Once the fundamental timber structure of the roof was completed it was covered in a roofing “felt” held in place by roofing batons nailed horizontally to the structure. These batons not only hold the roofing felt in place but eventually are also used to attach the roof slates. The type of roof that we have chosen to implement is a “cold roof” where the roof void is unheated and the thermal insulation installed to minimise heat loss through the roof is placed in between the roof joists, immediately above the room ceilings. Conventional roofing felt is impermeable therefore In a cold roof, without ventilation of the roof space, condensation will form on the underside of roofing felt and so the roof void will be damp, particularly in winter. Tyvek is a modern, spun polyolefin, non-woven fabric membrane manufactured by DuPont. In its form as Tyvek® Supro it is a permeable membrane which is used as a roofing underlay in pitched roof construction. It forms a secondary water shedding layer ( the roof slates or tiles being the primary layer of course) that also reduces the wind load acting on the slates and adequately resists wind blown snow and dust from entering the construction. Tyvek® membranes offer benefits over traditional impermeable roofing underlays by minimising the risk of interstitial condensation occurring within roof constructions Over the last 30 years or so, as we have become more aware of the need to conserve energy and so the required levels of insulation within roofs have become greater. This has had the effect of increasing the likelihood of condensation forming on the underside of the roofing felt. Prior to the introduction of modern vapour permeable membranes, the only way of reducing this risk was to introduce ventilation openings in the roof to effectively “change the air”. Since Tyvek® is a vapour permeable material as a roofing underlay it offers low resistance to the passage of vapour. A Tyvek® underlay permits water vapour within the roof space to permeate through to the batten space. Natural air movement through the roof subsequently allows any moisture-laden air to escape to atmosphere. Roofs with no provision for airflow beneath the underlay will be more energy efficient than conventional, ventilated roofs. The ability of Tyvek® membranes to provide the function of condensation control eliminates the need to ventilate between the underlay and the insulation. A non-ventilated Tyvek® system not only prevents excessive condensation but also offers substantial gains in energy efficiency.
At the centre of the roof hidden behind the four external ridges is a substantial flat roof. This part of the roof covers the central stair hall below which goes from the ground floor all the way up to the second floor ceiling so that a central galleried staircase is formed. A pair of polygonal roof-lights set into the flat roof provide natural day light to the stair hall. These roof lights are centred on the semi-circular walls at either side of the stair hall. From time to time debris from leaf fall will have to be cleared from the flat roof section and its associated gutters. To facilitate this a hatch has been constructed into the structure to allow access via a ceiling hatch in the main guest-suite bathroom ceiling, through the pitched roof above and onto the flat roof. The flat roof structure and its access hatch are shown in the photographs below.
Three chimneys are incorporated into the flat part of the roof. Two of these chimneys are active as they act as flues for the decorative flame effect gas fires in the dinning room and drawing room. The third is there to provide symmetry but also serves a useful function for providing a “penetrator” through the roof for the cables from the antennas that will eventually be installed on the roof. The photograph below shows the fixing the final stone into place of the last chimney to be built.
The water proofing of the flat roof was achieved by using Fatra fleece-backed PVC membrane adhered directly to the roof surface. The fixing method is quite simple. A polyurethane adhesive is applied to the roof and the Fatra membrane is then rolled out on to the wet adhesive by applying pressure using a roller or soft brush. The rolled out membrane is overlapped slightly at the edges. A water tight seal at the resulting seems is ensured by using a hot air welding gun to melt the membrane slightly at the overlap so that it can be welded together by applying pressure with a Teflon coated roller. Fatra rainwater outlets are specifically designed to work with Fatra membrane. Each comes with a flange of Fatra membrane to enable the outlets to be welded to the roofing membrane and provide a jointless sealed system. At the corners of the roof Fatra preformed corners are used and at every change in direction of the membrane Fatra coated membrane trims are used for reinforcement at those points.
The timber access hatch structure was completed by attaching a stainless steel hatch lid mechanism. Owing to the weight of this hatch the opening is assisted by incorporating a pair of gas-struts into the deign. To water proof the access hatch timber structure it is entirely encased in lead sheeting.
Turning back to the main ridges of the roof once the Tyvek membrane and roof batons were in place the primary water shedding covering of slates was started. The slates we chose were from the Penrhyn Slate Quarry located near Bethesda in north Wales. Welsh slate is still deemed to be the finest available. Cheaper slate is obtainable from Spain and China but it often is plagued with iron pyrites within the slate rock formation. This eventually oxidises to iron oxide or rust owing to exposure to air and water which then results in nasty brown streaks staining the roof. Welsh Slate is exceptionally durable. It is unaffected by normal extremes of temperature and is highly resistant to acids, alkalis and other chemicals. It retains its colour, even in UV light and is impermeable to water. It is non combustible and is compatible with all other building materials. Welsh Slate roofing material is available in two colours that reflect the true nature of its beauty. These subtle and elegant colours are further complemented by the distinctive natural texture of slate, creating an added dimension to any roof. The Penrhyn Quarry slate we have chosen has natural Heather Blue tonal variations which combines with colour of the lead used to waterproof the ridges and valleys of the roof form which in my opinion makes for a beautiful overall finish.