Questions

Is ventilating a roof only at high level sufficient?

It depends. The recommended method and amount of ventilation of the roof void depends on two critical factors: the degree of sealing or airtightness of the ceiling below the roof void and the type of roofing underlay used - whether water vapour permeable or not. If a ceiling contains a loft hatch without an effective seal, unsealed pipe or service or other penetrations then high level ventilation of the roof void should never be installed without adequate low level ventilation of the same void regardless of the type of underlay. In the absence of this low level ventilation warm, moist air from the living space can be drawn up into the roof void through the holes and gaps in the ceiling to replace air sucked through the ridge thereby raising the risk of harmful condensation occurring in the roof void. However, with careful design, specification and workmanship well sealed ceilings can be constructed to help meet the need for more energy efficient dwellings and other buildings. These better sealed ceilings allow the use of vapour permeable underlays (VPU's) for felting the roof void. With such construction - more prevalent in modern new housing - high level ventilation of the roof void only is sufficient to ensure adequate control of condensation risk even during the drying out stage of newly constructed buildings. Furthermore with a well sealed ceiling external air is encouraged into the roof void at low level through the tiles and laps in the underlay, in preference to moist air from the living space, thereby creating an effective method of ventilation without the need for an eaves ventilation product. From January 1st 2011, the NHBC require the use of high level ventilation when using a VPU in a cold pitched roof for all properties covered under its Buildmark Warranty.

Are there any Redland products for ventilating lift shafts?

BS 5655 requires the effective ventilation area at high level to be equivalent to 1% of the plan area of a lift shaft, e.g. if a lift shaft is 1.6 m x 2.4 m the ventilation area must be equivalent to 38,400 mm². This could be achieved by using a number of different product combinations.
Using DryVent Ridge, which provides 10,000 mm²/m and two ridge ventilation terminals, which provide 8,400 mm2 each, would be sufficient if the ridge ran along the 2.4 m length of the lift shaft. Alternatively nine 4.5k ThruVent tiles with a vent area of 4,500 mm² each could be installed or any other multiple combination equivalent to 38,400 mm².
However although this may work in theory, in practice it may be difficult to achieve in the roof area available especially with rafters at 600 mm centres. An alternative would be to form a truncated dormer with a louvre grille in the vertical face with the required ventilation area. This arrangement works well at the intersection of a ridge with a hip. The final choice will very much depend upon the exact situation directly above the lift shaft.

How should a room-in-the-roof construction be ventilated?

Using the roof space is an excellent way of creating additional space within the original building envelope. However careful attention must be paid to the ventilation and insulation requirements in this type of construction. Unlike in a house where the loft space is not used, in this type of construction the room-in-the-roof temperature is approximately the same as the rooms below. Depending on the precise position of the insulation in relation to the roofing underlay there may or may not be a cold roof void sandwiched in between which needs to be appropriately ventilated. For constructions where the insulation is either on top of rafters and/or fully filling rafters a vapour permeable underlay can be supported by and placed directly in contact with the insulation so long as the ceiling below is both well sealed and contains a suitable vapour control layer (VCL). If these criteria are met then for most common roof coverings, except less air permeable fibre cement and metal roof coverings, no further roof space ventilation is required. However for constructions where the insulation is partially filling and/or fixed below rafters or where the insulation is on top of and/or fully filling rafters but the ceiling below cannot be well sealed or does not contain a VCL, then the roof space between the underlay and the insulation needs to be ventilated or in the latter case a gap created (with counterbattens) and ventilated.  Where a sloped ceiling and insulation deviates from the line of the rafters a gap will be created between the insulation and the underlay which needs to be ventilated e.g. a mini-loft above a room-in-the-roof with a horizontal ceiling. In all of these constructions, the precise ventilation requirements depend upon a range of factors including the ceiling quality, the roofing underlay used, the roof covering, and other aspects of the construction such as whether timber sarking is present or not. It is always advised to contact Redland Technical Solutions to discuss the specifics of your project.

What does the Building Regulations state for eaves ventilation below 15°?

Approved Document C of the Building Regulations and BS 5250: Code of practice for control of condensation in buildings state that at 15º pitch or less the level of eaves ventilation should be 25,000 mm²/m regardless of whether the insulation is laid between the ceiling joists or the rafters. With Regent tiled roofs at 15º rafter pitch and less any ventilation system that only provides 10,000 mm²/m such as the RedVent Eaves Ventilation System should not be used. RedVent 25 Over-Fascia Vent should be used. At all other pitches the choice of eaves ventilation product depends on the location of the insulation in the roof structure.

Does condensation in a roof mean the roof ventilation system has failed?

Condensation in a roof space does not necessarily mean roof ventilation systems are not performing. Other reasons may include central heating expansion tanks in the loft having no lid, bathroom extract fans blowing steam and water vapour directly into the loft, soil vent pipes venting into lofts due to missing one way valves or poorly fitting loft hatches in bathrooms and kitchens letting high volumes of water vapour vent into the loft. Problems such as these need to be eliminated first before any attempt to investigate cold bridging or deficiency in the roof ventilation system are made.

Which products can be used to ventilate a roof with Delta tiles?

With Delta tiles there is a limited range of roof space ventilation products available. At the eaves, all soffit and fascia vents are compatible. However at high level, DryVent Ridge, DryVent Monoridge and the Top Edge Abutment Ventilation System are not suitable due to the height and sharpness of the Delta profile. There are also no vent tiles suitable in the Redland range. NB. Please note that Delta tiles are no longer manufactured by Redland.