RROJECT TITLE Project No; 18
Cavity Wall Gable end Incorporating Scaffolding CORE
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Airbricks and Ducting
To provide ventilation to foodstores and bathrooms an airbrick is built into the external wall and ducted across the cavity through the inner leaf.
Construction of the duct should provide a sealed unit to prevent cold air entering the cavity and the stagnant air from the cavity from entering the building.
Lightweight concrete or clay ducting units are manufactured in a range of shapes and sizes.
As with all openings through cavity walls the ducting requires a DPC tray and ‘weep holes’.
Sealing Cavity at Eaves level
The eaves are the part of the building where the walls and the roof come into contact. The cavity has to be sealed bringing the inner and outer leaves into contact.
A DPC membrane maybe specified at this point but it is not always considered essential as the overhang of the roof provides adequate protection. The contact with the roof and walls creates a ‘cold bringing’ situation which can be overcome by continuing the loft insulation over the top of the wall.
The soffit must be ventilated to allow a free flow of air into the roof space to protect timbers from fungal attack.
Wallplate Anchor straps
Galvanised restraint straps are required to secure the wallplate to the top of the inner leaf.
They may be cranked and built into the blockwork or fixed to the blockwork using plugs and screws. The distance apart should not exceed 1.2m.
Brick Features – Brick Knee, Soldier Course.
Brick and Tile knee’s
‘Brick knee’ is the term given to the corbelled brickwork used on gable end to close the void left by the overhang of the roof.
Current practice is to replace timber barge boards are with brick work features to reduce the ongoing maintenance costs of preserving timber.
Soldier Courses (Brick on end)
Soldier courses can be used as a decorative feature above door and window openings.
Care must be taken to ensure each brick is upright as errors are easily spotted.
Setting out Mitred Cuts
Method of Setting out Mitred Cuts
Where brick-on-end or brick-on-edge cappings are used on ramps or gable ends the mitred cuts should be set out full size and angles taken off the drawing with an adjustable bevel.
Sequence of setting out
Building Gable Ends
The two main skill areas involved in building gable ends are:
1. the alignment of the wall face
2. the alignment of the raking cut
Alignment of the Wall Face
As the gable is reducing in size on every course, it is difficult to erect corners. Temporary profiles or a ‘deadman’ can be used o speed up the process.
Alignment of Raking Cut
The angle of rake is established by the trusses. They also provide a means of securing lines to ensure the correct angle is maintained throughout the full height of the gable.
Stability of Masonry Walls
In most instances the roof framework will be required by building Regulations to give stability to gable walls.
This can be best achieved by correct bracing of the trussed rafters and the provision of adequate connection between the masonry and the trussed rafters.
This connection takes the form of 2m long galvanised steel straps (5mm x 30mm), fixed over at leased three trusses, and then built into the inner leaf of the cavity wall and turned through 90 degrees into the cavity.
They should be fixed on the underside of the rafters and on the top side of the ceiling joint.
The distance apart should not exceed 2m and they should be fixed to the timber member with a minimum of four screws or nails to each timber member.
Restraint straps should be positioned as shown above, including to separating walls. They should not exceed 2m centres.