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Architect : MUMA

Slate Consultant : Viv Stratton

Roofing Subcontractor : Forrester Roofing

 

Newlyn   Art Gallery was designed by local architect James Hicks and opened in 1895. It occupies an enviable position at the end of a public promenade overlooking Mounts Bay. MUMA’s refurbishment has created a new education room and entrance in a new building at the rear facing the sea. The new extension is clad in Trevillett slate hung in the traditional Cornish manner. A  ‘ scantle ’  was used to set out the battens and slates to achieve a continuously diminishing coursing without any banding. A scantle is a length of timber, usually one of the roofing battens, marked up as a gauge to locate each course.

 

Concrete   ground beams and a concrete floor slab sit on pile foundations. The first floor is also concrete with a perimeter ring beam supported on four internal circular concrete columns that allow the ground floor shop and café  to be surrounded by continuous silicone jointed glass. Above the first floor the structure is steel with timber studwork infill. The highly insulated walls have an inner and outer leaf of studwork with a 175 mm services zone in between. Both roof and walls are clad in slate and the coursing has been carefully set out to ensure the courses on the gable end line through with the courses on the roof.

 

 

 

The   slates have been wet-laid with a minimum triple lap (3½; gauge) on the walls and a quadruple lap (4½; gauge) on the roof to resist the severe Atlantic weather. Stainless steel screws have been used to secure the slates rather than the traditional oak pegs. Hydraulic lime mortar in a 1:2 lime:sand ratio was used without any cement, which might have caused efflorescence and staining of slates. The mortar was laid in a horseshoe-shaped bed to form voids so water cannot be drawn up into the roof by capillary action.

 

 

Integrating   the roof and walls into a single material idea gives the extension a scale appropriate to its civic function. From the first floor education space, a 9-metre wide strip window gives a sweeping view of sea and sky, a reminder of the qualities that attracted artists to this part of the world.

 

     

 

 

Drawing Labels :

 

 

 

 

1.     Structural frame    

Steel frame above first floor level with 203 x 203 mm x 46 kg UC (universal column) primary members. 300 mm thick reinforced fair-faced concrete first floor slab with 725 x 575  mm downstand perimeter ring beam. Four 300 mm diameter in-situ fair-faced concrete columns supporting first floor.

 

 

2.     Typical wall

Wet  ‘ scantle ’  laid random width Trevillett slate laid in diminishing courses. 25 x 50 mm horizontal tanalised softwood battens spaced to suit slate coursing and fixed to vertical battens with 25 mm stainless steel nails. 50 x 50 mm vertical tanalised softwood battens at 400 mm centres coinciding with studs behind to provide continuous ventilation gap. Breather membrane. 18 mm WBP plywood sheathing. 100 x 50 mm tanalised softwood studs at 400 mm centres with 100 mm mineral wool insulation between. 175 mm services zone with mineral wool packed in voids. 75 x 50 mm tanalised softwood studs at 600 mm centres with 75 mm mineral wool insulation between. Vapour barrier. Two layers 12.5 mm plasterboard with skim coat and paint finish.

 

 

 

 

3.     Slate cladding    

Wet  ‘ scantle ’  laid random width Trevillett slate laid in diminishing courses. Slate to walls laid at 3.5 scantle and slates to roof laid at 4.5 scantle. Slates fixed with stainless steel screws through pre-drilled holes, two holes for wall slates and one hole for roof slates. Hydraulic lime mortar laid in horseshoe pattern between slate courses.

 

 

4.     Corner junction

Code 4 lead soakers extending full height of each slate and 225 mm either side of corner. Slate edges to be mitred at corner.

 

 

5.     Roof

Wet  ‘ scantle ’  laid random width Trevillett slate laid in diminishing courses. 25 x 50 mm horizontal tanalised softwood battens spaced to suit slate coursing and fixed to vertical battens with stainless steel screws. 50 x 50 mm vertical tanalised softwood battens at 400 mm centres coinciding with studs behind to provide continuous ventilation gap. Breather membrane. 18 mm WBP plywood sheathing. 60 mm mineral wool insulation fitted over rafters. 200 x 50 mm tanalised softwood rafters at 400 mm centres.  200 mm mineral wool between rafters. 65 mm phenolic foam-backed plasterboard with integral vapour barrier taped and skimmed with paint finish.

 

 

 

 

6.     Roof eaves    

203 x 203  mm  x 46 kg UC steel eaves beam. 75 x 50 mm tanalised softwood upstand built off top of wall to support gutter. Code 4 lead flashing fixed to plywood below breather membrane and lapped over gutter lining. Black insect mesh to cover ventilation voids.

 

 

7.     Gutter

Fleece-backed reinforced PVC membrane gutter lining. 175 x 60 mm box gutter made from 18  mm WBP plywood. Continuous EPDM membrane to external face of gutter.  Zinc alloy facing (coated on the underside with a protective layer) folded over external face and ends of gutter with drip to line through with slate coursing.

 

 

 

 

8.     Window

Prefabricated box frame welded up from 550 mm wide x 12 mm thick mild steel plate fixed to steel structure at jambs. 18 mm WBP plywood external sheathing on tanalised softwood sub frame. Continuous EPDM membrane to external faces of projecting sheathing lapped behind breather membrane behind slates.  Folded zinc surround to front face of window. Window frame made from 60  x 60 x 4  mm stainless steel angles. Window casement made from 50 x 50 x 3 mm stainless steel box sections.  Stepped double-glazed sealed unit silcone - bonded to frame consisting of 6 mm toughened outer pane, 12 mm air gap, 6.4 mm laminated inner pane with low-E coating. Removable zinc-clad WBP plywood panels to head, cill and jamb external reveals. 12 mm thick Corian internal cill with 20 mm thick leading edge.

 

 

9.     Rooflight

120 x 120 x 10 mm RSA (rolled steel angle) welded to 203 x 203 mm x 46 kg UC steel ridge beam. Tanalised softwood carcassing to form 9000 x 900 mm rooflight opening. Stainless steel rooflight frame. Double-glazed sealed unit consisting of 10 mm self-cleaning toughened outer pane, 16 mm air gap, 12.8 mm laminated inner pane with low-E coating. Electric roller blind.

 

 

Cut-away section through west corner showing roof and wall construction

 

 

10.     Ridge flashing

Zinc alloy sheet (coated on the underside with a protective layer) flashed over heads of slates with mastic seal in between and held down with zinc clips at 400 mm centres. Code 4 lead soaker behind zinc and slate junction. Continuous EPDM waterproof membrane beneath zinc lapped over soaker. 25 mm thick tanalised softwood square-edged board deck at 5 degree pitch. 50 x 50 mm tanalised softwood firings at 400 mm centres to provide continuous ventilation gap. Breather membrane. 18 mm WBP plywood sheathing. Roof construction as 5.