Flat roofs, terraces and balconies

Also see:

7.1.10Thermal insulation and vapour control

Thermal insulation, vapour control and ventilation shall ensure satisfactory performance, and prevent the formation of condensation which could adversely affect the construction. Issues to be considered include:

  1. thermal characteristics of roof types
  2. types of insulation
  3. control of water vapour.

Thermal characteristics of roof types

Uninsulated roof – The temperature of the deck is at or close to that of the interior or exterior of the building whichever is the lesser.

Cold roof – The temperature of the deck is at or close to that of the external climate. Cold roof designs should be limited to roofs where it can be shown that effective cross ventilation can be provided to reduce the risk of interstitial condensation

Warm roof – The temperature of the deck is at or close to that of the building interior.

Inverted warm roof – The temperature of the deck is at or close to that of the building interior. The WFRL should be designed
and installed to collect and drain most of the rainwater to reduce water entering and cooling the insulation layer.

Condensation analysis should be calculated using an external temperature of -5C in accordance with BS 6229.

Types of insulations

Types of insulations and their use should be in accordance with Table 8.

Insulation materialAbbreviationBS EN StandardApplications Warm roofApplications Inverted warm roof ¹
Expanded polystyrene boardEPS13163yes ²no ³
Extruded polystyrene boardXPS13164yes ²yes
Rigid polyurethane foam boardPUR, or PU,13165yesno
Phenolic foam boardPF13166yesno
Polyisocyanurate boardPIR
Cellular glass boardCG13167yesno ³
Expanded perlite boardEPB13169yesno
Cork boardICB13170yesno
Fibreboard, bitumen-impregnated (used in conjunction with EPS with hot applied waterproofing)yesno
Composite boards or decks (e.g. Plywood + PIR)yesno
Resin bonded mineral wool rigid boardMW13162yesno
Mineral wool quilts ⁴MW13162nono
Vacuum insulation panels ⁵VIPyesyes


1 Thermal insulation used in inverted roofs should be designed in accordance with ETAG 031-1

2 Suitably protected when using hot applied waterproofing

3 Unless the product has an independent third-party assessment for use in inverted warm roofs

4 Not suitable for warm or inverted warm roofs and only used in cold roofs

5 Suitably assessed in accordance with Technical Requirement R3.

Insulation for warm roofs should be:

  • either: a) bonded or mechanically fixed to resist wind uplift in accordance with the manufacturer’s recommendations; where mechanically fixed, it should be installed using fixings of sufficient length to ensure adequate penetra- tion into the supporting structure, or b) part of a loose laid roof system which is ballasted to resist wind uplift
  • kept dry and installed in quantities which can be quickly covered if it rains (to aid bonding and to avoid trapping moisture)
  • dimensionally stable at working temperatures (e.g.100°C under metal roofs)
  • lightly butted to avoid gaps

Insulation for inverted roofs should be:

  • extruded polystyrene (XPS), or expanded polystyrene (EPS) suitably assessed in accordance with Technical Requirement R3
  • suitable for external use
  • ballasted to avoid floatation and wind uplift
  • suitable for the weight of the ballast and able to withstand anticipated traffic and design loads
  • protected by a breathable water flow reducing layer (WFRL) to reduce the cooling effect of cold water flowing into and under the thermal insulation and across the waterproofing layer.
  • calculated to take account of the cooling effect of rainwater seeping through the insulation board joints in accordance with BS 6229 and ETAG 031-1 and the dry calculated thickness increased to allow for this cooling effect.


A WFRL should be designed and installed to drain most of the water from the roof. It should be installed at right angles to the designed slope of the roof, starting at the bottom of the slope. All side and end laps should be a minimum of 300mm. The WFRL should be star cut around pipes and wrapped with additional piece of membrane, turned up at upstands and penetrations, and turned down at drainage locations/outlets.

Composite insulated timber decks should:

  • have two beads of sealant along each board joint at the foil under face (to maintain the integrity of the vapour control layer) in accordance with the board manufacturer’s recommendations.

Control of water vapour

The movement of water vapour within the roof construction must be controlled to avoid the risk of interstitial condensation. This can be achieved by providing an air and vapour control layer (AVCL) on the warm side of the insulation.

In a cold roof the addition of ventilation to the void(s) between the insulation and decking can also assist by removing water vapour from within the construction.

Warm roofs do not incorporate ventilation and rely entirely on the AVCL to avoid interstitial condensation. It is therefore essential that a good quality AVCL is used and that all laps, joins or penetrations are fully sealed. All AVCL’s should be fully checked for any damage just prior to being covered over. Any damage should be repaired using a full width section of membrane. Penetrations should be kept to a minimum and fully sealed to the AVCL.

In an inverted warm roof, the waterproofing layer also acts as the AVCL. Control of interstitial condensation also relies on the thermal insulation above the waterproofing layer being kept free from running or ponding water between the insulation and waterproofing layer and within joints between the insulation boards. This should be controlled by the correct installation of the WFRL laid over the insulation.

AVCL’s can be formed with one of the materials listed in Table 9.

Table 9: Materials for AVCL

Reinforced bitumen membranesBS 8747
BS EN 13970
Self-adhesive Polythene/ metal foil/ bitumen membranes for cold self-adhesionBS EN 13970
BS EN 13984
High density Polyethylene sheet with metal foil coreBS EN 13984
High density Polyethylene sheetBS EN 13984
Plastic and rubber sheetsBS EN 13984
12mm one coat mastic asphalt on glass fibre tissueBS 6925


Metal-cored flexible reinforced bitumen membranes are also suitable to form an AVCL.

AVCL’s to warm roofs should:

  • include at least one layer of reinforced bitumen roofing membrane (S2P3) below the insulation, fully or partially bonded to the structural deck as appropriate, or a suitable self-adhesive or torch-on membrane.
  • be self-sealing (e.g. self-adhesive aluminium foil-backed modified bitumen membrane) if penetrated by mechanical fixings,(e.g. those securing the insulation boards to the deck).
  • be sealed and lapped to the waterproofing layer (where compatible) at the perimeters and at each penetration, e.g. at outlets and pipes, rooflights etc. or extended up a minimum 50mm above the insulation and sealed to the wall, rooflights etc.
  • be fixed to resist the calculated wind uplift forces

AVCL’s for warm roof constructions supporting traditional hard metal roofs should have a minimum vapour resistance of 4000MNs/g and be fully supported.

Any fixings that penetrate the AVCL e.g. those securing insulation boards to the deck, should be carefully installed to avoid creating open perforations that would allow water vapour to pass through.

Cold flat roofs are difficult to detail correctly but, where used, they should be in accordance with BS 6229 and BS 5250 and have:

  • an effective air and vapour control layer at ceiling level
  • an unobstructed minimum 50mm ventilation space above the insulation
  • adequate cross ventilation (openings at both ends of each joist void equivalent to a minimum 25mm gap).
  • a maximum clear distance of 5m betwen ventilators on opposite sides of the roof.


All ventilation gaps should have protective mesh or grilles (nominal 4mm openings) to avoid entry of large insects and birds.