Green Roofs – Choosing the right metal deck and tray
With a focus on the use of structural decks and trays on green roofs, we highlight some of the main design considerations and deck attributes to look out for during the product selection process: –
- Decking systems in England & Wales must be designed and installed to meet the Building Regulations 2000 and Approved Document L2A 2010 (Scotland to be designed and installed to meet the Scottish Building Regulations 2007).
- Where used as a vapour control layer, the metal deck must be reasonably airtight so that the air permeability does not exceed 5m³/h/m² at an applied pressure of 50pa in accordance with the Building Regulations 2000, Approved Document L2A 2010.
- Deck profiles must achieve minimum Class C non-fragile assembly when tested to ACR(M) 001:2005 Test for Fragility of Roofing Assemblies.
- Fixing fasteners can be either carbon steel or austenitic stainless steel (Stainless steel fasteners required for aluminum deck and for fixing into timber supports)
- Fire rating of roofs is not generally applicable. However, for surface spread of flame, decks will achieve Class O to the Building Regulations, Class 1 to BS476: Part 7. Metal deck profiles are non combustible to BS 476 Part 4.
- Corus (Tata Steel) produce a range of nine trapezoidal decks from 35 mm deep to 210 mm deep, and three structural trays from 90 mm deep to 145 mm deep, in steel or aluminum, in various gauges and finishes.
- Decks and trays may be perforated to enhance acoustic absorption, assisting with reduction of reverberation time, thus minimizing the echoy effect within the room environment.
- Decks provide lateral restraint to support structure, or may provide full diaphragm, or stressed skin, design, which will minimize bracing structure.
- Decks must be designed to support all relevant loads, designed to British Standards or Eurocodes:-
(a) Dead loads: Calculate to BS 6399-1
(b) Imposed loads: Calculate to BS 6399-3
(c) Snow loads
(d) Diaphragm Loads: BS5950 Part 9
(e) Wind loads: Calculate to BS 6399-2
(f) Basic wind speed (Vb):
(g) Altitude factor (Sa):
(h) Direction factor (Sd):
(i) Seasonal factor (Ss):
(j) Probability factor (Sp):
(k) Terrain and building factor (Sb):
(l) Size effect factor (Ca):
(m) External pressure coefficients (Cpe):
(n) Internal pressure coefficients (Cpi):
(o) Dominant opening:
BS EN 1991 Eurocode 1: Basis of Design and Actions on Structures
Part 1 Basis of design
Part 2-1 Densities, self weight and imposed loads
Part 2-2 Actions on structures exposed to fire
Part 2-3 Snow loads
Part 2-4 Wind loads
BS EN 1993 Eurocode 3: Design of steel structures
Part 1-3 Cold formed thin gauge members and sheeting
If you require any guidance on any aspect of structural roof deck and trays, such as choosing the right deck for the right application, or simply require more advice on any of the above points, please contact the technical team on 0845 30 88 330