|INVESTIGATING THE NAVE ROOF
|The roof construction was investigated
by Terry Hughes on the 1st and 2nd of December 1998 (Hughes 1999). The
investigation covered an area of about two metres width from eaves to ridge
on both sides of the roof and was carried out from a scaffold at three
levels: at the eaves and at approximately 6' (1.8m) and 12' (3.6m) above
(Figs 20 and 21). This allowed all areas of the roof to be reached for
careful dismantling by hand. The main area of the scaffold at the eaves
was larger than normally required for roofing as it would later be used
for the main repair work when adequate space would be needed to sort and
redress the existing slates. Some of these were over 600 mm (24") square,
the largest weighing up to a 1 cwt (50.8 kg). The whole area under investigation
was covered and weather-proofed with scaffold sheeting. To ensure adequate
lighting for photography (plastic sheeting gives photographs a green cast)
and late working, two portable tripod mounted floodlights were used.
|The plant growth (lichens, mosses
and ferns) was so thick over most of the roof that the vertical and bottom
edges of the slates were often obscured (Figure 22). The stone slates were
therefore cleaned off with a scraper and stiff hand brush prior to progressive
stripping from the ridge. As each course was removed the exposed area below
was photographed using 400 ASA monochrome and colour negative film. The
photographs were taken from a point perpendicular to the slating wherever
the scaffold permitted and included a scale rule with 50 mm (2") divisions
(Fig 23) and a number 40 mm (1¾") tall for each course. Groups of
slates were also photographed laid out on the scaffold to record their
shape, degree of shouldering and the ratio of the length to the width.
All these factors are important in preventing the penetration of rainwater.
A description of features of each course were recorded into a dictaphone
as work progressed.
|The slate lengths below their fixing
holes, head laps and margins, the vertical dimension of the exposed area
of the slate, were measured at each course (Figs 23 and 32), except where
the courses had collapsed onto each other. In this situation only the slate
length was meaningful.
|On a roof of this type and especially
because of its deteriorated condition, measurements of head laps and margins
could not be precise. The margins varied considerably along each course.
Also, the head lap was somewhat theoretical. It is a dimension by which
the roof is set out rather than the precise head lap achieved which is
affected by the shape of the slate’s top edge and the position of the fixing
hole. The head lap achieved depends firstly on the position of the perpendicular
joint in the course above in relation to the curve of the slate head (if
it drops below the peg hole) and, secondly, on the position of the peg
hole laterally in relation to the perpendicular joint. For these reasons
it is not possible to work out any of the three measured dimensions from
the other two.
|In spite of these difficulties,
with care and a large number of measurements, it was possible to draw conclusions
about the construction of the roof, and the apparent intentions of the