design of external timber cladding

Design of external timber cladding to the BS 8605 series

Dr Ivor Davies

Wood Knowledge Wales, 28/11/17

External timber claddingPart 2: Code of practice for design and installation

BS 8605-2: 201X

External timber claddingPart 1: Method of specifying

BS 8605-1: 2014

[1] [2]


BS 8605-1: 2014 Moisture contentDurabilitySpeciesGradeGeometrical characteristicsWarpFixing resistanceDimensional stabilityReaction to fire

Etc.


BS 8605-2: 201X

AppearanceEnvironmentDurabilityRobustnessFireSafetyNoiseEnergySustainability

Oxidization

Leaching

Photo-degradation

Fungal staining

Erosion …

EHCN

EHCF

EVON

EHCF

3025201510

Moisture content of

external timber cladding

Moisture content (%)

[6]

Mean: 16 to 18%

Range: 10% up to the fibre saturation point for the species

MC specification in BS 8605-1: (16 ±4)%

Massive wall

Types of external wall

[3]

Face sealed wall

Screened &drained wall

SidingRainscreen

[4]

Photos: National M

useum of W

ales

Timber rainscreen cladding

Open jointed Closed jointed

Framed substrate Masonry substrate


Rigid insulation Flexible insulation within a frame


Interior functional

layers

Room lining

Services void (optional)

Air & vapour control layer

Exterior functional layers

Wet zone

Timber cladding

Cavity (drained & ventilated)

Breather layer

Core


Minimum gap?


Drainage gap at cavity base: ≥ 10 mm

Ventilation gap within cavity & at cavity head:

≥ 5 mm

Splashzone: ≥ 200 mm

Avoid water traps

Seal between breather layer

and window

Durability If wetting is persistent, ensure timber is decay resistant, taking account of cost of failure & ease of replacement

Deflection Minimize wetting

Drainage Allow rainwater to escape

Drying Ventilation for drying

Heartwood

Sapwood

Natural durability classes (fungi)

1Very

durable

2Durable

3Moderately

durable

4Slightly durable

5Not

durableJarrah

Western red cedarEuropean oak

UK larchBirch

Sapwood

BS EN 350

[9]

Use classes

Use Moisture Wood destroying class condition organisms in UK

1 Dry interior Minor beetle risk

2 Damp interior Dry rot + beetles

3 Exterior out Wet rot + beetlesof ground contact

4 In ground contact Soft rot + beetlesor fresh water

5 In sea water Marine borers

BS EN 335

[16]

Use class 3

Use class 5

3.1 Occasionally wet3.2 Frequently wet

1 2 3 4 5Use

class

Natural durability always sufficientNatural durability usually sufficient

Preservative treatment usually neededPreservative treatment essential

Natural durability probably insufficient, but…

12345

Matching natural durability to use class BS EN 460

Natural durability class (fungi)

[17]

1 2 3 4 5Use

class

Natural durability always sufficientNatural durability usually sufficientNatural durability probably insufficient, but…

12345

Matching natural durability to use class BS EN 460

Natural durability class (fungi)

Modified wood is specified in a similar way to naturally durable

timber

Wood preservation

Chemical modification

Thermal modification

Options for increasing biodeterioration resistance

Specify to BS 8417

[14 - 15]


Structural design of cladding should address:

1. Imposed loads from the cladding onto the wall

2.Self-weight of the cladding

3.Attachment of cladding to the wall

4.Movements and tolerances






With lightweight cladding, these are usually the responsibility of the cladding designer

External timber cladding is a semi-structural application and is therefore within the scope of the Eurocodes:

Basis of design: Eurocode 0Wind loads: Eurocode 1Timber connections: Eurocode 5

[18 - 20]

Rule of thumb guidance: ‘a smooth nail should penetrate the support by 2½ times

the board thickness & an improved nail should penetrate by twice the board

thickness’

This is not in accordance with Eurocode 5

Wind

Boundary conditionsAim:

Peak wind loads < 2.5 kN/m2

Minimize lateral loads

WindPlan section




Low risk: < 1.5 kN/m2

Medium risk: 1.5 to < 2.5 kN/m2

Lateral load

Axial load

Plan section




Conditions met?

2nd

batten layer?

Full design to Eurocode

5

Yes

No

Design claddingNo

Wind load <2.5 kN/m²?

