Making buildings: A new angle gets double roof extension past the planners

London’s new Wilde Aparthotel sits in a 1950s red-brick former warehouse on Middlesex St, once the site of Petticoat Lane market on the City fringes. On the edge of an ‘At-Risk’ Conservation Area, planning sensitivities meant that to extend two floors up, Nick Baker Architects had to think outside the box. NBA director Alyn O’Donnell and The Budgen Partnership structural engineer Ali Mohajeran discuss the design constraints and construction choices that resulted in its multi-faceted roof.

What was the project planning history?

Alyn O’Donnell: We’ve worked with the client on smaller residential projects for the last 12 years. In 2018 it approached us with a plan to convert this five-storey, 3100m² building into a 106-room aparthotel, for which it needed two extra storeys of hotel accommodation – 17 rooms – adding 500m² of internal area. 

While the building itself isn’t listed, its position on the north-west edge of the At-Risk Wentworth St Conservation Area meant planners were very sensitive to the building’s height and wanted to ensure any vertical extension was sympathetic to building and context. 

Before being bombed in WWII, the site had been a Victorian pub and we thought about butterfly and gambrell roofs when designing our two-storey extension. The planners opposed either brick or glass, leading us to develop our ‘folded arch’ proposal. This created a subservient profile to the existing building’s mass and broke up the roof form relative to other buildings in the area. Its double mansard gave extra surface area for dormers while setting back at an angle so there’s no defined roofline. It gained planning in February 2021.

What was the condition of the existing structure you were building onto?

AO’D: It’s a curious hybrid structure of steel columns and beams internally supporting 200mm thick pot and beam concrete floors, with the beams bearing out onto the loadbearing perimeter brick walls. The 15.5m ‘short’ side of the building has a column positioned mid-span  and these run across the ‘long’ face at 4.4m centres. 

Ali Mohajeran: The main challenge was to create the additional two floors plus roof for plant equipment without forcing a change to the existing foundations – the last thing we wanted to do was take up ground-bearing slabs and expose the foundations to underpin or reinforce the pad footings. We designed the underground drainage to avoid clashing with the existing foundations. It helped that the original building was designed for warehouse loadings of 5kN/m² and hotel loadings are markedly less than that at 2kN/m² plus allowance of 1kN/m² for lightweight partitions. The aim was to be as sustainable and economical as possible without compromising robustness and the required geometry.

Part of the gain was in reducing the dead load of the building. A 5th floor added in the 1970s was removed and the original fourth floor roof exposed and stripped back to its steel 406x140x39 UB beams. We sandwiched these between two new 356x171x57 UB steel I-beams to create the requisite rigidity for the loading of the new mansard structure, and then employed a metal web posi-joist system to replace the existing 5th floor and create the two new floors. At a dead load of 1kN/m², these weigh in at a fifth of the original concrete floors.

Why did you use steel for the primary structure rather than mass timber?

A O’D: We did think about a pre-fabricated mass timber system but we knew the building wasn’t true. Rather than being at right angles, the corner of the building was in fact about 85º and the structural columns we were bearing onto weren’t in alignment. We were concerned that facetted elements could arrive and might not fit on site as a result. This led us to think that the best way to deal with that was with a design approach that could allow on-site operatives to work around such inaccuracies.

AM: The main driver for us was that there was a lot of heavy plant hidden behind the roof-level mansard, some of it imposing a uniformly distributed load (UDL) of around 500kg/m² – which we felt couldn’t be supported by a panelised timber system. We thought the better option was to use a new primary steel-framed structure bearing off the existing with a secondary structure of timber rafters between acting as the carcass for the zinc cladding and forming the dormers.

Another reason for implementing steelwork was to provide enough vertical and horizontal ties to prevent disproportionate collapse. In addition to the adaptability capabilities of the timber rafters, they were more sustainable and readily available.

So why not steel for the carcass too?

