Self-Build Story

When I was a child, I grew up in a pub which had flat roofs. They were the old-style felt roofs which were quite common in the ’70s and ’80s. The pub, barely half a mile from where we are now building, was famous for having a huge collection of porcelain chamber pots hanging from the beams above the bar, and I remember them frequently being scattered across the floor to collect the drips which fell from the ceiling during particularly heavy rainfall.

For this reason I’ve never been a fan of flat roofs (which, owing to the need for drainage, are never actually ‘flat’ but rather very lightly sloping). So what do you do when your architect designs you a house with almost a dozen flat roofs, balconies and terraces, spread over five levels, all positioned directly above bedrooms and living areas?

With such a complex roof area to cover – and to alleviate my paranoia – we spent months researching and negotiating with some of the UK’s leading installers. Eventually we settled on a cold liquid-applied system, choosing to go with a Kemperol warm roof from waterproofing specialist Kemper System (kempersystem.co.uk).

Our architects advised us that, on paper, this appeared to be a superior solution and it required less insulation to achieve the same performance (150mm instead of 180mm). This meant it was also significantly cheaper — a saving of £10/m² against the inverted system we had initially considered. A real win-win situation as it was technically better and saved us around £3,000 on the project!

Kemper put us in touch with one of its most experienced approved installers, Neil Atherton of Castle Contractors Ltd (01772 861472), and after much consideration, we reached an agreement for approximately 300m² of roof.

ABOVE: 1. The new house, with its complex roof plan, begins to take shape. Note the unusual window openings; 2 & 3. Neil and Simon from Castle Contractors apply the polyester-based resin to the roof — it’s elastic enough to move with the building.

Neil’s first day on site didn’t go exactly to plan when the small winding country lane provided more problems for yet another articulated wagon. But a quick call to our ever-helpful local builders’ merchant (Roger W Jones: 01745 334402) and the wagon was rerouted. They agreed to store the 300m of 150mm-thick insulation board at no cost to us, delivering the materials as and when they were required — yet another reason for maintaining a good relationship with your local builders’ merchant.

Our main contractor Jim Evans (Arbour Developments: 07715 379793) had already prepared all the surfaces with a sand and cement screed, with falls to the drains which were to be dressed on the outside of the building with modern zinc hoppers and downspouts.

Neil brought along one of his most experienced roofers, Simon Cropper, and they got to work on the complicated system of roofs and terraces which he had anticipated would take around eight weeks to complete. If things ran to plan, they would finish the job just two days before the windows were due on site!

In principle, the build-up of layers which combine to seal the roof seemed quite straightforward. After drying off the surface with what looked like a giant blowtorch, the concrete deck was primed to receive a metal-lined self-adhesive vapour barrier. Neil assured us that once this was in place, the roof would already be watertight!

The insulation boards were then fixed to the vapour barrier with a polyurethane adhesive and the joints were taped and coated with primer. Finally the waterproofing membrane was applied onto the insulation — a cold liquid-applied process where the Kemp erol polyester reinforcement fleece was fully impregnated with the resin, bonding it to the substrate and creating a permanently elastic membrane which would, in theory, accommodate the stresses caused by thermal and structural movement of the roof.

Knowing how concerned I was from my childhood relationship with flat roofs, Neil doubled up the fleece which was saturated in a much thicker layer of waterproof resin than was needed. He also took the perimeter upstand to an average height of 250mm, which would stop our house filling with water if one of the roof outlets became blocked. In effect, what we now had was a network of roof areas similar to small, shallow self-draining swimming pools!

ABOVE: The ‘flat’ roofs have been designed to have a perimeter upstand of 250mm — effectively making them large, quick-draining shallow swimming pools. The intention is then to add timber decking to some of the roofs to make them useable (and make the most of the views)

We were told that because this membrane was very durable and root resistant, there would be no need for any other protection prior to the hard landscaping of iroko timber decking, washed pebbles and pavers. But with the windows, wood cladding, render and metal cappings yet to be in place, we were expecting heavy traffic over the coming months, and chose to protect the roofs further with a 3mm protective bitumen board.

Elsewhere on site, work was progressing at a steady rate as we strived to get the building watertight. The only non-flat roof on site, above the third floor guest bedroom, had already been finished with a stunning zinc roof. Despite choosing the more expensive matt-grey VM Zinc product over the glossier Rheinzink, NorthWest Metal Roofing Ltd (0151 228 6610) undercut all our previous quotes by a substantial margin.

The installation of the whole-house ventilation system, central vacuum, glass balconies and metal cappings are due to start any day, so Jim has begun landscaping his way down the slope so we can think about getting the heavy machinery off site. With the stonemasons, plasterers, joiner, electricians and plumbers also scheduled to be on site soon, things are hotting up for a hectic summer.

Flat Roofs: How to get the structure right

There are essentially three options when it comes to modern flat-roof design, known in the trade as ‘warm roofs’, ‘cold roofs’ and ‘inverted roofs’. All are somewhat misleading to the layman, although, in principle, there is a certain logic to them. In context, the roof of any building is susceptible to significant amounts of heat loss, which is why roof insulation is such a key consideration with modern building design, not to mention the Building Regulations.

When you insulate to prevent heat loss, the area immediately above the insulation becomes, you would hope, a pretty cold space, while the area below stays warm. Inevitably, any bridge between cold and warm air runs the risk of condensation — which occurs when the temperature of one surface (in this case the insulation) is cooler than the temperature of the air.

Quite simply, a cold roof is where the insulation is placed immediately above the ceiling but below the roof deck, with a vapour barrier (some form of impervious sheet) positioned between the ceiling and insulation. To prevent condensation, an air gap then has to be left between the insulation and the structural deck on which the waterproofing is applied — not ideal really, as it accepts that condensation will occur under the waterproof coating but provides a way of minimising it and preventing ingress to the house.

The other variation – an inverted roof – places the vapour control, waterproofing and insulation above the roof deck, but with the layer of insulation above the waterproofing. This means the insulation has no primary waterproofing protection and also requires relatively greater thickness of insulation to get the same Uvalue; it also limits the type of insulation that can be used.

Which is why we preferred the idea of a ‘warm roof’. As with the inverted roof, everything – vapour barrier, insulation and finally waterproofing – is placed above the roof deck, but in this case the waterproofing is above the insulation, meaning no risk of condensation on the underside of the deck, which then might somehow get into the building or cause hidden damage.

ABOVE: This plan shows the complexity of the roof design — meaning there is some 300m² of flat roofing to protect