Pitched Roofs

Introduction
The pitched roof is the most common type of roof construction used in new dwellings today. Pitched roof design will include a system of structural elements usually made from timber which provide support to a sarking felt, sarking board or breather membrane underlay. Over the underlay, tiling battens are nailed directly to the structural elements, providing support and a fixing point for a weather proof layer which normally consists of slate, concrete or clay tiles.

The primary function of a pitched roof is to shed water from a dwelling in poor weather conditions; however it has also become an increasingly important element when considering the most cost effective ways in which energy use in dwellings can be saved.

Pitched roofs have traditionally been insulated at ceiling level, i.e. between the ceiling joists. This results in what is known as a ‘cold pitched roof’. The enclosed roof space in many dwellings is increasingly being recognised as a potential area that can be used as a habitable space where insulation is placed in the slope of the roof, between rafters. This results in what is known as a ‘warm pitched roof’.

Thermal Performance
The U-Value of a pitched roof is calculated using BS EN ISO 6946, which is better known as the ‘Combined Method’ of calculation. With the new Building Regulations emphasis on the energy efficiency of a dwelling as a whole the insulation incorporated into a pitched roof has a major contribution to make towards the dwelling’s overall energy performance.

Insulation Envelope
There is a choice of insulation position in pitched roofs:

  • At joist level (Cold Pitched Roof)

  • At rafter level (Warm Pitched Roof)

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Ceiling Insulation (Cold Roof)
In the cold pitched roof method of construction, glass mineral wool insulation is placed at ceiling level between and over the ceiling joists. Cold pitched roofs can be ventilated or unventilated requiring the use of ‘vapour impermeable’ and ‘vapour permeable’ underlays respectively. Where ‘vapour impermeable’ underlays are used, such as traditional sarking felts, adequate ventilation of the airspace above the insulation is required to prevent condensation occurring within that airspace. Where ‘vapour permeable’ underlays are used, such as a breather membrane, any water vapour within the roof space must be allowed to dissipate through the pitched roof construction.

Condensation Risk
Condensation is caused when water vapour, generated in the warm living spaces of a dwelling, passes through the ceiling into the cold roof. When the water vapour comes into contact with cold surfaces such as the underside of the sarking underlay condensation will occur and the moisture associated with this condensation may lead to mould growth and even damage to elements within the roof.

Building Regulations set out what constitutes adequate levels of ventilation for new dwellings. BS 5250:2002 details the minimum solutions capable of ensuring adequate ventilation in different types of roofs.

Ventilated Cold Roof
Adequate ventilation requires that an unobstructed ventilation path is allowed between the insulation and the tiling underlay so that outside air can enter the loft space freely to prevent an accumulation of water vapour. Adequate ventilation is facilitated by openings situated at the eaves and the ridge of the roof. The size of the openings may vary depending on the roof pitch, span and configuration. The ventilation requirements for roofs with a double pitch are shown below.

Lean-to and mono-pitch roofs should have a continuous gap of 10 –25 mm at the eaves, depending on the pitch angle, and a continuous high level or ridge ventilation equivalent gap of 5mm.

Ventilation openings must be insect proof by applying a 3mm or 4mm mesh across the ventilation holes. The loft hatch should be draft sealed with the provision for compression catches or bolts to prevent air leakage from wind uplift. The loft hatch should also be insulated to a minimum depth of 50mm.

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Unventilated Cold Roof
This type of construction relies on natural vapour pressure differences across the roof construction. This allows water vapour from the roof space to transfer safely to the external environment. Condensation risks in unventilated cold roofs are minimised by including an airtight vapour control layer on the warm side of the glass wool insulation at ceiling level, the inclusion of an effective vapour permeable underlay in the roof construction along with a ventilated void of at least 25 - 50 mm between the underlay and the roof tiles or slates.

Thermal Bridges
To avoid thermal bridging, the roof insulation should butt up to or lap the wall insulation. In cold roofs, this may be achieved in practice placing a short length of insulation quilt over the wall plate (and cavity closer, where applicable) immediately before the tiling underlay is fixed. This avoids having to push the insulation into place from inside the roof once the roof covering has been completed.