Yes

Yes

Redesign or get advice

No Determine wind load

Conceptual design

Design 2nd

batten layer

Design 1st

batten layer

Design for robustness

Connections: vertical batten to studUse ring shank nails or screws

Pointside penetration of threaded part ≥ 8d

Plan section

Diameter d = 2.1 to 6 mmRing shank nails:

Pointside penetration

Fastener centres determined by diameter and wind load

Batten spacing 600 mm Board width 150 mm

Design wind load 1.2 kN/m2

Counter battens3 nails/m length (6/m2) Axial load/nail: 0.2 kN(1.2 ÷ 6)Battens2 nails/m length (4/m2)Axial load/nail: 0.3 kN(1.2 ÷ 4)

Cladding4 nails/m length (26/m2)Axial load/nail: 0.05 kN(1.2 ÷ 26)

Connections: horizontal batten to counter batten

Connections: horizontal batten to batten

Plan section

Pointside penetration

(incl tip)

Headsidethread length

Headsidethread length and batten spacing determined by screw diameter and wind load

Only screws are suitable

Nail connections: vertical batten to stud

Pointside penetration of threaded part ≥ 6d

Plan section

d = ≥ 3.5 mm dhead = d × ≥1.78

Spacing a1 12d 15d 5d

Spacing or end/edge distance

Fastener spacing as multiple of dPre-drilled

timber

Spacing a2 5d 7d 4dDistance a3c 10d 15d 7d Distance a4c 5d 7d 3d

a1

a4ca3c

a2

Examples of minimum spacings and unloaded edge/end distances for fasteners

[20]

Mean wood density (kg/m3)≤ 500 >500

Minimum batten thickness 38 mm to

maintain edge distances. This

determines cavity depth.

Lightweight cladding is fixed to and should move with the wall structure

Heavyweight cladding is separately supported and moves independently of the wall structure

Differential movement

Movement classes for wood species

Movement class

Across grain dimensional change due to moisture content fluctuation below the fibre saturation point

Small 1% for every 5% change in MC

Medium 1% for every 4% change in MC

Large 1% for every 3% change in MC

[21]

Compartment fire

Fire in a nearby but

non-adjoining building

Small fire near the

wall

Three fire scenarios affecting timber-clad facades

Peak heat release rate above window can be 70 kW/m2

Total energy release in front of the facade:1 to 1.5 MW

Duration of thermal exposure on facade: 15 to 20 minutes

Photo by IngvallMaxw

ell

The flame plume from a fully developed fire can spread 2 to 4 m up the facade irrespective of the combustibility of cladding materials

The plume might reach 6 m high in some conditionsPhoto by IngvallM

axwell

Typical flame height 2 to 4 m

Possible flame

height up to 6 m

Maximum allowable fire propagation up the facade before fire brigade intervenes

Performance criterion for timber-clad facades

[22]

Fire spread from a 5th floor flat onto the facade and externally to the eight floors above. The upper floors were engulfed in minutes. One person died.

Photo: BRE G

lobal

Irvine fire, Scotland 1999

Grenfell Tower fire, London, 2017. 71 people died after a compartment fire spread to the cladding.

Imag

e: W

ikip

edia

Com

mon

s

Time

Heat release

rate Flas

hove

r Fully developed

Decay

Reaction to fire

Sprinkler controlled

Fire resistance

Growth

A classification of how fire starts and

spreads up to and including

flashover

BS EN 13501-1

A classification of how long

construction elements can resist the power of a fully

developed fire

BS EN 13501-2

Reaction to fire

Fire resistance

[23 - 24]

Photo: US Forest Products Laboratory.

Floor >18 m?Combustible cladding

or insulation might be prohibited

Cavity barriers?

Spanning a compartment wall

or floor?

Junction should maintain fire resistance of the compartmentation

Use?

Facade near boundary?

External surfaces to be Euroclass B or A2

Assembly buildings etc need Euroclass B or C

surfaces

Almost always needed

Fire resistant room liningInternal sprinklers

Separating buildingsCavity barriers

Flame retardantsProjecting baffles

Fire resistant glazing

Options for limiting flame spread on facades






http://en.wikipedia.org/wiki/File:Sprinkler_-_by_Adrian_Sampson.jpg

Generally there are no restrictions on the reaction to fire class of timber cladding.

Tar coating

Flame retardants can improve the reaction to fire class of timber if needed.






Reaction to fire classes – indicative transposition

BS EN13501-1

BS 476series

Examples*

A1 Non combustible Inorganic materials

Limited combustibility

Products with small % of organic materials

A2

B O Flame retardant treated timberC 1, 2 or 3

D or E 4 or 5 Timber with no flame retardant treatmentUntested productsUnclassifiedF

* Classification is affected by assembly conditions

Exterior walls non-combustible < 3 m

Internal sprinklers

Euroclass B timber

Parapets not eaves

Insurance criteria for school & academy buildings

Typical criteria (facades):

Summerland fire, Isle of Man, 1973. At least 50 people died after a small external fire spread to the wall cavity.

