Flat Roof Air Gap

Flat roof joists 50mm air gap above decking surrey building control guidance note number 13.

Flat roof air gap.

A cold deck flat roof. R value of ccspf to be 40 of total assembly. Many properties use this option as there are no additional costs for scaffolding or organising how to gain safe access to the roof exterior. A boarded roof with plywood sheet providing structural strength over the rafters.

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. For both a warm and cold flat roof a vapour control layer must be installed. 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.

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. 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. 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.

Closed cell spray polyurethane foam ccspf at bottom of roof sheathing. An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it. Acceptable if above is not feasible. Roof sheathing to be air sealed.

2011 code of practice for control of condensation in buildings stipulates that there should be a ventilation gap between the insulation and the bottom of the flat roof deck of at least 50mm. If less analysis is required. Ci to be installed in multiple layers to limit insulation by pass from gapping of insulation. Foil lined plaster board on the ceiling will help because it forms a partial vapour barrier.

Ventilation air follows exactly the opposite path. You have various options. The insulation material is placed between the supporting joists leaving a gap of 50 60mm between the roof and insulation. This may involve the use of 50mm x.

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. On a warm flat roof the layer is installed underneath the insulation to stop moisture interacting with it through.