The Flat Roof

Chartered surveyor Ian Rock reaches the halfway point of his extension project and it’s time to put the flat roof on his smaller kitchen extension

Construction of our extension is now approaching the halfway stage. The second phase, the internal work, will be carried out by directly employing specialist trades combined with some DIY input. In the meantime, the two main outstanding jobs are finishing the flat roof to the small rear kitchen extension and installing the rainwater fittings.

Building the Flat Roof

One of the key design considerations with flat roofs is, paradoxically, the degree to which they slope. A decent fall helps prevent water ‘ponding’ and premature deterioration.

But what angle exactly? The Building Regs require a minimum slope of 1:80 but recommend building steeper at 1:40.

This is normally achieved by laying wedge-shaped strips of timber (‘firrings’) on top of the ceiling joists. Although sometimes cut on site, to achieve consistent quality it’s preferable to have the supplier precut them, which in our case Buildbase did for a small fee.

But I still had a nagging doubt about one aspect of the roof design. With 3.6m-wide bifold doors taking up almost the entire rear wall, I couldn’t help wondering whether the relatively slim IG lintel the structural engineers had specified to span across the open-

ing was up to the job of supporting all the roof joists. Similar designs I’ve seen employ hefty steel or oak beams. So to be on the safe side I phoned Anne who’d done our design calculations and she reassured me that the lintel was well within the recommended tolerances for the supported loadings.

Chippie Jason polished off building the flat roof structure, including the two raised skylight parapets. Then

the heavens opened, with torrential downpours delaying work until the following Saturday when he re-appeared to fit the

18mm OSB (oriented strand board) deck over the joists in defiance of the rain. This really didn’t bode well for the next key stage: building the insulated warm roof above the deck — a job that requires perfectly dry conditions. This takes the form of a ‘sandwich’ comprising a vapour barrier membrane laid over the deck, sheets of 100mm-thick Kingspan PIR insulation placed over it, and a top layer of 9mm plywood decking, similar to SIPs (structural insulated panels). Fortunately, the forecast for the week ahead predicted a lengthy spell of benign weather, so the following Monday work re-commenced.

When I got back home later that afternoon I was keen to check on progress and shimmied up to the roof where chippies Colin and Jason had just finished cutting a giant jigsaw of insulation boards.

But I noticed something was missing; they’d forgotten to lay the green polythene vapour barrier over the deck. Fortunately the insulation hadn’t yet been fixed in place, and it was a simple task to temporarily lift the boards and install the membrane. Disaster averted. The sandwich was then completed with the upper plywood deck secured in place using special 150mm long screws secured through the insulation into the OSB base deck below.

It then took Colin and Jason the best part of the following day to complete the installation of our two sets of bifold doors. This is a demanding task, not helped by manufacturer’s generic instruction leaflets that seem to bear little resemblance to the actual product. But it was great to see the bifolds in place, literally closing the door on cold gales howling through the embryonic extension.

Laying the Rubber

I’d selected artificial rubber (EPDM) for the flat roof covering because, as a single seamless roll, it’s hard wearing and leak-free, if laid properly. Placing the order with Rubber4Roofs was straightforward enough; we provided them with scale drawings, dimensions and photos, then a few days later a huge roll of rubber sheeting arrived together with a selection of adhesives, plastic upstand strips, and some lengths of PVCu fascia. The only additional requirement was a short strip of lead flashing to cover the junction where the roof abutted a wall.

Before the rubber is laid it’s essential that the deck is com-

pletely dry, with the temperature above 4°C and remaining frost free for at least 48 hours. The next day flat roof specialist Kevin and his team set to work unfurling the giant roll of rubber. The job progressed as planned although the rubber sheet turned out to be a shade shorter than was ideally required to form a generous lap under the tiles on the adjoining roof slope.

So with the crockery safely stashed out of harm’s way, the builders swung into action.

Having first supported the ceiling with Acro props, the

builders started hacking off the plaster, before cutting out chunks of masonry to accommodate engineering brick padstones at either end.

Unlike the super-sized steels we’d employed elsewhere in the build, this one was a comparatively docile beast amenable to manoeuvring by hand. Once installed, to get the beam positioned perfectly level the end bearings were carefully packed with slate offcuts. With the loadings now supported by the steel, the masonry wall below could be demolished and the props taken down. All that remained was to transform the sea of dust and rubble back into a pristine kitchen.