Eaves

What is the minimum pitch for Cambrian Slates with RedVent EavesVent?

The RedVent EavesVent tray fits over the rafter below the tiling batten. The front of the tray sits on top of the fascia board. At low pitches with tiles with small batten gauges such as Cambrian Slates, there is a negative fall on the tray (and hence underlay) between the bottom of the first batten and the top of the fascia board.
For this reason the minimum rafter pitch for Cambrian Slates when used with RedVent EavesVent is 25º.  Where rigid sarking and counterbattens are used this negative fall will be even greater as the tray sits below the sarking board. The thicker the counterbattens the greater the negative fall. Consequently it is recommended to contact Redland Technical Solutions for advice on the minimum pitch of Cambrian Slates with RedVent EavesVent when using rigid sarking.
The same issues exist with plain tiles due to their small gauge. As with Cambrian Slates it is recommended to contact Technical Solutions for advice on minimum pitch.

 

What is the purpose of an Eaves Comb?

The recommendation for filling in the corrugations of a profiled tile is to stop birds and rodents from nesting in the batten cavity. It has been found that the smallest bird or rodent that nests in house roofs has a body of over 16mm diameter. Some tiles have a corrugation low enough to leave a gap of less than 16mm so do not need any form of filler. However, where this is not the case an eaves filler is needed and it is important to use a good one. The most common product used to prevent bird and rodent access is a good quality eaves comb. The practice of filling the corrugation of the tile at the eaves with mortar is traditional in some parts of the country. However, unlike an eaves comb, this does not allow the eaves tile to be clipped to resist wind uplift if required in the manufacturer’s fixing specification. Moisture can also get trapped behind the fascia board which leads to rotting of the underlay as moisture is prevented from discharging into the eaves gutter.

 

Why is it important to fix the fascia board at the correct height?

If the fascia board is fixed too low the eaves course of tiles will lie at a steeper angle than the rest of the tiles above. Contact with the next course is along the top edge only making the weather bar arrangement in the headlap ineffective. If the fascia board is fixed too high the eaves course of tiles will lie at a shallower angle than the rest of the tiles above and may be below the minimum pitch for the tile. Additionally contact with the next course is along the leading edge only greatly reducing the effectiveness of the weather bar arrangement. In both cases the tiles in the first and second courses will not lie in the same plane. The tile clips for many tiles will also not fit unless all tiles lie in the same plane.

 

How is the correct length of insulation interrupters determined?

To satisfy Part L of the 2013 Building Regulations will require mineral wool insulation for most domestic dwellings to be more than 300mm thick when laid between and over the ceiling joists. The lower the roof pitch and the wider the soffit the longer an interrupter tray must be to ensure that an adequate eaves ventilation path is maintained between the roofing underlay and the insulation. With an insulation thickness of 300mm a tray, up to one metre in length may be necessary. Some trays such as those supplied with the RedVent EavesVent system sit on the fascia board and therefore the distance from the fascia to the wallplate must be added to the inclined insulation distance. This may be as much as two metres depending on the soffit overhang. It is important to choose a system that can be extended to any length. For rafter pitches below 30º extension trays will be required. The standard tray supplied with RedVent EavesVent extends 450mm from the front of the fascia board. Each extension tray adds 380mm to this dimension. Alternatively from the Rapid + Range, Rapid EavesVent Rafter Roll extends 800mm per single roll width.
With a 30º rafter pitch, no soffit and 200mm thick insulation, the tray must be at least 600mm long. It is essential to measure the inclined distance from the fascia board to the top of the insulation either on site or from drawings. When reroofing, it is advisable to fix the trays above the existing insulation to allow for any future installation of additional insulation.

 

How are open eaves ventilated when insulation is at rafter level?

 

According to BS 5250, Code of Practice for Control of Condensation in Buildings, unless you have both a well-sealed ceiling and a continuous vapour control layer below the thermal insulation positioned at rafter level then some form of ventilation should be provided to the air space between the insulation and roofing underlay. Where insulation partially fills the rafter space leaving an air gap between the insulation and top of the rafters, individual rafter spaces must be ventilated as each rafter space acts as an individual duct. With open rafters, as there are no fascia or soffit boards there is no simple means of introducing low-level ventilation. However, there are two possible approaches. In one method the RedVent 25 Over-Fascia Vent should be positioned between the rafters at the wall line venting directly into the 50mm void above the insulation just below the underlay. This is best achieved by leaving 50mm clear space between the structural wall and the top of the rafters. The vent should be cut to length to fit between the rafters and nailed to the top of timber trimmers also cut to fit between the rafters. Each of these assemblies can then be slid up the face of the structural wall between the rafters until the vent comes into contact with the underlay, 5mm below the top of the rafter. The trimmer should be fixed in this position to the wall or the rafters. Providing the trimmers do not come below the bottom of the rafters, this arrangement will give a neat detail at the junction of the wall and the roof. Alternatively, the use of ventilation tiles can be considered. These should be spaced so as to give the requisite amount of low level ventilation.