The Layered Roof
TV presenter and architectural designer Charlie Luxton explains why countless layers (and detail) have gone into creating his super-insulated, flat green roof
One of the biggest elements of any self-build is the roof. Getting it right is absolutely crucial. When you have a relatively simple piece of architecture like this house – a rear wall that sits into the hillside, a front façade and a roof is the sum of the architectural expression here – the roof is a massive part of the overall build and the design.
We used a pre-cast concrete system; on top of that there are multiple (seemingly endless) layers of material that go on in order to create our durable and well-insulated roof.
We started off with the concrete plank, and then used a primer on top. On top of the primer we installed a very thick torch-on vapour membrane, which is also a waterproof layer, to prevent moisture from the building rising into the insulation and condensating. A big part of the problems associated with early flat roofs, in the 1950s and 1960s, was condensation — so this is our solution.
Then we’ve laid on a Kingspan tapered insulation system. While the roof is flat, all the insulation is cut to fall, so the water runs back towards the outlets. That’s all done within the insulation layer, so you have to get somebody to survey the roof and then get the panels assembled for your roof — it’s quite a bespoke element.
On top of the insulation goes another layer of waterproofing, and on top of that is the most important element of what will be our green roof: the root-resistant layer. We’ve used Icopal Rootbar, which has a green hue because it has copper in it. That will stop the roots of the grass roof growing through the membrane, which, as you could imagine, would be catastrophic on a flat roof.
And there’s another four or five layers on top of this that are associated with the green aspect of the roof: fleeces, draining layers, gravel layers, soil and then the growing layer. Green roofs, while lovely and crucial to this piece of architecture, are time-consuming and complicated.
The roof ranges from about 150mm of insulation on one side to about 240250mm on the other — so it’s quite a fall. It means the roof has an average U value of 0.12W/M2K, which shows it’s a high-performing roof. It’s made a huge difference inside — you can feel the temperature change in the building and there’s been a reduction in condensation inside, too.
With the roof laid, we are fully weathertight — which means we’re ready to start inside.
Next month: The heating and ventilation systems
Complex Roof Structure
Charlie has devised a multilayer system that sits on top of a pre-cast concrete ‘plank’ for his green roof, including waterproofing, insulation and a root-resistant layer.