Slate & Render
Ian Rock makes major decisions on the roof and render for his mega 100m2 extension project as the process begins to run smoothly
Even with a fairly tight budget of £1,200/m2 for our extension, we’ve managed to include one or two ‘ luxuries’, such as underfloor heating and super-wide bifold doors. We chose solid oak bifold doors from Vufold, having negotiated a small discount for ordering two sets.
Amazingly they arrived on site within a week of placing the order — in marked contrast to all the hassle and delay getting our main windows delivered. But a word of warning: bifold doors are surprisingly heavy even in kit form and are obviously a prime target for theft, as well as being vulnerable to damage on site. So for the time being they’re safely stacked away in the garage.
Better Slate than Never
Although the main roof was completed last month, the smaller single-storey ‘ lean-to’ has, until recently, remained a skeletal forest of naked rafters. Last week, Anthony, our roofer, returned to make the most of the fine weather and started to fit the breather membrane underlay and battens. 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. Our local supplier came up with a competitive unit price of 99p (plus VAT) for slates containing small amounts of pyrites (iron sulphide deposits potentially prone to oxidisation and rust staining). So we opted to pay 20p apiece extra for better quality ones.
The slates were delivered as promised the following week. There was however a potential problem — one of my own making. To get the ‘ look’ you really want sometimes entails taking a calculated risk. 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.
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.
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.
But which one to choose? One of the best known manufacturers is K-rend, so I phoned their help-desk and also spoke to suppliers of rival brands, such as Parex and Weber. We needed a smooth but sandy finish replicating the texture, colour and thickness of traditional render on the main house. But to make an accurate assessment you really need to see and touch samples, and on their arrival it soon became apparent that even the brightest shade of white wasn’t quite white enough. Furthermore, the texture bore little resemblance to the old sand/cement, plus the coatings would be about 5mm thinner.
The best texture match was the Parex ‘Monorex GF’, but Sod’s Law dictated that the only supplies available were via the parent company in France, incurring huge expense.
I also got the impression that, unless done expertly, monocouche render can be problematic, with a number of complaints posted online relating to colour and durability. So, I didn’t want to take the risk of not having a professional install.
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.
The Render Mix
Godfrey, our main contractor, drafted in an ‘old school’ tradesman by the name of Martin, ably assisted by James, our multi-talented brickie. Both had plenty of experience working as a team applying sand/cement renders. Because ordering the materials was down to me, I got Martin to pop by a few days in advance to run through the shopping list and showed him the sculpted ‘ hood’ effect we wanted above most of the window and door openings.
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. The mix for the base coat (scratch coat) was 1:5 and the thinner top coat 1:3, with a total thickness of about 20mm. We also needed various lengths of beading in rust-proof stainless steel — the galvan-
ised type designed for internal use will rapidly corrode outdoors.
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.) Meanwhile, we’d endured further delays with the delivery of our long-awaited French doors; and because a couple of windows salvaged from the main house couldn’t be fitted quite yet, some openings had to make do with temporary wooden templates.
Icing on the Cake
The following Monday, Martin turned 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. Fortunately one of Godfrey’s boys was at hand with a hefty trailer to collect a couple of bags at short notice from the local Jewsons. Meanwhile Martin rigged up an electric hoist ready for winching supplies of mix up the scaffolding.
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.
There is something deeply gratifying about seeing rough masonry walls transformed with greenygrey icing like a huge houseshaped birthday cake. While I was busy admiring the hand-trowelled finish it suddenly dawned on me that no gap had been left at the top of the eaves between the protruding rafter feet. This needed to be cut back to provide the required 1cm ventilation gap to allow air to flow through the roof structure. Old-school trades are hard to beat for experience, but are sometimes a little behind the curve on matters like ventilation and insulation.
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. 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. The masterplan was at last bearing fruit.
Next month: Building the flat roof