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Part L's Impact on Roof Tiling by Chris Thomas

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Building Regulation Approved Document Part L has had quite a dramatic effect on the heating and insulation requirements for buildings, to reduce our consumption of fossil fuels and the flue gasses that help to erode the Ozone Layer, that helps to control our climate.
But what effect has it had on the construction of pitched tiled roofs?
Back in the 1950's when there was no requirement to insulate roofs and heating was mostly fuelled by coal, the roof space was only slightly colder than the rooms below. The lower room temperatures and the natural ventilation up the chimney flue helped to reduce the quantity of moisture in the air. As the thickness of insulation has increased, the greater the difference in temperature between the roof space, above the insulation, and the rooms below. This combined with the higher levels of moisture that warm air could absorb, so the risk of condensation forming in the roof space, or within the insulation, increases. The thicker the insulation the greater the condensation risk. Installing a vapour check on the underside of the insulation can reduce the risk, but under extreme conditions it will not be eliminated.
What effect does thicker insulation have on roofing?
  • British Standard 5250 Code of practice for the Control of Condensation in Buildings provides the best advice on what should be done. If the insulation is placed between and above the horizontal ceiling joists, the thicker the insulation the longer the corrugated rafter trays that helps to maintain an air space between the insulation and the underside of the underlay. The air space allows ventilation air in and out at the eaves, to remove the cool air containing excess water vapour before it can condense on the cold hard surfaces in the roof, such as metal truss plates.

At the eaves where the roof insulation meets the outer wall, the space diminishes down to the thickness of the rafter, which in most cases will be less than 100mm. The depth of the rafter tray may further reduce the rafter depth, which can be between 25 and 50mm. This may be restricted still further by the wall plate being notched into the rafter. The resulting space left for insulation may be less than 50mm. The risk of condensation forming at this cold bridge area is high.

  • If the insulation is positioned between the rafters, parallel with the roof tiles the quantity of insulation may be greater in thickness than the depth of the rafter. This may require an additional counter batten to be fixed to the top or bottom of the rafter to increase the insulation space. Alternatively a more expensive rigid insulation may be used that being a better insulator can be thinner to achieve the same thermal performance.
  • What is more likely is rigid insulation board placed above the rafters and counter battens fixed through it into the rafters below. Because each timber rafter is below the insulation there are fewer cold bridges and joints, making a thinner board more efficient.

At the ridge the insulation board needs to be mitred and sealed to prevent a V shape cold bridge occurring. At the eaves the rigid insulation needs to maintain continuity with the wall and will need infill slabs to stand between the rafters to prevent a cold bridge at the wall plate. With this arrangement any services, such as soil vent pipes or roof windows that penetrate the rigid insulation will need to be sealed to prevent a cold bridge.

In many instances the rigid insulation manufacturers recommend specific types of Vapour permeable underlay to ensure vapour seeping through the joints in the boards is able to escape into the batten cavity, and not condense on the underside of the underlay, and drain back into the building.

The use of vapour permeable underlay to improve the thermal performance of the roof will inevitably change the roof ventilation requirements. If you do not have roof space ventilation below the underlay then it will be needed above to ensure that there is a safe route for it to escape into the outside air. To meet the ever more stringent performance requirements, tiles are made to tighter tolerances (less gaps) to allow them to be laid at lower rafter pitches. Less gaps means the risk of water vapour condensing on the underside of the tiles and slates, or worse still the timber battens, the greater the need for batten space ventilation from eaves to ridge. Providing ventilation above the underlay is best done with a 25mm over fascia vent at low level and a continuous dry ventilated ridge system. If that is not possible, suitably spaced vent tiles with shortened or no underside pipes.

The use of vapour permeable underlays will allow the water vapour to pass into the batten cavity where it will condense on the underside of the roof tile/slates. Without ventilation at that point the battens and counter battens could be affected in the long term.


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Heating systems
The need for more efficient heating systems has seen the number of Gas Condensing Boiler types increase. The act of extracting more heat from the gas flame has resulted in lower flue gas temperatures. The lower the flue gas temperature the greater the need for a fan flue system. The cooler flue gasses, the higher content of water vapour and the higher fan velocities have allowed the boiler manufacturers to reduce the diameter of the flue and make them of plastic.
What has this to do with roofing?
Efficient building design requires the outer surface to be reduced to a minimum, making terraced properties with room in the roof more efficient. With less external wall surface, comes the compromise between services and windows/doors. The answer is to locate the gas-condensing boiler in the middle of the upper floor (airing cupboard) and take the boiler flue directly up and out of the roof slope or ridge. Some gas condensing boiler manufacturers are using standard inline vent tiles, licensed for use as a gas condensing boiler terminal, rather than a traditional flue pipe terminal. This may result in the choice of roof covering being dictated by the heating boiler terminal specification, rather than competitive price.

With the search for more efficient use of energy to heat buildings and hot water, we will see more buildings with Solar Water Heater or Photovoltaic panels that produce electricity, located on the upper parts of south facing roof slopes. This is likely to result in more systems being installed during construction or re roofing. At present the systems available are not liked by planners as they are not visually sympathetic to our traditional style of building, but some have been designed to be integrated into the roof hiding all the brackets and pipes. This trend combined with more compact housing units, may result in complete roof slopes being nothing other than Solar Water Heater or Photovoltaic panels.

It will not be long before the total south facing slope of most new roofs will be all solar panels. This roof was designed and installed by Solar Century.

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Whilst at first sight the impact of Building Regulation Approved Document Part L may not be very great on pitched roof coverings. In the medium to long term there could be a significant effect on the design and construction of roofs that will alter the specification and construction of the roof.
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