Flat roofs, terraces and balconies

7.1.5Flat roof, terrace and balcony structural design

Flat roofs, terraces and balconies shall support and transmit loads safely to the structure.

The structure of the flat roof, terrace or balcony should:

  • be produced by an engineer in accordance with Technical Requirement R5, and BS EN 1991-1-1, BS EN 1991-1-3, BS EN 1991-1-4 and BS 8579: 2020 for balconies
  • be designed to address both short term and long-term deflection to provide an effective drainage strategy with no back falls or ponding.
  • resist wind uplift by self-weight or by being anchored to the main structure – where required, holding down straps should be provided at maximum 2m centres at the perimeters.
  • have adequate provision for the additional loads where a flat roof is to act as a terrace, roof garden, for support of permanent service equipment, and for additional loads during construction.
  • have adequate provision for movement in larger roofs particularly where the span of the roof deck changes. e.g. in L-Shaped buildings; joints should be continuous through the vertical upstands, walls and edges of the building
  • include support steelwork and joists which are square, true and free from twists or sagging.
  • have adequate crack control/dimensional stability to avoid damage to directly applied AVCL and waterproofing layer, particularly liquid applied waterproofing.

Where joists and concrete roof elements are used to provide lateral restraint, they should:

  • have a minimum bearing of 90mm, or
  • have restraint straps at 2m centres (maximum) where joists or concrete beams are parallel to walls.