Fitting the Flat Roof for my 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.

Ian’s main extension is now mostly complete so he start to focus on the second, smaller extension for the kitchen 

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 pre-cut them, which in our case Buildbase did for a small fee.

The roof structure includes two raised skylight parapets

Unfortunately torrential downpours delayed work for a week when Jason, our chippie, 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 brightened and work re-commence the following week

When I got home later 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 completed with the upper plywood deck secured in place using special 150mm long screws secured through the insulation into the OSB base deck below

Laying the Rubber

Ian 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.

Before the rubber is laid it’s essential that the deck is completely dry, with the temperature above 4°C and remaining frost free for at least 48 hours. 

Similar materials are used for highly durable pond linings and when Ian came up slightly short with out rubber roll, he managed to buy an extra half metre strip along with some more robust corner ‘gussets’ for added strength around the projecting skylight surrounds.

After the roof installation was complete it then took Colin and Jason the best part of the following day to complete the installation of Ian’s two sets of bifold doors

Knocking Down Walls

The next job involved knocking through the existing kitchen wall to open it up to the new flat-roofed rear extension. This required a steel beam to be inserted before demolishing the small masonry wall separating the kitchen window and French doors.

With the loadings now supported by the steel, the masonry wall below can be demolished and the props taken down

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. 


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