As Technical Roofing Consultant John Mercer explains in his latest consultant case study, products, materials, standards and regulations change over time, so “it’s not always possible to simply replace like-for-like when renewing a roof covering.”
I recently carried out a roof inspection where fibre cement slates were being stripped and replaced. The homes – a series of tall, narrow townhouses – had been constructed around 40 years ago and many still had their original roof covering, though they were starting to show signs of age.
After 40 years in service, the slates were still in reasonable condition, but as is so often the case, it was the fixings that were starting to fail and slates were slipping out.
The roof I inspected, which was a mid-terrace, had started to leak, hence the reason for the re-roof. The original slates had been installed over impermeable bituminous underlay and there was no roof space ventilation. There was 100mm fibreglass insulation laid between the ceiling joists.
This inspection highlighted the problem that it is not always possible to simply replace like-for-like when renewing a roof covering. Not only do products and materials change over time, but so do the standards and regulations relating to design and installation.
Firstly, there is a legal requirement to improve the thermal performance of a roof to meet current Building Regulation requirements when replacing 50% or more of a roof covering. This can potentially increase the amount of water vapour in the loft space during cold spells, which, if not properly controlled and dispersed, could potentially cause harmful condensation build-up within the roof structure. Therefore, a means to remove excess water vapour must be fitted, normally either traditional roof space ventilation or a permeable underlay, or a combination of both.
Secondly, if vapour permeable or air and vapour permeable underlay is used, then the new fibre cement slates cannot simply be installed on battens directly over the underlay as before. Because fibre cement slates are close fitting, they do not provide enough natural ventilation for the water vapour that passes through the permeable underlay. Therefore, BS 5250, the British Standard Code of practice for the control of condensation in buildings, recommends that counter battens be installed, together with ventilation at eaves and ridge, to promote air flow through the batten cavity to prevent a harmful build-up of condensation. For the same reason, manufacturers of fibre cement slates also recommend the use of counter battens.
Unfortunately, fitting counter battens on this re-roof project would not be feasible as they would lift the new slates above the level of the neighbouring roofs. The slates are laid across the party walls of each adjoining property, therefore some form of bonding junction would be required to weather the joint between the new and existing neighbouring slates if the new slates were set higher than the adjacent slates.
A possible solution on this roof would be to use an impermeable underlay to prevent water vapour passing through the underlay from the roof spaces and reaching the batten cavity. This way counter battens would not be necessary. Traditional bituminous underlay is impermeable, though there are other types available which offer several advantages. One such product is Glidevale’s Protect A1 impermeable roofing underlay.
This is a modern, twin-ply polypropylene product which is durable and unaffected by temperatures changes. It is lightweight and has an absorbent under surface which prevents condensation drip off. It has also been tested to Annex A of BS 5534 and achieves unrestricted use for wind uplift throughout the UK and Ireland for batten gauges up to 345mm, delivering a high resistance to wind pressure.
The use of an impermeable underlay means water vapour would not pass through the underlay into the batten cavity. Instead, ventilation could be installed to control condensation risk in the roof space and the new slates could be interleaved with the slates on the adjoining roofs without the need for flashings or bonding gutters.
In practice, a dry ridge system could also be fitted to ventilate at the ridge. The underlay would be cut about 30mm short of the apex at each side to provide an air path from the roof space.
To drive air movement through the roof space, eaves or ‘low level’ ventilation would also be needed. Normally this could be in the form of a continuous over fascia or soffit ventilator strip, with rafter trays fitted to maintain a gap between the underlay and insulation inside the roof space.
The final problem to overcome with the roof under inspection was that the design of the eaves meant it was not possible to fit eaves ventilation. Therefore, when re-roofing, it would be necessary to fit slate vents in the first slate courses above the underlay, set at centres to provide the equivalent of a continuous 10mm wide air gap along the eaves.
• A seemingly simple strip and re-slate project may not always be as straightforward as it first appears. Always check the compatibility of the proposed materials with each other and current roofing practices.
• Remember that where a roof covering is to be removed and replaced, the roof insulation must be brought up to current Building Regulation thermal requirements.
• Pay particular attention to the roofing material manufacturer’s installation recommendations.
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