A O’D: Again, it was about on-site adaptability among other things. Here we used 200mm rafters with stone wool insulation between, 30mm plasterboard on the inside and 75mm batons, 18mm ply and nominal 0.8mm zinc thickness. With a Metsec system not only is it non-adjustable on-site, but we would need to account for the thermal properties of the framing. We would have had to insulate on the outside face, increasing the 325mm section we ended up with – and eating into the net areas. We didn’t want panels turning up on site that wouldn’t fit. Timber was more forgiving.

How did you generate the complex form?

A O’D: It was more about creating complexity out of relatively simple geometries. Each 4.4m structural bay forms a room and every other steel column rakes at one of two angles, meaning horizontal connecting steels also rake in and out to meet them. Most new steelwork for beams, vertical central columns and raking perimeter columns was conventional 254x254 UC sections. Set on the existing mid-span columns, central columns to the vertical extension were the heaviest available at this section size – 132kg/m – to withstand forces at the steel-to-steel connections.

To articulate the timber faceted bay geometry, we experimented with 3D software but found old-fashioned parallel projection in AutoCad was the simplest and most accurate way to work out the profile of each timber rafter. Once these were established, we double-checked them in a 3D model. Bespoke corners at both ends were done in SketchUp. This way we were able to create the joiner’s cutting schedules.

It was procured as a JCT traditional contract with CDP so the construction programme’s critical path was key. We looked at different ways to fabricate the envelope but in the end, via the contractor, we went with a timber frame manufacturer based in Lithuania. We sent our 2D AutoCad files; using these and the steelworker’s shop drawings they developed their own drawings and connections. From this, they CNC-cut a kit of parts, numbering and referencing them. These were packaged and shipped over and were assembled on-site by a local joinery sub-contractor.

What were your construction timescales?

A O’D: Steel erection took three weeks. Installation of the posi-joist flooring followed, working level by level, at a rate of three to four weeks per level (5th floor, 6th floor and roof); then the timber frame was installed. This was produced in four sections, with the main rafters fabricated first, then the framing to the rear of the building, corner sections forming the terraces, and finally the timbers forming the dormers. This allowed production to start early for the main sections of framing, while fabrication drawings for the dormers were still being developed. 

Each section took 8-9 weeks from the start of the approvals process to completion of installation on site. The approval process for the subcontractor’s fabrication drawings took about three weeks and once these were signed off, the turnaround on the fabrication of each section of the timber frame was two weeks including shipping, followed by three to four weeks’ installation time.  

How did you consider the detailing of the zinc cladding?

A O’D: We wanted to use a cladding system that could be worked on-site and so opted for zinc, which allows for bespoke finishes, shapes and junctions. We initially wanted to use single-welt interlocking ‘flat-lock’ zinc tiles throughout. But working with the manufacturer Rheinzink, it was apparent that the minimum fall for zinc tiles was 10º, which meant the upper levels of the faceted roof needed to be standing seam zinc sheet, which can be installed at a minimum fall of 3º. The falls are partially in place to prevent water staining the pre-patinated graphite-grey finish we specified.

Drainage from the upper sections of the roof is detailed to run to a shallow gutter positioned between each bay. This ensures that rainwater is controlled to prevent water staining the finish. These gutters run down to a deep perimeter gutter that is located behind the loadbearing brick facade at 5th floor level. This collects water from the whole roof – including the dormers, which throw water either side onto the main roof, before it runs into the gutter.

What about the dormer window design?

A O’D: To create a sharp line all around, we wanted a wide but slim framing detail to the dormers, which took quite a lot of engineering. We settled on a steel fin detail running at 200mm centres either side of the timber section over the window and door glazing sets to allow the zinc to run around it to make the deep frame detail. It runs either side to distribute load evenly as we had to allow for both snow loadings and possible human load for access or maintenance, so it needed a counterbalance to prevent a bending moment ‘twisting’ the timber section above the double-glazed frames.

As above, so below?

A O’D: We wanted to express the geometries evident on the roof level at ground level too. Cladding the ground level facade in green glazed tiles by Darwin Terracotta not only picked up on the site’s past history as a pub (we chose green as it is the livery colour of the Truman brewery) but the bespoke faceting we designed referenced the complex geometries that are going on above.