Continuous membrane roofs 1 General

Continuous membrane roofs have a continuous weatherproof covering which is also impermeable to water vapour from below. They are often, but not invariably, of low pitch, between 0° and 10°. They can be subdivided into three types.

a) Cold deck roofs have the insulation located below the roof deck, usually at ceiling level. It is essential that both a vapour control layer and adequate ventilation of roof voids is provided. Specific

recommendations on the overall size and location of ventilation openings are given in 8.4.3.2.2 and 8.4.3.3.1.

b) Warm deck roofs have insulation located above the roof decking. This type of roof relies on a high resistance vapour control layer below the insulation.

c) The inverted roof, also known as the protected membrane or upside down roof, is a type of warm roof deck construction in which the insulation is above the weatherproof finish. The weatherproof finish also functions as a vapour control layer. Design guidance appropriate to this particular roof construction is given in 8.4.3.2.3 and 8.4.3.3.3.

8.4.3.2 Framed continuous membrane roofs 8.4.3.2.1 General

These roofs have a structural frame, which may be metal or timber, supporting a continuous deck. There is a cavity between the frame, which can be ventilated.

8.4.3.2.2 Cold deck

A cold deck roof is shown in Figure 22. This type of roof should be avoided because interstitial condensation is likely and its effect on the structure and insulation can be severe. Where construction of this type of roof cannot be avoided, it is essential that moisture entering the roof is minimized and that adequate

ventilation is provided. If condensation does occur, it will normally be at the underside of the roof decking or waterproof finish.

The vapour control layer should have a resistance of at least 250 MN·s/g, and should have sealed laps to preserve the integrity over the whole roof. Gaps in the ceiling should be minimized and service openings should be avoided; if they cannot be avoided it is essential that they are sealed.

Ventilation openings should be provided to every roof void along two opposite sides of the roof and should

a) b)

Figure 21 — Ventilation positions for roofs with dormers

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excess of 5 m; in these situations, both the openings and airspace over the insulation should be substantially increased. Where fire stops occur which obstruct cross ventilation, it will be necessary to provide ventilation through the roof covering; to avoid this the use of a warm roof should be considered.

Timber not rated moderately durable or better should be preservative treated. During its life, it is likely that the decking will be subjected to periods of moist conditions; material should be chosen accordingly.

Surface condensation on the ceiling is unlikely to occur due to the fast thermal response, provided there is adequate insulation over the whole ceiling and steps have been taken to minimize thermal bridging, especially at external wall/ceiling junctions (see 8.6).

8.4.3.2.3 Warm deck

A warm deck roof is shown in Figure 23. Surface condensation is unlikely to occur due to the fast thermal response, provided there is sufficient insulation to maintain the vapour control layer above dewpoint over the whole roof and thermal bridging is minimized, particularly at external wall/ceiling junctions (see 8.6).

Interstitial condensation can occur on the underside of the weatherproof finish. To minimize this, a vapour control layer with a vapour resistance of at least 250 MN·s/g is essential, laid in hot bitumen with sealed laps. The VCL should be turned up around the insulation and bonded to the weatherproof finish at all edges and penetrations such as roof lights.

Key

1 Weatherproof roof finish 5 Vapour control layer

2 Roof decking 6 Ceiling

3 Ventilated air space 7 Structural member

4 Insulation

Figure 22 — Framed flat roof: cold type

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8.4.3.2.4 Inverted

An inverted roof is shown in Figure 24. Surface condensation is unlikely to occur due to the fast thermal response, provided that there is sufficient insulation to maintain the weatherproof finish above the dewpoint over the whole roof and thermal bridging is minimized, particularly at external wall/ceiling junctions. With this type of roof, rainwater seeping below the insulation will cool the waterproof membrane intermittently, increasing the risk of condensation on the membrane. This should be allowed for by reducing the thickness of insulation by 20 %, when calculations of the risk of interstitial condensation are carried out.

