Flat roofs used to be a right pain in the bum, and neither are they especially cherished over here.
I remember interviewing an estate agent in Darlington who told how an estate of 1930's semis was locally, and disparagingly, known as "Little Lebanon" because of the Mediterranean style flat roofs. (It certainly wouldn't be because of the climate there, that's for sure. Or the amazing Lebanese food, because there was none, sadly.)
When we bought our flat-roofed house it rather oddly came with planning permission to add a copper pitched roof (the place may as well have been designed to be gold-plated too, given commodities prices these days), partly I imagine because British folk are generally not spirit-of-Bauhaus enthusiasts.
Aesthetics aside there is performance to consider: visiting the Ennis House, designed by Frank Lloyd-Wright, for the first time on one rainy Los Angeles night, the Mayan-inspired, flat-roofed splendour was accompanied by a timpani of rain cascading from various points in the ceiling into saucepans below. Not quite what Frank had in mind, I'm sure.
Our own flat roof hasn't leaked since we have been in residence but you can tell it's got previous, so time to renew. And as our flat roof is so prominent, given it's site on the slope of a hill, we want to prettify it with an extensive green roof, the difference between intensive and extensive being that intensive is usually a blanket of plants like sedum and intensive means a roof garden.
It would be so helpful if we could all use plain language to start with for these things.
The same frustration applies to simply renewing a roof where things, for a moment, got rather unnecessarily complicated.
We have what's called a "cold roof": what this means is insulation is fitted below the roof deck, the plywood surface at the top of the roof but below the water-proofing. This is the most common and well-established type of roof construction.
The caveat with this arrangement is that it must be well ventilated, with vents set into soffits (the underside of the overhang of a roof), as any warm air trapped inside will react with the underside of the cold deck to create condensation.
These days sustainability champions champion "warm roofs": these are roofs where the insulation is fitted above the roof deck but below the waterproofing, meaning that the entire roof construction is all toasty, and also means that any moisture hitting this toastiness will not turn into condensation. It is argued that these roofs are the most efficient kind in terms of insulation and best suited to our Northern European climate.
Another difference between the two is several thousand pounds, the warm being the pricier option.
I only discovered this when I interrogated the costs roofing contractors were submitting, all of them having automatically quoted for a warm roof and all of them going on about firring pieces (wooden wedges) and needing an architect to redesign the roof structure and so on and so on - because of course if you fit a new layer on top of a flat roof it buggers up the falls, so you need to compensate by calculating new falls and new firring pieces, which in turn can bugger up your structural calculations for the weight of a saturated green roof!
I wondered whether the automatic price for warm had arisen as a result of our intention to install our sedum blanket but no, you can fit an extensive green roof over either a warm or cold roof below.
Chatting to one very experienced supplier about cold v. warm he advised that cold roofs can be just as efficient as warm so long as they are well-installed and well-ventialted, all complying with building regulations and how, in fact, some warm roofs can be inferior to cold if the installation is a bit iffy, which is not uncommon.
A final note on this subject I am probably going to go for an EPDM system over a PVC one, so while I might lose green points on the warm v cold debate I gain by using a system that emits less VOCs (volatile organic compounds), lasts longer, has a lower embodied energy factor (uses less energy from material extraction through to manufacture) and can be recycled with ease.