The new Merthyr base for the National Assembly for Wales is one of three new offices built in a scheme which is to decentralise the Assembly from Cardiff.

The three-storey building provides 7,600m2 of floorspace to house 450 new employees, which are intended to be sourced locally and the building is seen to be a key part of a local regeneration programme.

The 25° pitched roof was designed using a number of faceted hip and valley intersections in order to create a dramatic curved effect to the elevation which overlooks Merthyr Tydfil.

The specified Penrhyn natural Welsh slates (sized 500 x 300) were laid over Kingspan's KS1000TS insulated composite tile support system, which was in turn fixed to a steel portal frame.

The Project

As we were already technically proficient in the use of the Kingspan KS1000TS from many previous contracts, our experience was utilised by the Architect at the pre-contract stage as we were invited to review and (if necessary) revise all roof details in order to ensure an efficient process and achieve a technically correct roof.

Thanks to this experience, we were also well aware of the inherent logistics and handling issues of the KS1000TS system. However, the 12.55m roofslope of this design presented the longest roof which we had ever been required to tackle. Previously, due to the weight of the product, we had found that any lengths in excess of 6m were too heavy and unwieldy to accurately position on site - accurate positioning being vital in achieving a roof compliant

with thermal and air leakage requirements. Therefore, initial design considerations revolved around laying the panel system in two tiers, with a sealed joint between. Despite the time and cost constraints of this method, this was felt to be the only way forward especially bearing in mind the accurate cutting required to the eight hip and eight valley lines, over 12m and 8m long respectively.

The chance sighting of a trade magazine advertisement led to the research of an Oktopus crane lifting attachment, in order to combat the difficulties of handling long roof sheets. The Oktopus KT-B attachment uses vacuum powered suction frames to hold the roof panel (obviously any systems involving ropes or straps would fail as they would be trapped when the panel is fitting into place) and can accommodate sheets between 4 and 20m in length. The precise pitch control provided by this system meant that single sheet lengths could be accurately laid and a demonstration at our premises confirmed this.

The cladding work was scheduled for 25 days and was completed in 15. Due to our efforts and success on this project, Kingspan has incorporated this system into their recommended fixing method for KS1000TS.

Once the Kingspan KS1000TS panels had been installed, the small matter of battening and laying over 70,000 slates remained. The high profile of the project had led to the specification of Penrhyn natural Welsh slates and the workmanship of this finish was required to reflect this.

The sorting and grading of the slates was carried out at our premises, resulting in numerous pallets of "thicks", "mediums" and "thins" - all in accordance with the recommendations of BS 8000-6: 1990 to ensure that thicker slates are laid in lower courses and thinner slates in upper courses. Whilst sorting, a check was also made on the thickness of the slates from tail

to head, so that if there is a slight taper in the slate length, the thinner end is used as the head in order to position the nail holes. If this process is not carried out, it is impossible to produce a satisfactory finish however skilled the roofer is. The client actually visited our premises in order to view this grading process.

As our sorting and grading process is so effective and efficient, Spanish Slate Quarries recently filmed us in order to produce an advisory DVD highlighting the deficiencies in many roofing contractors' preparations for natural slate roofing and the correct method which should be undertaken.

The faceted hips which were used within the roof design to create the curve effect, required a close-mitred slate finish as hip cappings would have detracted from the required aesthetic impression. The fact that BS8000-6 1990 recommends a minimum roof pitch of 30° in sheltered locations when adopting a natural slate mitred hip detail (this roof was designed at 25°) was highlighted to the designer/specifier who maintained the detail as the Kingspan KS1000TS provided secondary weather protection.

In order to ensure that the cut slates remained in place in such an exposed location, a screw and washer was installed in each course to supplement the nail fixings. Due to the lengths of the hip lines and the viewpoints provided by the topography at the front of the building, it was vital that perfectly straight hips were achieved, as any deviation would be obvious and visible, even from ground level.