GENERAL
Cross-reference
General: Read with A90 General technical requirements.
PRODUCTS
Adhesives
Hot setting phenolic and aminoplastic wood adhesive: To BS 1203. Thermosetting wood adhesive: To BS EN 12765.
Aggregates
Aggregate for mortar (sand): To BS EN 13139.
Bales
Straw: Rye or wheat.
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Condition: Fresh, dry and free from seed heads, roots, leaves, flowers, insects, grasses and other contaminants. Bale compaction: Maximum compression, using approximately one third more straw than is customary for agricultural use. Moisture content: 15% maximum by weight.Damp proof course
Bitumen polymer: Agrément certified. Polyethylene: To BS 6515.
Slate: Natural slate.
Fasteners and strapping
Bolts
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Black hexagonal bolts, screws and nuts: To BS 4190.•
Carbon and alloy steel bolts, screws and studs: To BS ISO 898-1.•
Hexagon head bolts: To BS ISO 4016. Nails•
Aluminium: To BS 1202-3.•
Copper: To BS 1202-2.•
Steel: To BS 1202-1.Stainless steel fasteners: To BS ISO 3506. Stainless steel wire: To BS EN 10088-3. Twine
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Non-metallic (woven polyester) tensional strapping: To BS EN 13394.•
Polypropylene baling twine: To BS EN 12423.Lime
Building lime: To BS EN 459-1.
Manufactured boards
Oriented strand board (OSB): To BS EN 300.
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Type: OSB/3 load bearing board for use in humid conditions or OSB/4 heavy duty load bearing board for use in humid conditions.Plywood (structural): To BS 5268-2.
Wood fibre insulation board: To BS EN 13171.
Metal laths and beads
External render: To BS EN 13658-2. Internal plaster: To BS EN 13658-1.
Steel
Angles and flats: To BS EN 845-2.
Timber
Structural timber: To BS EN 338.
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Grading: To BS 4978 and BS EN 14081-1.EXECUTION
Workmanship generally
Bale density: Bales contain one third more straw than is usual for agricultural purposes. Temporary work: Provide temporary support or bracing to structural elements.
Embedded items: Build structural frames, door and window casings/ frame reveals, pipes, conduits, pins and other hold-down and bracing elements, into walls as the construction proceeds.
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Voids at or around embedded items: Pack with loose straw.Installing damp-proof courses
Installation generally: Form a continuous barrier to rising damp for the full width of wall.
Junctions: Preserve continuity of damp-proofing at junctions of damp-proof courses and waterproof membranes.
Laying bales
Moisture content at laying (maximum): 15%.
Daily progress: Establish leads at corners. Step back incomplete walls, do not tooth. Voids at dissimilar materials: Pack with loose straw.
Joints: Full flush type, with no open joints.
Bond: Running bond. Overlap units 100 mm minimum, preferably half a bale.
Pre-compression: Uniformly pre-compress the walls with strapping. Fit tightly so that it is difficult to get fingers under the strapping. Protect strapping with plastic tubing at wall plate.
Adjustment: For unframed construction, compensate for variations in compressed bale height by packing up the wall plate. Tying to frame, for framed construction: Strapping or twine stapled to the frame above and below bale.
Cut bales: Use baling 'needles' to bind both parts of a bale before cutting the temporary binding.
Inspection: Give notice at completion of wall bale placement including strapping, and before plaster is applied.
Installing fabricated mild steel lintels
Assembly: Weld flats to steel angles to form a cradle into which the bales can fit. Bearing depth: 400 mm, extending at least half a bale either side of the opening.
Location: Sit directly above window/ door casing/ frame reveal. Leave a 75 mm settlement gap.
Thermal insulation: Wrap tightly around lintel so that steel work doesn't come into contact with top head and straw.
Installing holding-down bolts
Cover (minimum):
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To face of bale: 75 mm.•
To end of wall: 150 mm.Structural integrity: Connect wall plates to footings.
Applying adhesives
Surfaces: Clean. Adjust regularity and texture to suit bonding and gap filling characteristics of adhesive.
Support and clamping during setting: Provide as necessary. Do not mark surfaces or distort components being fixed. Finished adhesive joints: Fully bonded. Free of surplus adhesive.
Installing flashings and weatherings
Lap joint treatment:
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Vertical and sloping flashings/ trims: End laps to be same as for adjacent sheeting.•
Horizontal flashings/ trims: End laps to be 150 mm, sealed and where possible arranged with laps away from prevailing wind. Laying: In long lengths, lap full width at angles and intersections and at least 150 mm at joints. Step as necessary for stepped footings.Earth plastering and rendering
Application on unframed construction: Only after all elements being supported have been completed.
Application on framed construction: When all built-in elements have been completed and the straw bales have been pre- compressed.
Embedded timber: Wrap 40 x 40 x 1.4 mm wire mesh around embedded timber elements that will be covered with plaster. Metal lath: Fix to exposed faces of bale walls at 450 mm nominal centres, and to wall plates.
Scratch coat: Apply as thickly as necessary, to even out the walls.
Brown coat: Apply after the scratch coat has set, to give the walls their final shape and form.
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Protection: After initial set, keep the surface moist for 7 days.•
Inspection: Give notice when the brown coat has fully set.Surface coat: Provide a 'rough' surface to the scratch and brown coats of plaster, through either a porous or scratched surface, to improve adhesion of the final coat of plaster.
Final coat: Apply as thinly as practicable, to give the walls their final appearance.