Flat roofs, terraces and balconies

7.1.7Concrete decks

Concrete flat roofs and balconies shall form a satisfactory substrate for the waterproofing systems. Issues to be considered include:

  1. structure and durability
  2. screeds to falls
  3. drying

Structure and durability

Concrete flat roofs should be constructed to ensure they achieve the required design, strength and durability, and be in accordance with BS EN 1992-1-1 and Chapter 3.1 ‘Concrete and its reinforcement’.

In-situ reinforced concrete decks should:

  • be formed using a mix which has low shrinkage characteristics
  • have accurately constructed and suitably supported formwork
  • have an even surface to receive the selected waterproofing layer (with adhesive bonded membranes the surface should be slightly roughened, wooden floated or lightly brushed, in accordance with the manufacturer’s recommendations).
  • be protected until adequately cured and dried
  • not contain additives that could affect the adhesion of any adhesive bonded waterproofing membranes


Liquids applied surface treatment onto the concrete to assist curing can adversely affect the bonding of the waterproofing layer. The compatibility of such liquids should be checked with the waterproofing layer manufacturer before use.

Precast concrete decks should:

  • have a minimum 90mm bearing (unless the design specifies „ a smaller dimension)
  • have allowance for continuity or anti-crack reinforcement to avoid differential movement between units and stress in waterproofing layer.
  • have allowance for movement approximately every 15m and at abutments
  • be installed to provide an even surface with no back falls
  • be grouted, as specified in the design.

Screeds to falls

Where falls have not been formed in the concrete deck, falls may be formed by the application of a screed. Sand/cement screeds used to form falls to concrete roofs should be:

  • free from ridges and indentations
  • laid on a concrete deck that has been suitably prepared to receive a screed
  • finished with a wooden float to provide a smooth, even surface for the air and vapour control layer and waterproofing finish.
  • to a minimum thickness in Table 7 where a cement/sand screed 1:4 (cement:sand) is used.
  • suitably dry and primed to receive the waterproofing system in accordance with the membrane manufacturer’s recommendations.

Lightweight screed should only be installed by specialist contractors and have a topping of 1:6 (cement:sand) 15mm thick.

Table 7: Minimum screed thicknesses

Location of screedNominal thickness (mm)
Bonded monolithically to in-situ or separate bonded to precast concrete140 (25 minimum)
Unbonded on separating layer70 (50 minimum)
Proprietary polymer modified screedsIn accordance with manufacturer’s recommendations


1     Precast units should be sufficiently rigid and properly grouted to avoid movement between units leading to cracking in the bonded screed. Movement joints/details
may be required over ends of precast units and at perimeters or abutments. The waterproofing layer detailing should take account of any movement joints.

2     Where a cast in situ concrete deck, designed and laid to falls, is found to have areas of ponding or back falls preparation work should be carried out to provide
effective drainage prior to the application of the waterproofing layer. Polymer modified screeds may be used to make up any depressions in the concrete roof to
avoid any ponding or correct any back falls. The adjusted areas should be resurveyed to ensure no ponding or back falls remain. The compatibility of the levelling
screed with the waterproofing layer should be confirmed by the waterproofing manufacturer.


Permanent waterproofing layer should not be installed until the deck has cured and dried sufficiently to avoid trapping potentially damaging moisture and to allow the application of the waterproofing in accordance with the membrane manufacturer’s recommendations. A check should be carried out on the moisture content of the slab to meet the membrane manufacturer’s recommendations. Where an adhesive bonded waterproofing layer is to be applied an adhesive bond test should be undertaken to check whether the concrete is sufficiently dry to achieve the correct bond, (usually around 28 days for fully bonded waterproofing or to manufacturer’s recommendations).


Permanent metal shuttering will significantly prolong the period needed to achieve an acceptable moisture content of the concrete before applying any waterproofing layer. Perforated shuttering can assist drying out, any perforation should be factory made. Mechanical extraction/dehumidifier can also assist in the drying process.