G20
G20 CARPENTRY/ TIMBER FRAMING/ FIRST FIXING
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TIMBER PROCUREMENT
Timber (including timber for wood based products): Obtained from well managed
forests/ plantations in accordance with:
- The laws governing forest management in the producer country or countries.
- International agreements such as the Convention on International Trade in
Endangered Species of wild fauna and flora (CITES).
Documentation: Provide either:
- Documentary evidence (which has been or can be independently verified)
regarding the provenance of all timber supplied, or
- Evidence that suppliers have adopted and are implementing a formal
environmental purchasing policy for timber and wood based products.
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STRUCTURAL SOFTWOOD NEW SECTION OF STRUCTURAL POST
Grading standard: To BS 4978, BS EN 14081-1, or other national equivalent and so
marked.
- Timber of a target thickness less than 100 mm and not specified for wet
exposure: Graded at an average moisture content not exceeding 20% with no
reading being in excess of 24% and clearly marked as ‘DRY’ or ‘KD’ (kiln dried).
- Timber graded undried (green) and specified for installation at higher moisture
contents: Clearly marked as ‘WET’ or ‘GRN’.
Strength class to BS EN 338: C24
Treatment: Tanalised or double vacuum treated and cut ends thoroughly treated
before fixing
- Standard: To NBS section Z12 and British Wood Preserving and Damp-proofing
Association Commodity Specification C8
- Type: CCA
WOOD TRIM AND CLADING SECTIONS
Species: Douglas Fir of Canadian source with FSC certification or ‘chain of custody
for forest products originating from a designated forest area.’
Standard: To BS 1186-3.
- Class: 2nd clear and better
Treatment: Kiln dried to moisture content between 12-15%
Fixing: Stainless steel screws where concealed, Stainless steel losthead screws,
punched and stopped to facing work.
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SELECTION AND USE OF TIMBER
Timber members damaged, crushed or split beyond the limits permitted by their
grading: Do not use.
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NOTCHES, HOLES AND JOINTS IN TIMBER
Notches and holes: Position in relation to knots or other defects such that the
strength of members will not be reduced.
Scarf joints, finger joints and splice plates: Do not use without approval.
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PROCESSING TREATED TIMBER
Cutting and machining: Carry out as much as possible before treatment.
Extensively processed timber: Retreat timber sawn lengthways, thicknessed,
planed, ploughed, etc.
Surfaces exposed by minor cutting/ drilling: Treat with two flood coats of a solution
recommended by main treatment solution manufacturer.
G
Structural/ Carcassing metal/ timber (continued)
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MOISTURE CONTENT
Moisture content of wood and wood based products at time of installation: Not more
than:
- Covered in generally unheated spaces: 24%.
- Covered in generally heated spaces: 20%.
- Internal in continuously heated spaces: 20%.
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BOLT ASSEMBLIES TO NEW SECTION OF STRUCTURAL POSTS
Designation: Stainless steel Coach Bolt fixings
Size: 12mm
Nuts and washers: Material grade and finish to suit bolts
Washer dimensions: Diameter/ side length of washers in contact with timber faces
to be minimum 3 times bolt diameter, with a thickness not less than 0.25 times bolt
diameter.
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BOLTED JOINTS
Bolt spacings (minimum): To BS 5268-2, table 81.
Holes for bolts: Located accurately and drilled to diameters as close as practical to
the nominal bolt diameter and not more than 2 mm larger.
Washers: Placed under bolt heads and nuts that would otherwise bear directly on
timber. Use spring washers in locations which will be hidden or inaccessible.
Bolt tightening: So that washers just bite the surface of the timber. Ensure that at
least one complete thread protrudes from the nut.
- Checking: At agreed regular intervals. Tighten as necessary.
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G20
TEMPORARY BRACING AND SUPPORT DURING CONSTRUCTION
Provision: As necessary to maintain structural timber components in position and to
ensure complete stability during construction. All to be agreed with CA prior to
construction.
H21
H21
TIMBER WEATHERBOARDING
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TIMBER WEATHERBOARDING TO ALL SURFACES AS DETAILED
Boarding:
- Quality of timber (exposed surfaces): To BS 1186-3,
Class: 2nd clear and better
- Species: Douglas Fir of Canadian source with FSC certification or ‘chain of
custody for forest products originating from a designated forest area.’ Paint
finished (M60)
- Profile: Tongued and grooved.
- Finished face dimension (overall width): to details
- Finished thickness: to details
- Moisture content at time of fixing: Kiln dried to moisture content between 12-15%
- Treatment: Preservative impregnation
Standard: To NBS section Z12 and Wood Protection Association Commodity
Specification C6
Type: Cuprinol Wood Preserver Clear
- Method of fixing to each support: Stainless steel screws where concealed,
Stainless steel losthead screws, punched and stopped to facing work.
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TREATED TIMBER
Surfaces exposed by minor cutting and/ or drilling: Treat with two flood coats of a
solution recommended for the purpose by main treatment solution manufacturer.
