Lack of ventilation leads to failure

We were asked to inspect a single ply membrane flat roof on a development of 9 terraced town houses in London constructed around 2012/13.

The roof was originally constructed as a cold roof and the design had allowed for the provision of cross ventilation above the mineral wool insulation. Unfortunately free flow of air had been blocked by the presence of party walls between the buildings as well the firings which had also compromised the airflow.

Air flow compramised by firings and insulation

Consequently, due to the lack of adequate ventilation interstitial condensation has been able to form within the roof build up leading to the complete failure of the existing plywood roof deck the entire roof area

Works in progress showing delaminated condition of plywood decking

Due to the complexity of the roof falls it was not feasible to replace the roof with a cold roof design and achieve adequate airflow, therefore the decision was made to replace the roof cold roof with a warm roof design.

An added complication was the restriction in roof finished level due to limited upstand heights. To over come this the firing pieces forming the original roof falls were removed, and a cut to falls insulation scheme was developed to recreate the falls.

The new warm roof system as designed incorporated a new roof deck fixed to the existing rafters, fully bonded vapour control layer, cut to falls insulation and two layers of reinforced bitumen membrane.

Completed Works

Contractor – Ifield and Barrett Roofing Ltd

Roof Design – Michael Kilbey Associates Ltd

Project Management – Michael Kilbey Associates Ltd

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