The gap between gable/separating walls and the first joist/rafter should be insulated to avoid thermal bridging. In cold roofs, the second layer of insulation should be butted up against the gable and separating walls to avoid thermal bridging.

Services in a Cold Roof
Large ‘cold pitched roof’ voids can be used to locate services such as water storage tanks & associated pipes, ventilation ducts, electrical cables and recessed light fittings. Where this is the case all gaps and holes around service that pass into the cold roof via the ceiling should be sealed to restrict the amount of water vapour that enters the loft space. All tanks and pipes in the loft should be insulated to prevent freezing, taking care not to insulate directly under the cold water tank. Ducting may have to be insulated to prevent moisture from condensing inside the ducts. In the case of recessed lighting sufficiently large enclosures may be required to dissipate heat from the lamps.

Upgrading an Existing ‘Cold Pitched Roof’

An overall insulation thickness of between 250mm to 300mm is recommended when upgrading the thermal insulation of an existing roof.

In most pitched roofs any existing insulation is likely to be between the ceiling joists and where this reaches the top of the joists, the new insulation should simply be laid across the joists at right angles. Where the existing insulation is below the joist height add extra insulation between the joists so that it reaches the top, then a second layer should be positioned at right angles to the joists.

Existing roofs are most likely to have an impervious sarking underlay, such as a bitumen felt, so it is very important to maintain the cross ventilation of the loft space from eaves to ridge as stated previously.

 

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Rafter Insulation (Warm Roof)
In the warm pitched roof method of construction, glass mineral wool insulation is placed at rafter level. Warm pitched roofs can be ventilated or unventilated requiring the use of ‘vapour impermeable’ and ‘vapour permeable’ underlays respectively. Where ‘vapour impermeable’ underlays are used such as traditional sarking felts an adequate ventilation space between the cold side of the insulation and the underlay must be included to facilitate the dissipation of water vapour to the external environment. Where ‘vapour permeable’ underlays are used, such as a breather membrane, the full rafter depth can be filled.



Condensation Risk
Condensation is caused when water vapour, generated in the warm living spaces of a dwelling, passes through the warm roof and comes into contact with cold surfaces such as the underside of the sarking underlay. If the moisture associated with this condensation is allowed to form, then mould growth and even damage to elements within the roof can result. The recognised way of preventing this is to install a vapour control layer on the warm side of the insulation, which limits the amount of water vapour that can enter the insulation layer from the dwelling. The vapour control layer must be continuous, well sealed at joints and placed behind services, such as electrical cables, to avoid puncturing.

Building Regulations set out what constitutes adequate levels of ventilation for new dwellings. BS 5250:2002 details the minimum solutions capable of ensuring adequate ventilation in different types of roofs.

Warm Roof (with impermeable membrane)
There is a requirement for 50mm ventilated airspace between the cold side of the insulation and the tiling underlay. Should the rafter depth be insufficient to accommodate both the required thickness of insulation and the 50mm ventilated airspace, the rafter can be battened out to increase the rafter depth or an insulated dry lining can be included, also providing the added benefit of minimising thermal bridging.

Ventilation openings should be installed at each and every roof void at both low and high level. At the eaves, ventilation openings should be equivalent to a 25mm continuous gap. At the ridge, the ventilation opening should be the equivalent to a 5mm continuous gaps each side of the ridge. A vapour control layer must also be applied to the warm side of the insulation.

Warm Roof(with permeable membrane)
Insulation fills the rafter space without airspace between the insulation and tiling underlay, which must be vapour permeable. In instances where a thin layer of insulation is used, thermal bridging can be eliminated by installing a thermal board. A combined airtight/vapour control layer should be placed on the warm side of the insulation. This not only makes the ceiling convection tight, but also restricts the amount of water vapour passing through it.

Where cables and piped services are to be installed, the plasterboard lining may be battened out to provide a suitable duct. The services should be routed on the inside of the vapour control layer that should be free from any puncturing.

 

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