In cold roofs, the structural and insulation layers are combined. This produces a thinner buildup at the cost of continuity and performance. One of the side effects is condensation under the deck. To combat it, you need ventilation.
Below I show two types of vent design between a horizontal roof and a vertical wall. The injection moulded plastic ventilator is a more basic and cheaper option that makes use of standard building materials. It is also more difficult to install wrongly. This, in particular, is an important consideration, as a failure of a cold roof vent can result in a failure of a roof.
The profiled metal sheet ventilator comes with more features and complexity. A specific version has to be ordered for different roof coverings, and each version has various options of steel grade and fire rating. For every one fixing in the plastic ventilator, this type needs three. Being more flimsy, it is also more susceptible to wind uplift and rattling.
DRAWING LEGEND:
- Roof insulation
- Roof rafters with an unobstructed ventilation void in between.
- Angled components for creating required roof falls; typically, timber firring strips.
- Roof deck; typically, OSB or plywood.
- Waterproof roof cover, e.g., EPDM.
- Stop roof deck 25 mm short of the wall.
- Cant strip
- Upstand, at least 150 mm tall to minimise water ingress.
- Battens for creating a void between upstand and wall.
- Extend flashing pass ventilator.
- Ventilator with integrated insect mesh, injection moulded polypropylene.
- Welt lower edge of flashing around bottom edge of ventilator.
- Profiled sheet abutment ventilator, stainless steel. In windy or coastal zones, thicker 1 mm 316 grade steel should be specified to counter wind uplift and corrosion better.
- Spacer feet/fixing bracket.
- Flashing, dressed over ventilator; e.g., lead.
- Intumescent fire strip. Optional if required.
- Insect mesh, e.g., vinyl-coated GRP.
- Abutment, e.g., parapet, building wall.
- Flashing inserted into mortar 25 mm and repointed. If cavity tray is needed, install it above flashing.
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