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All materials and components have a
different weight and depending upon who you talk to, they will tell you
that weight is either good, bad, or indifferent to the performance of
the roof covering.
Generally products that are lighter are easier to
lift, transport and install and therefore make the product prior to
installation cheaper than heavy products. But once installed weight is
less of an issue and often weight can be an advantage. Often heavy
things are thicker and therefore take up more space, but this may be
offset by other properties that are inherent in the material such as
sound deadening. By looking at the effect of weight and the factors that
are affected it can be seen where weight is good or bad. Different types
of materials have varying densities that are fairly constant throughout
the life of that material. Density is easiest to understand as the
weight of one cubic metre of material. One cubic metre of pure water
weighs 1,000kg, therefore we refer to it as being 1,000kg/m3.
But what about the other materials we use in roofing?
Lead is 11,389kg/m3, more than 11
times heavier than water. Steel is 7,480-8,000kg/m3 depending
upon the grade of steel.
Slate is 2,691-2,800kg/m3, depending upon
the deposit.
Aluminium alloy is 2,560- 2,800kg/m3 depending
upon the alloy.
Concrete is 2,162-2,403kg/m3 depending upon the mix.
Glass is 2,000-8,000kg/m3 depending upon the type of
glass.
Plastic is 1,380-2,300kg/m3 depending upon the
type of plastic.
Fired clay is 1,362kg/m3.
Wood is 350-1,370kg/m3 depending upon the species.
From this list we can see that slate and aluminium are
heavier than concrete, and plastic is heavier than fired clay, which
seems a little strange. We think of concrete as being heavy and
aluminium as being light. The truth is that metals and slate have a
greater strength for their weight than concrete, and therefore we tend
to use much less aluminium, or slate, to achieve what would be possible
with concrete. Because concrete is made from abundant and relatively
cheap materials, we can afford to use more of it to achieve the same
strength. Therefore the overall product weights more because it is
thicker.
It is fair to say that strength is also a function of
design, and therefore by designing in ribs and flanges, strength can be
induced where it is needed. With plastics, air bubbles are often
introduced to reduce weight without impairing the strength. Often other
materials are introduced as fillers to reduce the total content of the
plastic materials such as pvc, which, being a petrochemical product, is
expensive. But for this discussion we are looking at materials in their
basic form.
Thickness
Products that are thin can bend easily while products that are thick are
generally more rigid. There are exceptions to every rule, such as roof
slates which are very rigid for their thickness of 4- 10mm. Solid
plastic, or steel, of the same thickness would be less rigid. Rigidity
in this instance is the deflection over a given span for a given load.
Some materials, such as lead sheet, are unable to span over any
distance, unless they are fully supported, as, over time, they deform.
Performance
There are many climatic conditions that act on a roof and need to be
resisted, from sunshine to wind and rain. But as far as weight is
concerned the two major factors are dead load (+), which is the total
weight of the roof on the structure below, and wind uplift (-), which is
the effects of wind trying to suck the roof covering off.
For dead load, weight is bad as the more weight there
is the stronger the structure below has to be. For wind uplift the
situation is the opposite, the more weight the better. Think of the
effect of a light breeze on the papers on a desk if the window is left
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If the pages are loose they will blow
around, if they are bound together as a book then the pages may flutter
but the book is unlikely to blow away. If the book is replaced with a
block of wood of the same size it will not move unless there is a storm
force wind. If a steel plate of the same dimensions replaces the block
of wood, higher winds would be needed to move it. The higher the wind
speeds the heavier the component needs to be to resist the wind pressure
and suction. As the wind gets stronger and faster, so the aerodynamic
effects of the shape of the product on the roof, and the shape of the
roof itself becomes more important.
A lightweight product can be given more
resistance to wind uplift by fixing the tile or slate to the roof
structure using mechanical fixings in the form of nails, screws or
clips. Whilst minimal dead load can be supplemented by mechanical
fixings, maximum dead load can only be resisted by a substantial roof
structure, which costs extra money.
Earlier we said that thicker components tend to be more rigid. But
thickness has other effects. Firstly thicker tiles and slates need
longer nails or screws to fix them to the battens as in most cases the
fixing needs to penetrate the batten between 15 and 23mm to achieve the
maximum holding down force.
The thicker a tile or slate, the more each course of
tiles or slates will kick up on the course below. Long thin slates may
lay at only 3° less than the rafter pitch, while short thick plain tiles
may lie at between 10-13° less than the rater pitch. Whilst all
manufacturers quote rafter pitch parameters in their literature, the
critical figure is always the true tile pitch. It is true that
variations in head-lap will alter the true pitch angle, but not
significantly and this is accounted for in the manufacturer’s
recommendations. In addition to the tiles or slates lying at a shallower
angle, thicker slates and tiles have an increased step height on the
leading edge. Whilst visually this can make a big difference, in high
winds the step height of the tiles or slates, facing the wind will
create a rougher surface for the wind to blow over and can create more
turbulence, which can create some uplift.
Conclusion
Weight and thickness are interrelated, and using them in the right place
is important when deciding what material, fixing, or component should be
used. In most instances this experience has been determined through
custom and practice, but for new products, systems and materials this
has to be carefully considered.
Tips
- Lightweight slates or tiles need
more fixings to resist wind uplift.
- Expensive materials are generally
formed into the thinnest and lightest shape possible to keep the
weight and cost down.
- If you change the thickness of a
component you will affect the fixing length as well.
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