Roof Design

Profiled metal deck

typically consists of galvanised steel, coated steel or aluminium that is factory formed into a trapezoidal profile. Forming the profile provides the specific span and load capabilities of the deck that provide a sound base for the insulation and waterproofing system. To avoid reduced drainage performance the mid-span deflection of the metal deck should not exceed 1/200 of the span under uniformly distributed loads. The choice of thickness, profile, acoustic perforation and finish of the decking will be dependent on the required span, roof construction, imposed dead and live loading and any aesthetic consideration such as a decorative exposed underside.

Typical material specifications include: Galvanised Steel; minimum recommended thickness 0.7mm to BS EN 10147:1992 Fe E280G Z275. Gauge range 0.7-1.2mm.  Coated Galvanised Steel; minimum recommended thickness 0.7mm to BS EN 10147:1992 Fe E220G Z275. Gauge range 0.7 – 1.2mm. Plain Aluminium; minimum recommended thickness 0.9mm to BS EN 485-2:1994 AA3004 H34 and BS EN 485-2:1994 AA3103 H16. Gauge range 0.9-1.2mm. Special fasteners may be required where the waterproofing system is mechanically fixed to the deck. Coloured Aluminium; Minimum recommended thickness 0.9mm to BS EN 485-2:1994 AA3004 H34 and BS EN 485-2:1994 AA3105 H25. Gauge range 0.9-1.2mm. Special fasteners may be required in mechanically fastened applications.

Structural concrete decks

can be classified as either reinforced (cast in-situ), precast, pre-stressed or lightweight aerated. Each will have a different effect upon cost, contract period and performance. Cast-in-situ concrete deck is a site poured concrete slab incorporating reinforcing steel interspersed transversely and longitudinally throughout the slab, typically this type of concrete will require a minimum of 28 days to cure prior to roofing works commencing.  Precast and Pre-stressed concrete decks consist of panels made offsite and craned into place, if screeded the screed will require a drying time, if not screeded the roofing system can be installed immediately. Lightweight concrete can be defined as a type of concrete which includes an expanding agent in it that increases the volume of the mixture while reducing the dead weight.

Timber decking

is generally classified into pre-formed sheets and timber boarding. Pre-fromed sheets dominate the new construction market whilst timber boarding is commonly found in re-roofing situations. When buying either product type the timber should be responsibly sourced. Orientated Strand Board (OSB) is a wood panel composed of timber strands orientated in cross directional layers and bonded together. Only grade 3 OSB manufactured to BS EN300 and certificated by the British Board of Agrément or BRE Global should be used in roofing, with a minimum recommended thickness is 18mm. Plywood should conform to BS EN 1995-1-1 Eurocode 5 Design of timber structures and BS EN 636 Plywood specifications Service class 2 ‘humid conditions’. For more demanding application Service class 3 ‘Exterior conditions’ should be used. In all cases minimum thickness should be 18mm. New timber boarding should be a minimum 25mm thick and, ideally be tongued and grooved to maintain its resistance to deflection after natural shrinkage. Woodwool is a rigid timber and cement based medium weight panel, which may be reinforced with steel edge channel for longer spans. Woodwool cement slabs should conform to BS EN 13168 and be a minimum thickness of 50mm.

Composite decks

comprise a polyisocyanurate (PIR) insulation core factory adhered onto a profiled steel liner sheet with a variety of top sheet options: Insulated panel with a membrane top sheet; waterproof membrane factory-applied. Insulated panel supplied with a 50micron foil top sheet; waterproof membrane mechanically fastened on site. Insulated panel supplied with a polyester/cellulose mat reinforced with glass fibre; waterproof membrane adhered on site. Insulated panel with plain galvanised steel top sheet; waterproof membrane mechanically fastened or adhered on site. Insulated panel with plastisol/polyester coated steel top sheet; waterproof membrane mechanically fastened or adhered on site. Structural Insulated Panels Systems (SIPS) generally comprise a Pre-fabricated insulated core of polyisocyanurate (PIR) or Polystyrene (EPS) sandwiched between two skins of plywood (BS EN 636 Service Class 2 minimum) or Orientated Strand Board OSB3to BS EN 300.