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SURFACE TREATMENT
Finishing system: Before fixing boards, apply first coat of specified system to all
surfaces. Apply liberally to end grain.
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FIXING BOARDING
General: Fix boards securely to give flat, true surfaces free from undulations,
lipping, splits, hammer marks and protruding fasteners.
Movement: Allow for movement of boards and fixings to prevent cupping, springing,
excessive opening of joints or other defects.
Heading joints: Position centrally over supports and at least two board widths apart
on any one support.
Nail heads: Punch below surfaces that will be seen in the completed work.
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H71
H71
LEAD SHEET COVERINGS/ FLASHINGS
02
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ROOFING TO TOP OF TOWER
Underlay: Geotextile underlay- between 200 and 220 gm /sq m
Lead: Rolled lead Sheet
- Thickness: Code 6
Joints in direction of fall: Hollow core rolls
Spacing: As shown on the drawings
Cross joints: Welts as shown on the drawings
- Spacing: As shown on the drawings
Intermediate fixings: Capped screw fixing with allowance for thermal movement as
detailed. And stainless steel clips.
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60
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WEATHERING TO SURROUND OF HATCH
Underlay: Waterproof building paper to BS 1521, class A.
Lead: Rolled lead Sheet
- Thickness: code 6
Joints:
- Cross joints: Welts
FLASHINGS TO BASE OF TOWER
Lead:
- Thickness: Code 6
Dimensions:
- Lengths: Not more than 1500 mm.
- End to end joints: Laps not less than 100 mm.
- Upstand: Not less than 85 mm.
- Cover to roof: Not less than 150 mm.
Fixing:
- Top edge: Lead wedges at every course.
- Bottom edge: Clips.
Material: Stainless steel
Spacing: 300mm centres
MATERIALS AND WORKMANSHIP GENERALLY
Lead production method:
- Rolled, to BS EN 12588.
- Machine cast: BBA certified.
Identification: Colour marked for thickness/ code, weight and type.
Workmanship standard: To BS 6915 and latest editions of 'Rolled lead sheet. The
complete manual' published by the Lead Sheet Association.
Fabrication and fixing: To provide a secure, free draining and weathertight
installation.
Marking out: Do not use scribers or other sharp instruments to mark out lead
without approval.
Solder: Use only where specified.
Finished leadwork: Fully supported, adequately fixed to resist wind uplift but also
able to accommodate thermal movement without distortion or stress.
Patination oil: Apply smear coating to all visible lead, evenly in one direction and in
dry conditions.
LEADWELDING
In situ leadwelding: Not permitted.
H
Cladding/ Covering (continued)
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TIMBER FOR USE WITH LEADWORK
Quality: Planed, free from wane, pitch pockets, decay and insect attack (ambrosia
beetle excepted).
Moisture content: Not more than 22% at time of fixing and covering. Give notice if
greater than 16%.
Preservative treatment: Organic solvent as section Z12 and Wood Protection
Association Commodity Specification C8.
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H71
UNDERLAY
Handling: Prevent tears and punctures.
Laying: Butt or overlap jointed onto a dry substrate.
- Fixing edges: With copper or stainless steel staples or clout nails.
- Do not lay over roof edges.
- Turn up at abutments.
Wood core rolls: Fixed over underlay.
Protection: Keep dry and cover with lead at the earliest opportunity.
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FIXING LEAD SHEET
Top edge: Secured with two rows of fixings, 25 and 50 mm from edge.
Fixings:
- Nails to timber substrates: stainless steel (austenitic) clout nails to BS 1202-1.
Shank type: Annular ringed, helical threaded or serrated.
Length: Not less than 20 mm or equal to substrate thickness.
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CLIPS
Material:
- Stainless steel: Cut from 0.46 mm sheet to BS EN 10088, grade 1.4301(304),
terne coated if exposed to view.
Dimensions:
- Width: 50 mm where not continuous.
- Length: To suit detail.
Fixing clips: Secure each to substrate with either two screw or three nail fixings not
more than 50 mm from edge of lead sheet. Use additional fixings where lead
downstands exceed 75 mm.
Fixing lead sheet: Welt clips around edges and turn over 25 mm.
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WELTED JOINTS
Joint allowance: 50 mm overlap, 25 mm underlap.
Stainless steel clips: Fix to substrate at 450 mm centres.
Overlap: Welt around underlap and clips and lightly dress down.
M60
M60 PAINTING/ CLEAR FINISHING
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GLOSS PAINT EXTERNAL TO EXISTING JOINERY AND METAL WORK
Manufacturer: ICI Dulux
- Product reference: Weathershield Exterior Gloss System.
Surfaces: External timber facades
- Preparation: as clauses 30, 32, 37, 39, 61, 65, 66, 68.
Initial coats: Prime unprimed joinery or factory primed joinery which has become
eroded with Weathershield Exterior Preservative Primer+. Prime any bare metal
with Dulux Metal Primer.