Key

1 Weatherproof roof finish 5 Unventilated air space

2 Insulation 6 Ceiling

3 Vapour control layer 7 Structural member 4 Roof decking

Figure 23 — Framed continuous membrane roof: warm type

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8.4.3.3 Concrete continuous membrane roofs 8.4.3.3.1 Cold deck

A cold deck roof is shown in Figure 25. This type of roof should be avoided because interstitial condensation is likely to be serious. Where construction of this type of roof cannot be avoided, it is essential that moisture entering the roof is minimized and that adequate ventilation is provided. If condensation does occur, it will normally be at the underside of the roof decking or waterproof finish.

Surface condensation is unlikely to occur due to the fast thermal response provided there is sufficient insulation to maintain the vapour control layer above the dewpoint over the whole ceiling and thermal bridging is minimized, particularly at external wall/ceiling junctions (see 8.6).

The vapour control layer should have a vapour resistance of at least 250 MN·s/g with sealed laps. Gaps in the ceiling should be minimized and service openings should be sealed.

Ventilation openings should be provided to every void along two opposite sides of the roof and should be equivalent in area to a continuous opening of not less than 25 mm at each side. A 50 mm (minimum) unrestricted airspace should be maintained between the underside of the roof deck and the top of the insulation. Adequate cross-ventilation can be difficult to achieve with spans in excess of 5 m; in these situations, both the openings and airspace over the insulation should be substantially increased.

It is essential that construction water should be allowed to dry out, e.g. by delaying the installation of the vapour control layer and internal finishes.

Key

1 Topping or ballast layer 5 Unventilated air space

2 Insulation 6 Ceiling

3 Weatherproof roof finish 7 Structural member 4 Roof decking

Figure 24 — Framed continuous membrane roof: warm type inverted

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8.4.3.3.2 Warm deck

A warm deck roof is shown in Figure 26. To minimize surface condensation, the heating system should be matched to the construction, because the thermal response is slow and therefore constant low output heating is recommended. Sufficient insulation should be provided to maintain the vapour control layer above dewpoint and as thermal bridging is difficult to avoid at the perimeters; care should be taken at external wall/roof junctions, gutters and roof lights (see 8.6).

Interstitial condensation can occur on the underside of a weatherproof finish. To minimize the amount of condensation, a vapour control layer is essential and it should have a vapour resistance of at least 250 MN·s/g with sealed laps and be turned up around the insulation and bonded to the weatherproof finish. If possible construction water should be allowed to dry out, for example by delaying the installation of vapour control layer and internal finishes. Suspended ceilings can delay drying out or cause condensation problems (see 8.4.6).

Key

1 Weatherproof roof finish 5 Insulation

2 Roof screed (if required) 6 Vapour control layer 3 Structural concrete deck 7 Ceiling

4 Ventilated air space

Figure 25 — Concrete continuous membrane roof: cold type

Key

1 Solar reflective chippings 4 Vapour control layer 2 Weatherproof roof finish 5 Roof screed (if required)

3 Insulation 6 Structural concrete deck

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8.4.3.3.3 Warm deck inverted

An inverted warm deck roof is shown in Figure 27. To minimize surface condensation, the heating system should be matched to the construction, because the thermal response is slow and therefore constant low output heating is recommended. Sufficient insulation should be provided to maintain the

waterproofing layer, which acts as the vapour control layer, above dewpoint and as thermal bridging is difficult to avoid at the perimeters; care should be taken at external wall/roof junctions, gutters and roof lights (see 8.6).

With this type of roof, rainwater seeping below the insulation will cool the waterproof membrane intermittently, increasing the risk of condensation on the membrane. This should be allowed for by reducing the thickness of insulation by 20 %, when calculations of the risk of interstitial condensation are carried out.

Suspended ceilings can delay drying out or cause condensation problems (see 8.4.6).

8.4.4 Profiled metal roof cladding