Verulamium Hypocaust Building, St Albans

In the 1920s a well-preserved mosaic ﬂoor was uncovered during excavations of the Roman city of Verulamium in modern day St Albans. Playing ﬁ elds were laid over the archaeological area to protect it and the mosaic was left exposed in a drab utilitarian brick building. In 1999 muf won a competition to replace it with a new structure.

Disturbance of the ground has been minimised by using 24 micro-piles, threaded perforated steel tubes 3 cm in diameter which are drilled into the earth and pumped with grout. The galvanised steel frame is ﬁ xed to the piles via circular concrete pile caps. Gabions ﬁlled with ﬂints form a continuous wall retaining the earth around the building’s perimeter.

Photo credit: Jason Lowe

Photo credit: Muf

Architect: muf architecture/art
Consulting Engineer: Atelier One

The inner wall leaf has a framework of Masonite beams with sheathing on either face that braces the steel structure. Glass-ﬁ bre reinforced concrete (GRC) rainscreen cladding panels are suspended from the Masonite structure on stainless steel angles. A standard concrete panel would be 100 mm thick but the GRC panels only need to be 30 mm with ribs for extra strength, so are much lighter. GRC is extremely durable making it ideal for a building in a public park.

Eighty-six 1.6 m wide panels were pre-fabricated in different shapes to follow the slope of the ground. Finely crushed oyster shells were used as aggregate in the concrete, a common practice in ancient Rome, and more shells were rolled into the surface before the concrete had cured in the mould. The shells give the exterior a pearlescent shine and provide a rough surface to deter climbing and grafﬁ ti. Shaped rubber forms were placed in the concrete moulds to make the rosette windows.

Photo credit: Jason Lowe

Plan with section and area of detail

shown in red, scale 1:300

At either end the roof kicks up to allow light in through a glazed clerestory strip. The glazing is frameless with a single laminated pane spanning from wall to roof and the roof provides no structural restraint to the top of the wall. On the elevation facing the town there is no steel in the wall so the timber structure acts as a cantilever with a full moment connection to the steel beam at its base which makes it rigid. Mirrored acrylic is ﬁ xed to the sofﬁt on the approach side to give glimpses of the mosaic to approaching visitors and allow people inside an inverted reﬂ ection of the town.

1. Roof

Sedum planting in 80 mm soil.Drainage mat.80 mm extruded polystyrene insulation. Bituminous felt waterproof membrane. 18 mm WBP plywood roof deck.200 x 50 mm softwood roof joists on galvanised hangers.Steel frame primary roof structure. Ceiling void.2 layers plasterboard suspended ceiling. Acrylic mirror glued to underside of plasterboard.

2. Roof ridge

PPC aluminium fascia.Polished stainless steel mirrored sofﬁ

3. High level windows

Steel channel at head ﬁ xed to timber joists to allow for +/-5 mm roof deﬂ ection.Single laminated glazing in 1200 mm widths. Aluminium channel section at bottom ﬁ xed to timber structure.

4. Top of wall

Aluminium ﬂ ashing dressed down into gutter. Aluminium gutter.

5. External wall

Glass ﬁ bre reinforced concrete (GFRC) rainscreen cladding panels with oyster shell inlays and rosette-shaped cut outs.Stainless steel shelf angles with dowels to locate cladding panels bolted to timber frame. Nominal 75 mm ventilation gap.Breather membrane.18 mm OSB external sheathing.200 mm Masonite beam structural framework to inner leaf ﬁxed to steel frame. 200 mm blown cellulose ﬁ bre insulation. 12 mm plywood inner sheathing.25 mm softwood battens.12 mm birch faced plywood lining.

6. Windows

Rosette cut-out in external cladding panel. External steel framed double glazed inward opening window.Internal frameless single glazed opening window held on patch plates. Glass fritted from transparent behind rosette openings to opaque behind solid wall.Birch faced plywood window lining.

7. Walkway

Precast concrete slabs bolted to steelwork. Steel balustrade uprights made from 2 x 20 mm thick ﬂ ats bolted to 20 mm steel plate bracket below through notches in precast slabs.60 mm diameter stainless steel handrail.

8. External wall/ground junction130 x 4 mm stainless steel plate fascia skirting. Galvanised steel perimeter cavity closer. Geotextile drainage membrane.50 mm rigid polystyrene insulation board.Flint ﬁ lled gabion retaining wall.Soil reinforcement around perimeter.

9. Foundations

450 mm diameter concrete pile caps. Micropiles injected with grout.

10. FloorOriginal Roman mosaic ﬂoor on tile pillar hypocaust.