Roof Slates Melting in the Sun

A manufacturer produced an imitation Welsh roofing slate using slate particles bonded together with resin.  The manufacturer had the product independently tested by a reputable independent body and published the results.  The tests showed his man-made slate to be a serviceable product.

In time failures became apparent.  Initial inspection of one poorly performing roof showed many slates to have concave curves, and the tiling to be loose.  It was decided to re-roof with the same materials, which the slate manufacturer agreed to provide free of charge. 

The remedial work commenced in early summer. 
The new tiles were checked and appeared to be fine.  After the first slope to be renewed was about one quarter tiled, many slates were found to be loose.  Initially, poor workmanship was suspected and the nailing was checked.   Some nail heads were found to be proud of the slates. 

Under the midday sun, the foreman noted that he could bend and straighten new slates.  This plasticity was not apparent in the ‘dished’ slates already laid. 

Further study revealed that when the new slates were first heated in the sun they became plastic.  This ‘second’ heating fully cured the resin, so setting the slates in the shape they adopted in the midday sun.

The tiles were fixed as centre-nailed slates.  As the slating progressed the load on the lower tiles increased.  Under their first exposure to full sun the tiles softened and deformed to settle down onto the slating battens under the load of the slates in the courses above.  This allowed the tiles to become loose and made it appear that the nails had not been drive fully home.  One they cooled, the tiles set and no longer softened when hot.

For each production run, the manufacturers sourced the products used on the open market, unaware that the resins they bought could exhibit significantly differing characteristics despite each supply having the same generic description.  The tiles failed when the resin was sufficiently different to that used in the development and initial testing of the product for it not to be fully cured in the manufacturing process.  The result was that many tiles were delivered to building sites as a rigid material only to become plastic when heated, after which they cooled and became rigid. 

The manufacturer went out of business rather than face up to the cost of replacing the defective material he had supplied.  This was a third-generation resin-bonded man-made slate.  The technology was not new.  A fundamental error in manufacture was made which was not discernible by any normal site check.  It came to light because of the re-roofing work carried out in hot weather under close supervision.