 For years it has been taken for granted
that tile batten nails are 65mm long by 2.65, 3.0 or 3.35mm diameter
steel wire nails. But why?
Nail penetration
Since September 1997 it has been recommended that all battens used
for roof tiles and slates should be a minimum of 25mm thick to allow the
tile nail to get a full 25mm of grip. If we deduct the batten thickness
from the length of a 65mm long nail, the remaining 40mm of nail
penetrates into the rafter or counter batten. If batten thickness was
increased, nail penetration into the rafter would be reduced, so a
longer nail may be needed.
The figure of 40mm penetration for the batten
nail is very traditional, as in reality every roof will have a different
minimum nail penetration requirement. For instance, the nail penetration
for plain tile roofs could be less than for an interlocking tile roof.
It is better to have a nail that is longer than needed as there is a
risk that if the nail penetration is too short, serious roof damage may
occur during hurricane force winds.
Wind uplift load
Batten nails are one of the primary fixings for the transfer of wind
uplift loads on the roof into the roof structure. Wind suction acts on
the tiles, the tiles transfer some of the load via the nails or clips to
the battens, and the battens in turn transfer the load to the rafters
and so on down to the foundations. The size of the load that is
transferred from the batten to the rafter will depend upon four main
factors:
- Rafter centers. Roofs with rafters at
400mm centers have 30 percent more batten nail fixings than those
with rafters at 600mm centers.
- Batten gauge for the tiles. Plain
tiles with a batten gauge of 100mm will have 71 percent more batten
nail fixings than an interlocking tile with a gauge of 340mm.
- Dead load of the roof tile. Part of
the wind load on the exposed surface of the tile will be resisted by
the dead load of the tile. This will depend on the weight of the
tile, the distance to its centre of gravity and the rafter pitch of
the roof slope.
- Anticipated wind uplift load. This is
the predicted load that will act on the roof once in a 50 year
period, as defined by calculation in BS 5534, Code of practice
for slating and tiling: Design 1997.
Remember that battens take 100 percent of
the wind uplift load acting on the underlay when the underlay is
positioned directly under the batten.
By calculation, the design load on one
square metre of roof slope can be found. Having deducted the dead load
of the tiles the remaining load should be divided by the number of
batten nail fixings per square metre of roof, to define the withdrawal
load on each nail. |
Nail grip
The action of a wind uplift load is to pull the nail out of the
rafter. The amount of grip the nail has in the timber is critical and
determined by three main factors:
- Exposed surface of the nail. The more
exposed surface that comes into contact with the rafter timber, the
greater will be the grip. The exposed surface area is defined as the
circumference of the nail, times the nail penetration depth, less
the nail point. A nail with a larger diameter provides more grip
than one of a smaller diameter.
- Surface finish of the nail. Nails that
are smooth are easier to pull out than those with an improved
surface, such as ring shank grooves or a spiral square section.
- Timber grade being nailed into. Higher
density timber will grip the nail better than a lower density
timber.
BS 5268: Part 3: Structural use of
timber, can assist further with the resistance of different fixings to a
withdrawal load.
European standard
In January 2000, European standard BS EN 10230: Part 1: Steel
wire nails for general applications, became effective. Now all steel
batten nails should be supplied to the new standard. The 2.65mm diameter
increases to 2.7mm, and 3.35mm increases to 3.4mm. Whilst these figures
are not large, roofs constructed using batten nails to the old British
Standard 1202: Part 1: 1974 may be fit for purpose, but may not truly be
said to be constructed in accordance with current British and European
Standards, and this fact may invalidate a manufacturer's guarantee. The
specification for nails used with the new generation of nail guns fall
into the same category, especially with regard to diameter and head
design.
Galvanized nails
The new European Standard for steel nails states that the nail
diameter is measured before galvanizing. Logic says that for bending and
compressive strength the pre-galvanized dimension should be used. For
the withdrawal load on the nail, the post-galvanized dimension may be
used.
Batten nail calculations
Whilst there are so many variations to be considered, experience
tells us that for most homes in populated parts of the UK a 65mm long
batten nail is adequate. But as soon as the building rises above three
storeys, or the roofing material becomes lighter, and the exposure of
the building requires a higher level fixing specification, it is likely
that the size and type of batten nail will need to be calculated. Major
tile manufacturers will automatically calculate the tile batten nail
size at the same time as the tile fixing specification, others may not.
Advice
Seek the advice of the tile manufacturer early in the specification/
tendering process. Also check that the batten nails being used comply
with the new BS EN 10230 specification. |
