Flat Roof Air Gap

The problem with celotex is that it does not completely seal the voids so you get some air in and when that hits the cold flat roof water condenses hence the manufacturer wanting the air gap for the moisture to evaporate.

Flat roof air gap. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. A boarded roof with plywood sheet providing structural strength over the rafters. Acceptable if above is not feasible. Let s start with the only situations with between rafter insulation where you do actually need a 50mm ventilated air gap over the insulation a loft conversion where there is a bitumen felt in place.

A cold deck flat roof. This is placed over the. Building regulations state that a ventilation gap of at least 50mm must be in place between the insulation and the underside of the flat roof deck. Rising along the underside of the angled purlin until it reaches a gap then rising up to the next purlin and so on to exit at the ridge or gable.

In a concrete deck for example this gap could be created by fixing 50x50mm timber battens at 600mm centres and attaching the insulation to. Ventilation air follows exactly the opposite path. Roof sheathing to be air sealed. Continuous 5mm gap at ridge depending on the type of roof you are considering here s some technical details you may find useful.

If less analysis is required. To prevent condensation forming when the insulation is placed at ceiling level a minimum of 50mm gap is provided for ventilation above insulation. An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it. Many properties use this option as there are no additional costs for scaffolding or organising how to gain safe access to the roof exterior.

The map at right which is based on table r806 4 of the irc lists the minimum r values required to prevent condensation in unvented assemblies in various climate zones. Flat roof joists 50mm air gap above decking surrey building control guidance note number 13. The insulation material is placed between the supporting joists leaving a gap of 50 60mm between the roof and insulation. Foil lined plaster board on the ceiling will help because it forms a partial vapour barrier.

On a warm flat roof the layer is installed underneath the insulation to stop moisture interacting with it through. R value of ccspf to be 40 of total assembly. You have various options. Flat roof u value 0 18 w m2k 2 warm deck flat roof the type of insulation for this is usually of a rigid type and the thickness will vary depending on the manufacturer s specifications.

This may involve the use of 50mm x. Closed cell spray polyurethane foam ccspf at bottom of roof sheathing.