There comes a point in any project when the niceties of the design process collide with the harsh realities of the construction world. The crunch duly arrived when, with mounting dismay, I realised that rumours of dire materials shortages were proving well founded. With Thermalite blocks about as rare as moon dust, we needed to draw up a contingency plan quickly.
The trouble is you can’t just substitute any old block. A key part of complying with Building Regs involves meeting thermal performance targets to minimise heat leakage, while also ensuring the walls are strong enough to support the building.
A chance discussion with a structural engineer threw up a possible alternative in the form of a brand of block called Plasmor Fibolite. Unfortunately I wasn’t the only person in the world to have figured this out; most of the available supplies had been ravaged by panic buying.
There was nothing for it but to get on the blower to the manufacturers. As far as I could make out, stock availability was largely dependent on a combination of luck and geography. Plasmor’s regional office responsible for supplying our part of the world (Buckinghamshire) reported zero stocks for the near future.
Then a stroke of luck. It so happened that I’d recently opened a trade account with Buildbase in Oxford who, as it turned out, were supplied by Plasmor’s Cheshire regional office. I picked up the phone to them and, wonder of wonders, a full artic load was scheduled to arrive in a fortnight’s time.
Pulling out all the stops, the local sales manager printed out 25 ‘reserved’ stickers with our account number emblazoned on them. As a welcome bonus, the blocks could be drawn down as and when we needed them rather than being dolloped onto the garden in one enormous mountain. Things were looking up.
Tweaking the Design
We managed to negotiate a reasonably competitive price for the quantity of blocks required (around £1 per block), a decent result bearing in mind that the suppliers held all the cards.
Changing a key component at this point in the proceedings risks making a mockery of finely nuanced design calculations, prompting a return to the drawing board. We knew that Fibolites were fine from a load-bearing perspective, only needing to be standard 3.6N, as opposed to the stronger 7N. Although not quite as good in terms of heat loss performance as pure thermal blocks, they still scored a respectable K value of 0.24W/m°C (compared to around 0.15W/m°C for the best thermal blocks).
Upgrading the cavity wall insulation from the original glass mineral wool to 50mm-thick Kingspan phenolic foam more than compensated, giving us a final U value of 0.25W/m2K (including external render and internal dry lining), comfortably exceeding the 0.28W/m2K target. The online Concrete Block Association U value calculator proved helpful in refining our calculations which were duly copied to building control.
Working to a Schedule
If there’s one thing that can scupper an otherwise successful build, it’s failure to coordinate deliveries of materials with labour on site. So I needed to prioritise items with longer lead times to synchronise with the programme of works.
If there is one component above all which has a sullied reputation for monumental delays it’s windows and glazing. To minimise delivery delays it helps if your design uses standard sizes and styles. But it’s still advisable to place your order at least six weeks in advance.
Ordering the Suspended Concrete Floor
Next on the list for early ordering was the suspended concrete ground floor structure. The beams have to be custom-made, so I’d emailed a set of plans a couple of weeks prior to local firm TT Concrete. Like many manufacturers, they provide a complimentary design service, and within a few days I’d received detailed drawings from them showing the dimensions and layout of all the floor beams (more complex than you might imagine).
As well as negotiating a competitive price (a shade over £1,000 + VAT for the full 67m2 internal ground floor) and confirming delivery times (up to three weeks), it was very useful to speak with chief estimator Ian Lee to check details like the number of air vents, the minimum permissible depth of void under the floor (150mm) and the simplest way to deal with the oversite ground underneath the floor (a tough weed control membrane covered with granular material).
Once the deal was done, the beam and block drawings were copied over to building control. As well as various lengths of beams, the price includes cranked ‘periscope vents’ and a pack of thin slip bricks for infilling between the vents. However we didn’t want cheap-looking plastic air bricks and set about independently sourcing traditional terracotta ones.
The following week Steve from Buildbase dropped by again, this time with a car boot laden with brick samples. You can’t beat seeing bricks in the flesh before you buy. Although the extension walls are of white rendered blockwork, the contrasting red brick detailing evokes a traditional farmhouse vernacular, lending a pleasing sense of lightness to the overall design.
After mulling the options over for a while we plumped for traditional plain red ‘multis’, a relatively soft brick, for the decorative arches and corbels. For below the damp-proof course area, between the ground and the bottom of the render, we selected Northcot multi-red rustics; although not an exact match, you’d do well to spot any meaningful difference once they’d been laid. A much tougher brick was needed for the foundation walls below ground level, a role for which Class B ‘semi-engineering’ bricks are perfectly suited.
I was pleased to learn from Steve that it was okay to order exact numbers, rather than aiming for the nearest multiple pack size, and, should we over-order, Buildbase could accept returns (subject to a handling fee). But the shortages hadn’t entirely gone away. The less urgent Class A engineering bricks and full-width 7N trenchblocks specified by the structural engineers to build columns supporting the huge steel beam over the living room were impossible to find.
We’d also hoped to source some 300mm-wide trenchblocks for the lower foundations to avoid the hassle of later having to backfill subterranean cavities. But the shelves were bare. And there was the question of finding suitable blocks to infill the floor beams. Then, out of the blue, Steve announced that they were expecting an imminent delivery of Thermalite Hi-Strength 7N blocks.
Would we like some? I nearly bit his hand off.