Tanking and studwork in the North Wing

Ric finished rendering the bottom metre of the north wing before he headed home and as soon as it dried out, Jil painted on the primer and by the end of the following day we had got the whole lot tanked. We did prep on the foundation between north and west wings so we can get some on the studwork up and temporarily clad it with OSB to keep rain and wind out. Jill peeled back the DPM, we trowelled on a bed of mortar, then DPC on top, then taped the DPM back up over it. The following day we tanked it, for good measure.

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We carried on with the studwork in the north wing, finishing the north wall in a day and building the studwork across the foundation the day after. We arranged the studs either side of rooflights so that the studs were the other sides of the trusses from the rooflight.

We have yet to go back and rework the three rooflights we added in to earlier studwork, where one stud is nailed up in the way of what will be the rooflight reveal. We will join a timber to the truss side and run it u to the truss, then cut out the original at the right height to be below the reveal.

The only other real peculiarity was immediately to the left of the front door, where we unwisely placed our incoming water supply and a foul drain right where we need to build studwork. We carved lumps out of the timber to work round it, but will have to work out how to prevent them showing once we have boarded up. This leaves about 1 days-worth of work on the south wall to get that out of the way, on our next rainy day.

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Tanking and studwork in the north wing

Ric has rendered some of the north wing and, as we were still waiting for rooflights to turn up, we followed him, tanking sections of wall and then building studwork over that. It is non-structural, so we had one baseplate, ran studs up to our rafters and nailed them down. We put uprights either side of openings, a lintel across the top and short studs from the lintel up to the rafters. We ran out of wood about a third of the way along the length, so ordered another 100 lengths of 145×47 timber.

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Tanking the east wing walls

We are working to get our east wing habitable for next winter, and we need to get our timber studwork built around the external wall. Which means we must get the wall tanked up to 1m, over the render that Ric had applied last year. We need to tank across the floor by 1.5m as well, but that will wait until we need to build the floor up and screed it. We are using the same stuff that we lined our window sills with – we paint a primer on the render, wait for it to dry out, then use tanking membrane that more-or-less sticks to the primer. The membrane comes in 1mx20m rolls and is surprisingly heavy – over 30kg. It comprises an outer polythene skin and a rubbery, tacky backing. We used it last year under the studwork that holds some of the east wing roof trusses up, it has been well trodden ever since, but seems tough as old boots. We also used it to line our window sills.

We did the tanking in two stages. We primed a 400mm strip round the edge of the concrete slab and 100mm up the wall, then cut lengths of membrane in half lengthways into 500mm strips. We followed the advice to leave the primer 1-2 hours, but found by experience that that was not long enough, overnight was much better. Probably because the weather has been cool-ish. Anyway, we folded the tanking strips to get a neat fit right into the corners of the wall & floor, then laid the tanking onto the primer. We used a hot air gun on a medium setting to get good contact in key areas where we thought it might not stick properly.

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The next stage was to prime up the walls, wait for that to go off, then apply full-width lengths of tanking over the 100m upstand from the first stage. The membrane sticks very well over other membrane, so we had to get it right first time! The membrane mostly stuck well enough to the primer, but we knew from last year that gravity would do its thing and that eventually it could well simply fall away from the wall. We fixed that by screwing a 30mmx4mm PVC beading along the top edge. We turned the top edge of the tanking over on itself under the beading and used stainless steel screws into wall plugs. A couple of times we needed to loosen the ends of pieces of beading and discovered that the membrane bonded to it very effectively – I had to use a screwdriver the lever them apart. Most of the work involved simple straight runs, the tricky bits were getting good results in the external door opening & our big window reveal, and lapping it onto our previously-tanked window sills. So it took a day and a half and we were glad to see the back of it – the primer is stuffed with VOCs and the membrane was heavy and unwieldy. We found out that it really needs to be done in dry weather. Any dampness really reduces the sticking power of the membrane onto the primer.

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Our first home-grown window sill!

Getting our granite lintels delivered was quite a turning point for us: We cannot get our roof trusses installed until we rebuild our walls; we cannot rebuild walls without getting windows and doors formed; we cannot form windows and doors without sills; we could not make sills until the lintels arrived. Our building warrant had specified concrete sills – this did not appeal to us at all. Ric convinced us that we could convert lintels into sills without too much effort & cost. So as soon as we got the lintels, Ric got to work. He used the bathroom window, in the east wall of the east wing, to test out his ideas…

1.  He cut a 1.6mx212mmx100mm lintel to the width of the window + 50mm.
2.  The sill has to slope forwards to shed rainwater, Ric used the angle grinder to trim the front face to be vertical once the sill is in place. He used the bush hammer on the sawn front and ends to get a suitably rustic roughened finish.
3. He used a simple wooden jig to guide him using the angle grinder to score irregular lumps on  the base to a level and used the medium breaker with a chisel tip to clean it up.
4. He trimmed the front of the underside to be horizontal once the sill is in place, then cut a groove along the length, as a drip-line.
5. I installed tanking membrane over the base of the window opening. This was laid on a single coat of primer and run down the inside of the wall by a few cm. We need to tank the lower metre of the wall (and 1.5m of floor) before we build the timberwork to hold the wall insulation, we will run the tanking up and overlap onto the window tanking, to make a continuous barrier to water.
6. I used a 6mm double-sticky-sided foam strip to create a cushioned base to lay the sill on. We laid five strips down the sill area, doubled up to 12mm.
7. We placed the sill over the adhesive strips and adjusted it to be horizontal along the length. It had a sensible slope downwards.
8. I cut three lengths of 25mm x 1mm galvanised steel strapping, drilled holes in the back of the sill and into the top of the concrete behind the sill, then screwed the straps in, to stop the sill being able to slide forwards.

And that was it. When we get round to it, we will squirt foam under the sill to seal it and spread the loading on the sticky foam seals – although I was rather surprised that it really did not compress a great deal under the load. The window frame will lap over the back of the sill by a few cm, we have plenty of space behind to run insulation and the window ledge under the rest of the window frame.