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The story goes that clay hip tiles were
moulded on the upper thighs of ladies working in the factories making
tiles. These tiles were tapered such that they lapped over each other.
Ladies thighs, hips, can you see the association. The problem with this
story is that a valley is the opposite of a hip and valley tiles are
also tapered like hip tiles, so why are valley tiles not called hips?
Tapered hip tiles are still available in Europe but
here in the UK we have opted for straight hip tiles for use with
interlocking tiles and slates, and with plain tiles where the rafter
pitches are different.
What is a hip?
A hip is an external junction between two inclined roof slopes that are
in the same horizontal plane. The plan angle between the two roof slopes
can be any angle from 1 to 179 but is generally 90º, also any angle
greater than 135º is difficult to form as the thickness of the roof
construction is difficult to close off.
The rafter pitch of the adjacent roof slopes can be any
angle but is generally the same as differing rafter pitches which can
create complications. At a hip with identical rafter pitches rainwater
will run away from the hip on both sides, but if the pitches are not at
90º and the rafter pitches are different then this may not apply and
water may run towards the hip on one side, creating a problem.
Provided that the plan angle is 90º, and the rafter
pitches are the same, then it is fairly simple to weather the joint
using clay or concrete hip tiles, as they will sit square and central
over the hip rafter, but with differing rafter pitches the hip tiles
will not sit square, and with an angle pattern this will be visually
evident, therefore it is best to use angle hip tiles on roofs with equal
pitches.
Because the dihedral angle between the two roof slopes
is greater than for the two roof slopes meeting at a ridge, hip tiles
are shallower. This means that for curved hip tiles the top surface is
reduced from a half round to a third round. This also means that the hip
tiles are narrower than a ridge tile.
To allow for the hip tiles to lap over the tiles on
each adjacent roof slope by 75mm and for the vertical cut of a high
profile of a double Roman or pantile profile, the hip tile needs to be
approx 220mm wide.
If the rafter pitch is greater than 60º then the
third round hip tile will not be able to span the junction correctly,
and a half round ridge tile should be used. The exact pitch at which the
change takes place will depend upon the roof tile and hip tile profile.
To prevent the hip tiles sliding off the roof it is
essential that a 6mm thick galvanised or stainless steel hip iron is
twice screwed into the hip rafter. This can be a problem if there is no
hip rafter. If there is a hip batten located up the full length of the
hip then the hip iron can be screwed to the hip batten. The mortar
bedding should be continuous along all four edges of the hip tile to
achieve 50mm of surface contact of both surfaces. It is essential that
the mortar is placed under all four edges before the hip tile is placed
and levelled into position as if the mortar is buttered under the top
edge after it has been placed, the mortar will not adhere to the under
surface and a crack will form letting rainwater in under the hip tile.
The cut tiles that form the edge of the roof slope at
the hip must each be mechanically fixed as they are edge tiles. If the
head of the tile is cut off then additional nail hole fixings will be
required, to allow the cut tile to be nailed into a parallel batten, or
if interlocking half tiles are available they can be used to make the
edge cut tile bigger. |
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Where the tiles on the roof are all clipped
it may be difficult to clip the cut edge tile and therefore it is
essential that the hip tiles are all mechanically screw fixed to a hip
rafter or batten to hold down the cut tiles especially on the right hand
side of the hip.
Above a true hip rafter pitch of 45º (55º rafter pitch) all
hip tiles must be mechanically fixed regardless of the use of mortar
bedding. This will require the hip rafter to be raised to the top
surface of the tile battens and the tile battens fixed to noggins up the
side, or a hip batten fixed above the tile battens and strapped down to
the hip rafter between the tile battens. If each ridge tile is
mechanically screw fixed to the hip batten, or rafter, through the
centre of each hip tile then there is no need for a hip iron at the
bottom.
Where there is a deep trough profile in a tile, dentil
slips should be used to reduce the effective mortar bed to no more that
15mm of wet mortar between the hip tile and the roof tile. If the mortar
is greater than 15mm there is the risk of slump in the mortar which will
pull away from the underside of the hip tile and leave it vulnerable to
water ingress and wind damage. The amount of visible dentil slip is
optional.
At the apex of two hips meeting a ridge the hip tiles
should be cut to a tight mitre using a full tile and bedded onto a lead
saddle and screw fixed into the hip batten/rafter. The ridge tile should
be mitred into the hip mitre but may have to be packed up to meet the
hip mitre. The amount of lift will depend upon the pitch of the roof and
the plan angle of the hips.
Conclusion
Provided the two roof slopes meet at 90º and the two roof slopes are the
same rafter pitch then the construction of the hip using a mortar bedded
hip tile over the junction is fairly straightforward. But outside of
this condition the construction of the hip becomes more complicated and
prone to problems.
Tips
- Wherever possible design and
construct the roof with 90º plan angles and equal rafter pitches.
- All cut edge tiles must be
mechanically fixed to the roof battens as reliance on mortar to hold
the small cuts in place will compromise the security of the hip
system.
- In exposed locations and on steep
rafter pitches all hip tiles should be mechanically fixed.
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| Compiled
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774 |
