Although the main extension’s roof was completed last month, the smaller single-storey ‘lean-to’ has, until recently, remained a skeletal forest of naked rafters. Because of the relatively shallow slope, plain tiles like the ones used on the main roof aren’t an option since they require a minimum pitch of 35°.

If you rule out sheet metal, the choice narrows to either large interlocking tiles, natural slate or artificial slate.The former are rather bulky and can look a bit clumsy, and artificial slate can sometimes appear excessively shiny and uniform. Natural slate, on the other hand, is incredibly hardwearing and looks the business.

The king of slate is the legendary Welsh variety but unfortunately it comes with a fairly hefty price tag, so a good second choice can be Spanish slate. They were delivered as promised the following week.


The week before, Anthony, Ian’s roofer, returned to make the most of the fine weather and started to fit the breather membrane underlay and battens.

There was however a potential problem. I made the decision to prioritise proportions and aesthetics knowing that the minimum stipulated pitch for our slates was 22°.

The trouble is, any steeper than 20° meant that roof would abut the wall at a higher level, eating into the upper wall window space. I reasoned that since this was a very sheltered single-storey roof it was a risk worth taking. But as a precaution we increased the headlap (the amount the slates overlap each other) to a generous 125mm as an added barrier to water ingress.

Project Complexity

The main complexity in this part of the extension project, however, is what I call the ‘Mussolini balcony’ — a small inset flat roof to the front of the side gable that allows the French windows to sit lower.

This requires a thick covering of Code 5 lead sheet laid in twin sheets to accommodate expansion. Good quality leadwork is a joy to behold, but lead doesn’t come cheap — our 3m x 760mm roll cost £184 (and weighed a jaw-dropping 85kg).

For the main roof I’d sourced lead flashings at bargain prices online. But the downside of buying materials on the web became apparent when I needed to return surplus unused rolls incurring an astronomical £45 ‘re-stocking’ charge.

slate-roofing on extension

Ian’s chosen Spanish slate was a calculated risk but was completed quickly

Choosing the Render

The extension is designed to replicate the traditional white-painted render on the existing house so the next task was to start rendering the glaringly naked Fibolite blockwork walls, their naturally rough face providing a suitably ‘grippy’ key for the render.

The main downside with render as everyone knows is that it needs to be redecorated every seven or so years, which is why I’d looked into the manufactured, through-coloured ‘never paint again’ monocouche variety.

After investigating, I got the impression that, unless done expertly, monocouche render can be problematic, with a number of complaints posted online relating to colour and durability. 

Had we been building a new home from scratch I wouldn’t have hesitated to take the monocouche route. But because it’s essential to get a perfect match for the extension we decided to play safe and revert to traditional sand/cement. On the plus side this slashed the upfront cost of materials by nearly £1,000.

Preparing for the Work

Before rendering could commence, the remaining window and door frames first needed to be installed. To protect the frames from being engulfed in render they needed to be taped, with the glazed casements and doors remaining safely stored away for the time being.

When it comes to fitting window frames, builders sometimes have a tendency to use brackets cut from spare metal straps kicking about the site, which strikes me as a bit amateurish. It also risks forming a ‘coldbridge’ across the newly fitted cavity closers.

A better option is to use countersunk concrete screws of the type used to secure bifolds. (On the subject of bifolds, fitting the massive rectangular frames is a major task best carried out by skilled chippies because it’s essential that the unwieldy frames are fitted perfectly square and are securely fixed, especially the top rail which carries most of the door loadings.)

exterior of extension with white render

Deciding against a monocouche render, Ian opted for the more traditional sand/cement-based render. The scratch coat consisted of a 1:5 (sand/cement) mix, with a top coat of 1:3 for a 20mm finish.

The required mix for rendering was 50/50 soft ‘Leighton Buzzard’ fine, sharp sand and standard building sand, with a smattering of plastic fibres for added strength and Martin, our tradesman recommended by Godfrey, our main contractor, was up at the crack of dawn to make a start on the base coat.

Delving into one of the jumbo bags of sand, his face fell. It turned out it wasn’t the special super-fine variety I’d ordered but standard builders’ sand which we already had copious supplies of. Buildbase apologised for the mistake but needed two days’ notice to come and replace them.

applying render to extension exterior

Ian specified that Martin rendered a ‘hood’ effect over the window and door 

By the end of the week most of the rendering was complete, in large measure thanks to being blessed with perfect weather — dry but not excessively hot to cause shrinkage, and not too cold to risk frost damage.

To keep costs down, decoration isn’t included in the contract. So having consulted Martin on render drying times, and with the scaffolding still in place, I took advantage of the fine weather and spent the weekend applying two coats of white Sandtex masonry paint over a primer of stabilising solution to the upper walls.

exterior of extension with scaffolding

“The masterplan was at last bearing fruit,” says Ian

There is something deeply gratifying about seeing rough masonry walls transformed with greeny-grey icing like a huge house-shaped birthday cake. That evening as the sun set, I stood back in the garden drinking in the view. For the first time the extension was starting to look like an integral part of the house. 

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