there and when it is time to renew it.

2Can the structure carry it?

Weight is the first question, because wet growing medium is far heavier than dry. Approved Document A treats any increase above 15% in the load a roof carries as a significant change. Where the structure cannot cope, strengthening or replacing it counts as a material alteration. The Planning Portal puts its own threshold for heavier coverings at 15 per cent or more. Either way, a green roof is the kind of change that calls for a structural check and a conversation with building control.

Only an engineer who has seen how the roof is really built can give you that answer. Older roofs, timber decks and light steel sheeting often have little spare capacity. Concrete decks usually have more, but even they have to be looked at where the load lands.

It also affects what you plant. A thin sedum blanket is much lighter than a deeper planted roof with shrubs, and the heavier option may need support through beams and columns, not just a stronger deck.

3Falls, outlets and where the water goes

Green roofs hold water, but they still need to drain. The deck below should still be laid to falls so that water not taken up by the planting runs to outlets rather than ponding on the membrane. IKO, citing the 2018 edition of BS 6229 (since revised), gives a recommended minimum fall of 1:80, with the roof designed to 1:40 to allow for tolerances and deflection, and that logic applies under planting as much as anywhere.

Outlets and gutters are the weak point. They need to stay clear of soil and roots, so they are normally fitted with inspection chambers or a clear stone border around them, which lets you see and reach each one. A blocked outlet under a green roof can fill the whole build-up before anyone notices the water.

Edges matter as well. Parapets, upstands and flashings are reached through the planting, so ask how they will be kept visible and accessible. Our page on overcladding a metal roof deals with a different roof type, but the same principle applies: whatever you add, the original edges and drainage points must still work.

4Keeping it alive and keeping it sound

Planted roofs are not maintenance free. Sedum needs a look every so often for weeds, bare patches and seeds from nearby trees, plus the outlets cleared of debris. In a dry season it may need water until it establishes. On a seafront or exposed site, ask the supplier how the planting mix copes with sea weather.

Access is part of the design. A green roof that nobody can reach safely will be neglected. Edge protection, a fixed ladder or hatch, and a firm path of paving or stone for walking mean inspections happen without trampling the planting or putting a foot through a fragile section.

Fire detailing is worth a question. Dry planting burns, so ask the supplier what the system specifies at roof edges and around openings, and what your building insurer expects. If you plan solar panels on the same roof, see solar panels on a commercial roof before settling the layout, since the two can share a roof but need planning together.

5Planning, permission and who does the work

A green roof can involve several parties: a roofer for the waterproofing, a green roof supplier whose system and warranty apply, possibly an engineer and sometimes the landlord or a planning officer. Ask for the supplier's installation guidance up front, so the roofer is working to it.

Whether permission is needed depends on the building and its location. A commercial roof, a listed building or a conservation area all change the answer, so check with the Adur & Worthing planning team before work starts. Building Regulations may also apply because of the added load.

For a hall or place of worship, the consent routes differ again; see church and village hall roof repairs. If the building stays open during the work, read roof work while the business stays open as well. More on commercial roofs generally is on the commercial roofing page.

Our written quote arrives once the survey is finished. Whoever prices a green roof, the figure should make clear what the planting system includes and which waterproofing the roofer is supplying.