Carved in stone
By Carolyn Willitts
Designing creatively with stone requires more than just a great construction detail; understanding the material is key to achieving a fantastic end result.
The new entrance forecourt to the award-winning refurbishment of the Grade II* listed Liverpool Central Library, by Austin-Smith:Lord, was designed to open up the space to the street, create a wow factor and signify the transformation of the library.
Adrian Hazelwood, the landscape architect who designed the concept as part of a competitive bid to secure the work, says, ‘We were looking for something bold to set us apart... The library is in the middle of a line of very grand, large-scale buildings, so creating a new entrance that was both clearly visible but didn’t detract from the architecture, was quite a challenge’.
Two key elements achieved this: the literary carpet that names many books, films and music titles that can be found in the library; and the wall, which provides strong visual signage and connection to the street frontage. Both elements were constructed from granite and a great deal of time was spent working with the supplier, Hardscape, which is well- known for creating bespoke art features from stone. As the landscape architect responsible for the landscape design through planning to construction detailing, I worked closely with Anthony Collins from Hardscape so that we could push the technical properties of the stone to achieve our concept, from the initial choice of materials to the detailed design and the creation of a tight specification.
Chris Moor, the landscape architect who worked on the project during construction, explained that most of the complications on site were to do with the setting out and how to get all the separate elements to line up perfectly. The paving had to be designed so that the joints lined up with movement joints every 6m in the concrete slab below. Because of this the coursing of the granite in the literary carpet had to line up with the coursing of the surrounding sandstone. It also had to line up with the stone steps, the stone seating, the stone tactile paving, the slot drains, the lighting columns and the lengths of the LED in-ground strip lighting that ran along the edges of the literary carpet. In a different situation, without the need for movement joints, the paving coursing and step joints could have been staggered which would have allowed for much more tolerance on site.
I met up with Anthony at the library twelve months after the defects liability period, and it looks great. He said he was most pleased with how the individual elements come together to create a complete environment. It is one that both adults and children enjoy, and this is, after all, why we go through this complex process when we design.
The granite wall
The granite wall was the most complicated item to achieve technically, consisting of polished Kobra granite cladding pieces with large hand-carved letters in relief. The granite was polished right up to the edge of the letters, which were sandblasted, with a rounded edge.
The cladding was 40mm thick and had a 50mm recessed shadow gap. The letters were 700mm high and projected out by 40mm. The 100mm thick coping incorporated anti-skate profiling which projected vertically above the finished stone level.
We consulted an ex-skateboarding champion when designing the anti-skate measures that the client had requested for the library. He recommended that we only used defence measures where there was genuinely a risk of stone being damaged, rather than using a scattergun approach and firing stainless steel balls at everything in sight. The wall was the perfect height for skateboarders and BMX bikes, and some defence was required to the top of the wall on both sides. A projection of 5mm above finished stone level is enough to stop a skateboard, so we used the letters themselves on the entrance side and hand-carved projections in the stone on the other. This meant that our defensive measures against damage were an integral and positive part of the function and form of the wall.
The literary carpet
We chose a dark and a light granite to create the literary carpet. Because they are the same stone type, they have the same properties which ensures the entire surface wears down at an equal rate over time. The text was water-jet cut out of both the Royal White and the Crystal Black granite. The cut letters were pocketed 25mm deep into the 80mm thick flags. Joints around the letters were 1mm maximum and filled with resin on the underside and grouted on the face of the slab. Tolerances were tight around the letters, as well as in the flags themselves as the coursing had to line up perfectly with the courses of the surrounding York stone.
Hardscape developed a set of rules to work with when designing the layout of the text within each individual flag, for example no waterjet cutting within 20mm of the edge. There were some very special cases, however, where we agreed to have a letter within 10mm from the edge of the slab to suit the paving joint, for example between the ‘o’ and the ‘p’ in the word ‘Philosopher’s’ in ‘Harry Potter and the Philosopher’s Stone’.
A stepped approach was required within the frontage to cope with the level changes, as William Brown Street has a steep slope of 1:15. Dark granite nosings, cut from stone in China, were fitted to the sawn York stone step units in the quarry in Leeds, to ensure a neat finish and joints that lined up before delivery to site.
The York stone for the steps and the surrounding paving was Woodkirk sandstone from the Woodkirk Quarry in Leeds. It was chosen because the grey-buff colour worked well with the existing stone of the library, while also complimenting the contemporary mid and dark grey granites.
The stone seats
The seating was introduced into the design, following discussions with the access officer, to reduce any risk that could come with tapered steps being a trip hazard. Coordination was required between the stone supplier, the granite quarry in China, and the sandstone quarry in Leeds, to ensure that the tops of the seats would fit perfectly on to the base pieces. The York stone cladding was 75mm thick, and the end pieces were 300mm thick to give the benches a solid appearance.
The bench tops were single pieces of Crystal Black granite 2400mm x 800mm x 150mm. Hardscape had to reserve good quality large pieces of granite from the quarry well in advance because it is not usually recommended to use single pieces of dark granite that are so large, as the dark stone comes out of
the ground in smaller pieces than lighter coloured granites.
Tips for designing with stone
• Anthony Collins from Hardscape says ‘Remember that stone is a natural material that comes out of the ground and has natural imperfections. This is what makes it such a beautiful material to work with’.
• Granite is a dense stone, but edges can be vulnerable. A subtle chamfered edge can prevent pieces getting chipped on site.
• Different quarries have different skills. Check with your supplier that the granite you want comes from a quarry that has the skills you need.
• Dark granites come out of the ground in smaller pieces than lighter granites, which should be taken into consideration when designing. This is why the darker the granite, the more expensive it gets.
• Make the most of your supplier’s expertise; they have the knowledge to know whether what you want can be done. If they don’t know, they can speak to the quarry.
• Each material has its pluses and minuses. Limestone can have up to 75% wastage due to the size and shape of the block and how it fits on the saw, which is why it is more expensive in flag size than sett size. This understanding of materials is key
• Why use natural stone in generic concrete sizes? There’s no need. If you can wait 6–8 weeks for it, granite can come in almost any size.
Tips for creating text in stone
• Respect the kerning (letter spacing) of
the font. Don’t move your letters to fit the joints, move the joints to fit the letters.
• If you are using a non-standard font (we used Proxima Nova) you need to purchase it, as does the supplier who is creating the stone cutting drawings, as does the client who needs a licence in order to use it. Your colleagues need it on their computers too, just in case one day they amend the drawing, pdf it and issue it for construction without realising that the text has reverted to Arial. Remember to check issued drawings for the correct font!
• For waterjet cutting, agree the minimum gap you can design between letters, and between the edge of a letter and the edge of the piece of stone with your stone supplier.
• Print some of your paving out at 1:1 and walk up and down it in the office. Then get your client to walk up and down it too.
It uses up a lot of paper but everyone can agree the sizes of the font and the paving.
• Triple check your spelling and get others to check it too. Even after many checks an accent was missed off a letter in a Spanish word in the literary carpet and had to be added two days before opening, because the word meant something unsavoury without it...
Carolyn Willitts used to work for Austin-Smith:Lord and is now director of Carolyn Willitts Design, based in Manchester.