- Number of coats: 1 coat (if required as above).
Undercoats: Weathershield Exterior Flexible Undercoat.
- Number of coats: 2 coats.
Finishing coats: Weathershield Exterior High Gloss.
- Number of coats: 1 coat
GLOSS PAINT EXTERNAL TO NEW JOINERY
Manufacturer: ICI Dulux
- Product reference: Weathershield Exterior Gloss System.
Surfaces: New factory primed or unprimed joinery. External timber facades.
Preparation: as clauses 30, 37, 61, 65, 68.
Initial coats: Prime unprimed joinery or factory primed joinery which has become
eroded with Weathershield Exterior Preservative Primer+. Prime any bare metal
with Dulux Metal Primer.
- Number of coats: 1 coat (if required as above).
Undercoats: Weathershield Exterior Flexible Undercoat.
- Number of coats: 2 coats.
Finishing coats: Weathershield Exterior High Gloss.
- Number of coats: 1 coat
COATING MATERIALS
Manufacturers: Obtain materials from any of the following: ICI Dulux, Akzo Nobel
Decorative Coatings, Farrow and Ball (Specialist surfaces or paint colours).
Selected manufacturers: Submit names before commencement of coating work.
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SURFACES NOT TO BE COATED
BELL AND BELL ACCESSORIES, INTERNAL TIMBER, PROTECT ROOF BELOW
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SURFACES TO BE CLEANED BUT NOT COATED
BELL AND ACCESSORIES
Suspected hazardous materials: submit method statement.
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PREPARATION GENERALLY
Standard: In accordance with BS 6150.
Risk assessment and method statement for hazardous materials: Prepare for
operations, disposal of waste, containment and reoccupation, and obtain approval
before commencing work.
Preparation materials: Types recommended by their manufacturers and the coating
manufacturer for the situation and surfaces being prepared.
Substrates: Sufficiently dry in depth to suit coating.
Efflorescence salts, dirt, grease and oil: Remove.
Surface irregularities: Provide smooth finish.
Organic growths and infected coatings:
- Remove with assistance of biocidal solution.
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M
Surface finishes (continued)
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32
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35
37
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M60
- Apply residual effect biocidal solution to inhibit regrowth.
Joints, cracks, holes and other depressions: Fill with stoppers/ fillers. Provide
smooth finish.
Dust, particles and residues from preparation: Remove and dispose of safely.
Doors, opening windows and other moving parts:
- Ease, if necessary, before coating.
- Prime resulting bare areas.
PREVIOUSLY COATED SURFACES GENERALLY
Preparation: In accordance with BS 6150, clause 11.5.
Contaminated or hazardous surfaces: Give notice of:
- Coatings suspected of containing lead.
- Substrates suspected of containing asbestos or other hazardous materials.
- Significant rot, corrosion or other degradation of substrates.
Risk assessment and method statement for hazardous materials: Prepare for
operations, disposal of waste, containment and reoccupation, and obtain approval
before commencing work.
Removing coatings: Do not damage substrate and adjacent surfaces or adversely
affect subsequent coatings.
Loose, flaking or otherwise defective areas: Carefully remove to a firm edge.
Alkali affected coatings: Completely remove.
Retained coatings:
- Thoroughly clean.
- Gloss coated surfaces: Provide key.
Partly removed coatings: Apply additional preparatory coats.
Completely stripped surfaces: Prepare as for uncoated surfaces.
FIXTURES AND FITTINGS
Risk assessment and method statement for hazardous materials: Prepare for
operations, disposal of waste, containment and reoccupation, and obtain approval
before commencing work.
Removal: Before commencing work remove: Bell and bell accessories.
Replacement: Refurbish as necessary, refit when coating is dry.
WOOD PREPARATION
General: Provide smooth, even finish with lightly rounded arrises.
Degraded or weathered surface wood: Take back surface to provide suitable
substrate.
Degraded substrate wood: Repair with sound material of same species.
Heads of fasteners: Countersink sufficient to hold stoppers/ fillers.
Resinous areas and knots: Apply two coats of knotting.
Defective primer: Take back to bare wood and reprime.
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STEEL PREPARATION
Corrosion and loose scale: Take back to bare metal.
Residual rust: Treat with a proprietary removal solution.
Bare metal: Apply primer as soon as possible.
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COATING GENERALLY
Application: In accordance with BS 6150, clause 9.
Conditions: Maintain suitable temperature, humidity and air quality.
Surfaces: Clean and dry at time of application.
Thinning and intermixing: Not permitted unless recommended by manufacturer.
Priming coats: Apply as soon as possible on same day as preparation is completed.
Finish:
M
Surface finishes (continued)
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M60
- Even, smooth and of uniform colour.
- Free from brush marks, sags, runs and other defects.
- Cut in neatly.
Doors, opening windows and other moving parts: Ease before coating and between
coats.