THE BUILD IT EDUCATION HOUSE: PART 7
Some complex design details and minute tolerances throw up a few challenges on site – but the Build It Education House roof is now on! Chris Bates updates us on progress
Chris Bates gives the latest update on our home building project at Graven Hill, as the SIPS roof is installed
Right: A chippy created templates for the three gable ends; the most complex of which (shown in the foreground) is for the angled wall at the side of the house. Opposite page: The ICF crew’s last job on site was to pour concrete into the gables. While it’s more complicated than a standard pour, the three sections were completed in just a few hours
We’re now entering an exciting phase of the Build It Education House project. Once you’ve topped out your new home with the roof structure, you’re basically in the dry (minus the windows and doors). So not only can you see the full form of the building you’ve put months of work into designing and realising on site; but you’re tantalisingly close to being able to start the fit-out works inside and out. Here’s what was involved in getting our roof installed.
Why did we specify SIPS?
Quite simply, structural insulated panels felt like a natural fit for our project. We’d already chosen a modern insulating concrete formwork (ICF) walling system, attracted by its energy efficiency qualities and the fact that it works well for basements (find out more about this construction method on page 90). So we wanted to complement that with a similarly high-performance roof.
A factory-made product, SIPS panels can be used for the whole shell of a house – but we’re reserving them for our roof. They are quick to build with and can deliver excellent insulation and airtightness performance in a relatively thin profile. That latter point was what tipped the scales for us over more conventional attic trusses. We want to create a conversion-ready roof void to help future-proof the finished house; but the rather tight 8.1m ridge height available under our Plot Passport means that we need to eke out every spare millimetre of head height. So this thinprofile, pre-insulated roofing system made a lot of sense.
Our chosen panel supplier, SIPCO, was tasked with translating the architect’s drawings into a workable roof plan, via their own structural engineer. It was at this point we discovered the triple-aspect roof was going to be a bit more complex than we’d imagined. That’s partly because of the amount of open-plan space we need on the ground floor and basement. As a result, SIPCO’S design includes a number of load bearing steels and glulam beams – along with a structural attic floor (rather than the normal first floor ceiling). The latter includes special kerto laminated veneered lumber (LVL) plates to which interlocking fillets will be fixed to prevent the main roof from spreading.
By this point, the steels were already in place and our first floor walls were built, so the main pre-roofing works were to install the attic floor, pour the three gables and fix the wall plates. This work was shared between the ICF builders and our main contractor, Scott Hunt from Drewett & Hunt, who has been much more heavily involved now that the house is approaching weathertight.
Pouring the gables
When using a pre-manufactured system like SIPS, you’re dealing with very tight tolerances. So tight, in fact, that the ICF crew were asked to get the building within 2mm of perfect for the wall plates and gable ends (the triangular bits of a wall that go up into the roof apex), to ensure the SIPS panel installation would go smoothly.
There was quite a bit of to-ing and fro-ing involved in co-ordinating this. Most builders would probably say 2mm over a whole building is a pretty tall order; so when you’re getting to this kind of accuracy on your own dream home, the buck will ultimately stop with you. We’ve experienced that common self build situation where different suppliers require exact tolerances from others; but don’t necessarily deliver on those themselves. That can lead to finger-pointing when things don’t work out perfectly. Communication up front is clearly the key.
In the end, we agreed to finish the first floor concrete pour about 20mm low at the eaves. This would allow us to put in a mortar bed and laser-level the wall plates to get them as close to dead-on as humanly possible. With the gables, we decided the best approach was to get one of Drewett & Hunt’s regular chippies on site to draw up templates for the ICF installers to work from.
The trickiest part would be shaping the gable on the angled external wall, as the intersection with the roof pitch creates a compound angle along the wall thickness. But something got lost in translation between trades here, and unfortunately we didn’t spot it until we were already manoeuvring the SIPS panels onto the roof, well after the concrete had been poured (more on this later).
Pouring gables is a slightly more complex operation than for standard storey-height ICF walls. Wet concrete won’t just sit at an angle without something to hold it back; so the crew used a slightly drier mix combined with small sections of plywood as shuttering, screwed into the top