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.
Tyvek placed over the roof rafters and held in place by the roofing batons nailed place horizontally, ready to enable the slates to be attached.
Tyvek placed over the roof rafters and held in place by the roofing batons
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.
The flat roof timber construction. Note the Tyvek membrane and roofing batons fully installed on the ridged part of the roof in the background.
The timber flat roof access hatch structure is shown here.
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.
Fixing the last chimney stone in place.
Here the construction of the roof-light upstands, the flat roof access hatch and two of the chimneys can clearly be seen.
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.
Spreading the polyurethane adhesive before laying out the fatra membrane.
Detail of the fatra roof water outlets welded in place. These outlets allow rainwater to drain out of the drainage gullies of the flat roof area. The outlets are connected to drain pipes which pass through the pitched roof void and to the outside of the house to the rainwater hoppers and drain pipes of the roof guttering.
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.
The stainless steel access hatch with its supporting timber sub-structure encased in lead for water tightness.
A close up of one of the roof-lights showing how it is mounted on to its up-stand and how the fatra membrane provides water proof integrity.
The almost completed flat roof. Note the installation of the air-conditioning heat exchangers and the air extraction fan for the kitchen.
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.
The slates were fixed in place using copper nails. At the ridges of the roof as slate will not bend over the angle of the ridge waterproofing is achieved by a lead strip which conforms to the shape of the ridge and overlaps the slate on either side of the ridge. To help form the lead and maintain it in position “mop-sticks” are nailed over lead straps on to the apex of the ridge. These are clearly visible in this photograph.
This photograph shows the lead waterproofing of a ridge and how it is formed over the ridge “mop-stick”. The lead flashing strip is held in place by bending over the lead straps that are shown under the mop-sticks in the previous photo. Note the welded end piece over the mop stick at the bottom centre of the picture.
Similarly at the valleys of the roof where the slate planes meet lead is placed in the valleys so that water is shed from the slate into the lead formed valley and then down the valley form into the gutter. This photograph shows how that lead valley is formed.
An aerial view of the roof with slates in place awaiting completion of the lead flashing to the ridges.
A view of the house with the roof completed after the scaffolding had been removed prior to re-erecting it to be suitable for the renderers. Note that the guttering is not quite complete, requiring lengths of guttering to be cut to the exact length to fill in the gaps.