Page 3 ... Class 3. In ground or fresh water, MC permanently. >20%. Soft rot &
beetles. Class 4. In sea water, MC .... Current: EN 14915, BS 5534, BS 1186-3.
Timber Cladding & Shingles Ivor Davies
What are the moisture conditions in timber façades and roofs? What technical issues are affected by moisture? How are these issues managed? What are the implications for fire safety?
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What moisture conditions?
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Exposed, Vertical, Closed, front closed Coated joints, no coating) Panel EVCN (exposed site, vertical boards, (average Moisture Content,total totaldaily Rain Fall and average Temperature) Average daily moisture content, rainfall, and average daily temperature.
35.00 30.00 Support Battens Board Edges
25.00
Defect Free Timber Near Defects Temperature
20.00 15.00 10.00
Rain Fall 5.00 0.00
05
.1
0.
9. .0 17
.0 30
05
05
05 8.
05 .0 12
25
.0
7.
8.
05
05 07
.0 19 Date
.0
6.
7.
05
05 6. .0 01
14
.0
5.
4. .0
26
.0
08
05
05
05 4.
05 3.
.0
21
.0
03
13
3.
2.
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.0
1. .0
05
05
05
-5.00
26
Moisture content (%), Rain fall (mm) and Temperature (˚C)
40.00
What technical issues? Fungal decay & insect attack Dimensional changes Weathering Corrosion Event/seminar title
Class 1
Under cover, dry MC permanently20%. Beetles wet & dry rot
Class 3
Not covered but not in ground contact, MC frequently >20%. Wet rot & beetles
Class 4
In ground or fresh water, MC permanently >20%. Soft rot & beetles
Class 5
In sea water, MC permanently >20%. Marine borers
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Biodegradation: EN 335-1&2 Use classes
At MC < 30% timber shrinks as it dries and expands as it wets ‘Shrinkage’ describes dimensional change due to initial drying ‘Movement’ describes size change due to ongoing MC fluctuations
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Dimensional changes
Leaching (water soluble extractives) Bleaching (UV degradation of lignin) Staining by non-decay fungi (Aureobasidium pullulans) living off lignin breakdown products + water: the hyphae are grey Event/seminar title
Weathering
At MC >20% all timbers are corrosive, mainly due to release of acetic acid The most corrosive timbers are: Douglas fir, western red cedar, oaks, sweet chestnut, & several Australian hardwoods
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Corrosion
Risk management
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Prevent wetting wherever possible
Detailing-fordurability
Where wetting occurs ensure rapid drying wherever possible If wetting is persistent ensure timber is either: naturally durable, preservative treated, wood modified, or replaceable. Consider cost of component failure or replacement
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Outer rainscreen Cavity: drained & (usually) ventilated Wind barrier: also water resistant
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Controlling biodegradation: rainscreen cladding
Ventilation gaps: 0 mm 4 mm 23 mm
Ventilated cavities stayed drier on wet sites Unventilated stayed drier on dry sites Event/seminar title
Controlling biodegradation: cavity design
Class 1
Class 2
Class 3
very durable durable moderately durable
Class 4
Class 5
slightly durable
not durable
Use class 1 Use class 2 Use class 3 Use class 4 Use class 5 Event/seminar title
Controlling biodegradation: EN 460
Beware sapwood inclusion
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Controlling biodegradation: durable timber
Preservation: ‘CCA alternatives’: CCA and chromium based treatments withdrawn September 06 due to non-support in the BPD. Thus: copper azoles, ammonical copper quaternary, copper HDO, boron based products, organic biocides. BS 8147 + WPA 2007 manual (specification C6) Thermal modification e.g. ThermoWood, Plato Wood, etc
Selection of preservative
Site practice
Chemical modification e.g. Acetylation, Furfurylation, etc Event/seminar title
Controlling biodegradation: treated timber
Built 1834 Local Scots pine (i.e. durability class 3-4) >170 years
Built 1984 Scandinavian whitewood (i.e. durability class 4) failed 2001: 25mm
Current: battens create Event/seminar title ventilated cavity
Traditional: sarking boards but no battens
Controlling biodegradation: detailing shingles
Boards installed too dry or with inadequate expansion gaps Event/seminar title
Boards installed too wet or with inadequate shrinkage provision – particularly important with green timber
Dimensional change
Cupping because boards installed too wet
Install boards as soon as possible after board conversion Keep it simple Use overlapping or open joints Ensure joints can accommodate anticipated shrinkage
Green oak shrinks across width by 4-7% as it dries
Use single fixings if possible or drill holes oversize Support battens at 400mm
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Dimensional change
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Weathering
Maintenance?
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Selection of coatings?
Weathering: if you don’t like it use a surface coating
Corrosion cell with galvanised steel in Douglas fir
304 grade stainless steel normally OK but use 316 grade near the coast
Use galvanised fixings with most timbers but austenitic stainless steel or phosphor bronze with high risk timbers Event/seminar title
Corrosion: fixing materials
Fire safety
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1. Fire spread from an adjacent fire (e.g. neighbouring but nonadjoining building)
2. Flame source in contact with the façade (e.g. burning rubbish)
3. Fire spread to the façade from a room in the building
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Fire safety: 3 fire scenarios affecting timber façades
Boundary distance + limits on combustible materials in some locations
Limits on spread of flame class + cavity barriers
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Fire safety:
Internal sprinklers
Cavity ventilation to promote drying
Barriers to fully block fire spread in cavity
Intumescent horizontal barriers + vertical barriers made of wood >38mm thick
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Fire safety: conflicts in UK fire codes?
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Fire safety: detailing
Current: EN 14915, BS 5534, BS 1186-3 TRADA: External Timber Cladding, 2nd Edn. 2007
Designing the Timber Façade
Davies I et al. Timber Cladding in Scotland 2002 Nash G. Timber-framed buildings in Wales, 1995 NFRC TB34 Wooden Shingles 2003 Forthcoming: Code of Practice for External Timber Cladding (in preparation) Davies I, Designing the Timber Façade (in preparation) Event/seminar title
Further information
Ivor Davies
Timber Cladding & Shingles Ivor Davies
