Case 2

Good Practices

To prevent blisterings from occurring

Vents are openings installed to allow air or a liquid to escape from somewhere. Roofing systems are vented to permit escape of moist air. For one or more of these components, venting might achieve some of the following objectives:

1. To permit release of moisture during construction.
2. To permit subsequent dying of a wet-applied component.
3. To prevent accumulation of moisture.
4. To remove moisture that has accumulated.

The only valid objective of venting a hot-applied bituminous built-up membrane is to remove excess moisture on or in the felts during construction. Small amounts of moisture trapped within a membrane can result in blisters and other flaws that promote premature membrane deterioration. Many roofing felts are perforated to allow moisture to escape during hot mopping of the membrane.

The amount of moisture present in a cured concrete deck will usually be so low that it is of no great concern. Consequently, cured concrete decks seldom create moisture problems. They should not be sealed at their base since downward drying by evaporation into the air within the building is desirable.

Concrete decks dry more slowly when cast or corrugated steel forms which remain in place. If roofing is applied to such a deck while it is still wet, and no vapor retarder is placed above the concrete, deck moisture may move upward into the insulation.

Some consideration 

Whether flat roof are constructed of concrete or timber, they need to be laid to adequate falls and incorporate a vapour barrier. The generally accepted minimum fall is 1 in 80, but the latest recommendation is 1 in 40 to make allowance for any inaccuracies on the site and possible deflection of the roof structure.

A problem with concrete roofs stems from moisture trapped in lightweight concrete screeds, which are incorporated to provide falls and insulation. Vapour barriers need to be provided in most roofs to protect insulation against water vapour from inside the building. Vapour barriers are not recommended under concrete screeds as the entrapped moisture which take years to dry out, is likely to be more troublesome than that from condensation.

Dry-type lightweight aggregate screeds, such as perlite i hot bitumen, are preferable to wet screeds. An alternative is to formthe roof slab itself to give the fall and to use dry insulation such as minerial/glass wool slabs or expanded plastics. A vapour barrier, preferably of one layer of bitumen felt laid in hot bitumen, should be provided between the structural slab and the dry insulation.