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Global warming, we are told, could make
lightning a more common climatic condition that we should be prepared
for. While not every roof needs a lightning protection system, those
that do need to resist lightning, deluge rainfall and high winds. To
achieve this, it is essential that the lightning protection system is
integrated with the roof covering.
All lightning protection systems should comply with BS6651 and be
designed, installed and tested by specialist lightning protection
engineers. Often this is either done after the roof has been completed,
or in two phases: after the felt and battens, and on completion of the
Problems can occur at a later date if there is no
coordination between the roof installation and the lightning protection
system. Even where there is good coordination between the trades, there
are fixing problems that need to be solved. When re-roofing is
undertaken around an existing lightning protection system, it should be
checked on completion of the works by a qualified lightning protection
There are three main systems:
- Air terminals rise above the ridge
and protect an area around it; some of which have ionising devices
that positively attract lightning streams to them. Each air terminal
is then connected to the earth by the shortest route.
- Tapes are conductors that are
exposed above the roof covering, located along all ridge, eaves,
verge and hip lines; all connected as a grid to one or more earth
- Strike pads have tapes distributed
below the tiles/slates, and only the strike pads are seen above the
Air terminals are copper rods 0.5m to 2.0m long, often with a multiple-
pointed end that extends up into the air. The plate at the base of the
rod takes the full force of the wind on the rod.
The recommended fixing method for the air terminal
bracket is to fix it to the apex of the ridge tiles using 38mm-long wood
screws and wall plugs. This recommendation may result in the ridge tile
and air terminal being blown over. It is better if the air terminal
bracket is screw-fixed directly to the roof structure, and the 15mm
copper rod is passed up through the joint between two ridge tiles. Or
with a dry ridge, up through a hole in the centre of a ridge tile and
sealed with mastic to prevent water tracking down the rod. If the
bracket is fixed directly to the roof structure, then the conductor tape
is to be distributed below the roof covering.
The tapes are soft copper or aluminium strips, some of which are coated,
and vary in size from 20mm x 3mm to 50mm x 6mm in section. They are held
onto the roof with plastic DC clips that are either glued onto the outer
face of a tile or slate with a round pad, or fixed using a slate
holdfast, which is a DC clip on the end of a 500mm-long aluminium strip.
The fixing of the DC clip is critical as it has been known for hurricane
force winds to lift the tape at a vulnerable point, like a corner, and
cause it to flail around doing further damage to the roof and the
The adhesive pads are effective on smooth surfaces,
such as resin slates, FC slates, some profiles of smooth faced concrete,
and clay tiles; but they are not suitable for granular-faced tiles,
tiles with no large flat surface, or natural slates, as the adhesive can
pull off the outer flake of slate.
Slate holdfasts are popular in most
situations as they will work with almost any type of slate or tile. With
natural/FC slate and interlocking slate – like Cambrian – the fixings
should be installed when the tiles or slates are laid. The correct
procedure is to nail the aluminium tape to a batten; either through the
gap between the tiles, or using the head nail fixing of an interlocking
tile. The DC clip should hang just below the leading edge of the upper
tile so as not to obstruct it, yet allow space for the tape to be
inserted in the clip. If the DC clip is too far below the leading edge
of the tile above, the wind can lift and bend the aluminium strip. Too
often the holdfast straps are pushed up between the slates or, at best,
bent over the head of a tile to stop it sliding out. This places the
full wind uplift load from the conductor tape onto the tile above the
holdfast strap and, if not nailed or clipped adequately, may pull the
tile off with the conductor tape.
Each strike pad, like an air terminal, will protect an area around the
pad, so they are located at regular intervals along the ridge, eaves,
verge and hip of the roof. The tapes are distributed below the roof
covering and short tapes ‘T’ off through the roof covering to a plate –
often a square tape clamp.
At a mortar-bedded hip, or ridge, the tape is often fed
through the mortar joint, but elsewhere the tape needs to pass either
through the head-lap of the tiles or through a vent tile. Either way,
unless a groove is cut in the tile, or a slot is cut through the
ventilation grill in the vent tile, the tape will kick up the tile
This can be a problem at low rafter pitches, when it
can affect the ability of the roof covering to keep out wind-driven rain
and large insects.
While most inclined valleys will not require protection, tapes will
often either be laid down the valley as an easy route from the ridge to
the eaves, or will cross the valley to get between one eaves and
another. Where this occurs, the flow of water down the valley can be
disturbed and water can track along the conductor tape instead of going
into the gutter.
It is essential to connect all metal valley, metal roof
windows and metal ridges to the lightning conductor tape, but an
aluminium conductor tape should not come into contact with a lead sheet
valley as the electrolysis will eat away the aluminium tape.
It is common for aluminium gutter systems to be used as the
lightning protection at the eaves. In this situation, cable bridges
between each section of guttering are required.
- Slate holdfast lightning protection
tape fixings should be nailed or screwed to a batten.
- Air terminals should be screwed
directly to the roof structure.
- The tape serving a strike pad should
not be kicked up a tile or slate as it will affect its performance.
- Aluminium conductor tapes should not
come into direct contact with lead sheet valleys or flashings.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774