Flames entering a cavity can extend 5 to 10 times their original height irrespective of the combustibility of cavity linings

Photo: Liverpool Echo

Cavity barrier to block fire spread

Ventilated cavity to promote drying

Horizontal cavity barriers: use

intumescent strips with specified fire

resistancePhoto: Tenm

at

Vertical cavity barriers do not need through-

ventilation: use timber battens ≥ 38 mm thick

(Non-combustible barriers needed above 18m)

Plan section

≥ 38 mm Cavity barriers for timber rainscreens

Interior Interior

Exterior

e.g. mineral wool batt

e.g. 2 solid timber cavity

barriers≥ 38 mm wide

Fire resistance of separating wall and floor junctions must be maintained

Cavity barriers for timber rainscreens

Plan section

InteriorInterior

Inte

rior

Exterior

References1. BS 8605-1:2014. External timber cladding. Part 1: Method of specifying. London: BSI.

2. BS 8605-2 (in prep.). External timber cladding. Part 2: Code of practice for design and installation. London: BSI.

3. STRAUBE J. and BURNETT F. (1999). ‘A review of rain control and design strategies.’ In: J. Thermal Insulation and Building Envelopes. July 1999: 41-51.

4. NASH G. (1995). Timber-framed buildings in Wales. Cardiff: National Museum of Wales.

5. NORE K (2009) Hygrothermal performance of ventilated wooden cladding. PhD thesis. Trondheim: Norwegian University of Science and Technology.

6. DAVIES I., FAIRFIELD C., STUPART A. and WILSON P. (2011). ‘Moisture conditions in timber cladding: Field trial data’. In: Proc. Inst. Civ. Engnrs., Construction Materials. 165(5), 265-279.

7. BYGGFORSKSERIEN (2008) Exterior walls over terrain. Features and principles of construction. Building details 523.002.. Requirements and recommendations [in Norwegian]. Oslo: SINTEF.

8. DIN 68899-2: 2012. Wood preservation - Part 2: Preventive constructional measures in buildings. [In German] Berlin: Beuth Verlag GmbH.

9. BS EN 350:2016. Durability of wood and wood-based products. Testing and classification of the durability to biological agents of wood and wood-based materials.London: BSI.

10. MOORE J. (2011). Wood properties and uses of Sitka spruce in Britain. Edinburgh: Forestry Commission.

11. JONES D., DAVIES I., SHARPHOUSE P. and SUTTIE E. (2013). Grading UK grown larch for durability. IRG/WP 13-10809. IRG/WP: Stockholm.

12. BS EN 350-2:1994. Durability of wood and wood-based products. Natural durability of solid wood. Guide to natural durability and treatability of selected wood species of importance in Europe. London: BSI.

13. HUMAR M, FABCIC B, ZUPANCIC M, POHLEVEN F, OVEN P. (2008). ‘Influence of xylem growth ring width and wood density on durability of oak heartwood.’ In: Int. Biodeterior. Biodegrad. 62(4):368-371.

14. BS 8417:2011+A1:2014. Preservation of wood. Code of practice. London: BSI.

15. WOOD PROTECTION ASSOCIATION (2011.) Modified wood specification manual. Castleford: WPA.

16. BS EN 335:2013. Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products. London: BSI.

17. BS EN 460:1994. Durability of wood and wood-based products. Natural durability of solid wood. Guide to the durability requirements for wood to be used in hazard classes. London: BSI.

18. BS EN 1990:2002+A1:2005 - Eurocode. Basis of structural design. London: BSI.

19. BS EN 1991-1-4:2005+A1:2010. Eurocode 1. Actions on structures. General actions. Wind actions. London: BSI

20. BS EN 1995-1-1:2004+A2:2014. Eurocode 5: Design of timber structures. General. Common rules and rules for buildings. London: BSI.

21. BRE (1992).The movement of timbers. Watford: BRE

22. BART B., KOTTHOFF I. and WEIDERKEHR R. (2009). External construction – cladding. [In German] Zurich: Lignum – Dokumentation Brandschutz.

23. BS EN 13501-1:2007+A1:2009. Fire classification of construction products and building elements. Classification using test data from reaction to fire tests. London: BSI.

24. BS EN 13501-2:2016. Fire classification of construction products and building elements. Classification using data from fire resistance tests, excluding ventilation services. London: BSI.

25. BS EN 14915:2013. Solid wood panelling and cladding. Characteristics, evaluation of conformity and marking. London: BSI.


BS 8605-2: 201X


BS 8605-1: 2014

Dr Ivor Davies

Tel: 0131 455 2397Email: [email protected]: www.napier.ac.uk/isc

design of external timber cladding

Documents