Putting a chimney or flue pipe through a Sedum green roof

What is a Sedum roof?

A Sedum roof is a type of green roof. The roof is designed to be able to take the extra weight of plants, growing medium and water and the plants grow on top of the roof requiring relatively little maintained. There are about 400 species of Sedums found in the Northern hemisphere. Sedums are a good alternative to grass when making a green roof for a variety of reasons. The plants are often hardier than grass and can survive with less growing medium - so the roof weighs less when wet. They are hardy and suited to the harsh conditions on a roof where there is little growing medium, low water, hot sunshine. A Sedum roof can look very nice indeed and is relatively easy to install.

Putting a flue pipe or chimney through a Sedum roof

Sedum roof are becoming ever more popular and many customers have come to us wanting to put a twin wall insulated flue pipe through their Sedum roof so that they can install a stove. This is relatively easily done.

We have a tried and tested method of putting a flue pipe or chimney through a Sedum roof.

Tall cone flashing under the Sedum roof membrane

This method is good for use when the Sedum roof has not yet been installed. Our tall cone flashing has an upstand of 350mm which easily clears the water retentive material layer, the growing medium, etc. Our tall cone flashing is installed under the water proof membrane and the membrane is brought up around the tall cone flashing. Thus the integrity of the membrane remains intact apart from a hole at the top of the tall cone flashing.
Here the membrane is tied back tight to the flashing, sealed to the sides with the appropriate sealant and a storm collar sits above to stop rain entering from above.

Tall cone flashing on top of the Sedum roof membrane - retrofit

It is also possible to install our tall cone flashing directly on top of the membrane and then seal the flashing to the membrane with a suitable adhesive. This is sometime necessary when the Sedum roof has already been installed and you want to put a chimney through it for a stove at a later date.

It is a good idea to make an upstand under the membrane to try to discourage water ingress. You can cut out shaped sections of material to fix in under the membrane to give this upstand. The sections need to be relatively small as you will have to put them in through the hole in the membrane that you have cut for the flue pipe.

The problem with this method is that you rely on the seal made by the adhesive to stop water coming in, rather than relying on the membrane itself.

Drainage around the flashing

In both cases the growing medium, etc around the flashing should be replaced with gravel to improve drainage and discourage plant growth right nextto the flashing and a short gravel channel made downhill to lead the water away from the flashing.

Tall cone flashing

Prices for the tall cone flashing are shown near the bottom of our Selkirk twin wall flue page.

For further technical advice, chimney and flue design and consultancy please contact us on 0845 226 5754

If you would like us to design your flue system then please go to the flue design service section of this site.