Old meets new from quarry to rooftop

• 
• 1st October 2008
• Quarrying
 

Providing traditional, high quality roofing slate for both refurbishment projects and new builds, Welsh Slate's Penrhyn and Ffestiniog quarries in North Wales have been operational since the early 18th century. What was once a commodity is now a highly prized building material and careful handling of the reserves is essential to ensure continuity of supply for a variety of projects.

In the redevelopment of heritage buildings, balancing the tricky partnership between preserving the character and history of the original building and bringing it up-to-date takes careful planning and an eye for detail.  Get it right, and the building not only becomes fit for purpose but maintains its architectural appeal.  Get it wrong, and it may well disappoint in terms of both function and form.  What's more, it only takes one corner to be cut, or one construction material to be specified out of place for that delicate balance to be upset.

The £36 million renewal project at London's St Martin-in-the-Fields complex, due for completion this year, is certainly a case in point.  The current church building dates back to 1726 and there has been a church on the site since 1222, so there was a clear need to protect St Martin's heritage. This is no ordinary church, however, known as the ‘Church of the Ever Open Door', St Martin-in-the-Fields has been using its crypt and Victorian burial chambers to offer help to vulnerable homeless people since the first world war.  As a centre of community outreach, the church provides invaluable support and advice for thousands of homeless people and the local Chinese community every year.  In addition to refurbishing the church, the renewal project aims to create purpose-designed spaces to help the work of The Connection at St Martin-in-the-Fields and the community resources to continue in flexible and inspirational surroundings.

"The old crypt and burial chambers provided a labyrinth of small rooms," explains Geoff Hunt from main contractor, Costain, "but these were impractical and pretty dilapidated.  The response from Eric Parry Architects, who designed the scheme, was to demolish the old vaults and replace them with a new two-storey basement to provide new kicthens for the crypt Café, music rehearsal rooms, a parish hall, a Chinese Centre (Ho Ming Wah) and a small chapel.  The North Range building, which houses the vicarage, church offices and The Connection at St Martin's, has also been refurbished and extended and, here, traditional materials have been used to recreate the original roof, preserving the character of the complex."

The traditional roof that has been installed on the North Range building extension belies the modern interior beneath and matches it perfectly with the roof on the original building, which has also been repaired as part of the roofing project.  Using ‘Heather Blue' slate tiles from Welsh Slate's Penrhyn Quarry and ‘Blue Grey' slates from the company's Ffestiniog Quarry, the roof has been built up on a metal frame using the existing trusses and close boarding.  This approach has allowed the roof to be insulated and provided a ventilation chamber behind the slates to create a diffusion zone.

Installed by roofing contractor, NDM, the roof has a mixture of Heather Blue and Blue Grey slates, an attention to detail that will please even the most sensitive of conservationists and ensured that the extension to the North Range building complied with planning consent.

The roof was specified in this way to ensure a like-for-like addition to the existing building. The way in which Welsh slate is produced for projects like St Martin-in-the-Fields reflects this dichotomy between the old and the new.  The four North Wales quarries still producing Welsh slate - Penrhyn, Blaenau Ffestiniog, Pen-yr-Orsedd and Cwt-y-Bugail - are now owned by Welsh Slate and the company uses a combination of cutting-edge modern technology and traditional skills to quarry and process the slate.

The two quarries that produced the slate for the St Martin-in-the-Fields project share many of the same processes but, geologically, from different periods.  Traditionally a slate mine, Ffestiniog is now being reworked as an open quarry and the product is taken from the old slate pillars that were previously left intact as roof supports.  Penrhyn, by contrast, is a more traditional open cast quarry, where Welsh Slate works the material in a progressive manner on a bench-by-bench basis.

"At both quarries we use low level blasting with black powder," explains Welsh Slate's production director, Mark Hodgkinson, "and because of the nature of the material managing the explosives is a very skilled process.  Welsh Slate has many natural geological faults and a natural breaking point so it has to be encouraged from the quarry face - too much force and the product would be damaged."

Once the slate has been quarried it undergoes a selection process at the quarry itself.  The material is visually sorted to select slate that is free of faults.  The quarry team also uses a ‘pecker', a piece of mobile plant not unlike a giant chisel, to gently ‘peck' the slate and check for faults hidden beneath the surface.  This not only ensures a more thorough selection process on site but also allows the team to reduce the slate to a thickness of 450 mm, preparing it for processing at the sawmill.

Both quarries have their own dedicated sawmills located less than 1 km from the quarry itself.  On arrival, the slate is cut into cuboid blocks using precision Van Vorden and Pedrini machinery with 1-1.2 m diameter diamond-tipped blades.  The length and width of these blocks is slightly oversized by around 5% to allow for the dressing process, later in production.

Once the slate has been machine-cut into blocks, it is palletised using vacuum lifts and transported across the saw mill site to the workshop where Welsh Slate's time-served team of craftsmen take on the job on hand splitting it into slates.

"In the hands of our craftsmen is where Welsh Slate's heritage as a traditional material is really defined," Mark continues. "This is a highly skilled job that has remained the same for hundreds of years and generations of Welsh slate craftsmen have been proud to handle the material that is so typical in Welsh buildings and so widely used elsewhere."

Welsh Slate employs 22 craftsmen at the larger Penrhyn Quarry and six at the Ffestiniog site.  This head count includes a number of apprentice craftsmen, learning a trade that takes years to master to ensure that the company can continue to offer high quality, hand split slate in the future.

"This has always been a skill that has been passed on from generation to generation," Mark adds, "and our more experienced craftsmen take pride in helping to train the youngsters, it's an important part of their job both for the team on site and for the company's commercial future."

Any slate that cannot be split naturally by hand - usually the lower grade material - is split using industrial saws on site. All the rivings - or undressed slates - are then palletised once again ready to be moved across site for dressing.  For standard products, this process is automated using equipment that chips away at the slates' edges to create the traditional ‘dressed' appearance.  However, for bespoke orders like the slates used at St Martin-in-the-Fields, the slates are individually dressed by the quarries' more experienced craftsmen.

"Once installed on a building, Welsh slate is extremely durable, strong and impermeable," Mark adds, "which is why it has stood the test of time as a roofing material.  During processing, however, it's an extremely delicate product to deal with and that's why we have to combine the most advanced modern equipment with the very best traditional skills."

Welsh Slate

Penryn Quarry
Bethseda
Bangor
Gwynedd
LL57 4YG
UK

T: 01248 600656