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H21 Timber Weatherboarding (Checklist) G#11946

GBE H21 Timber Weatherboarding Checklist

GBE > Encyclopaedia > Code > Checklist > G#9439

GBE H21 Timber Weatherboarding Checklist


  • Timber is a natural material with natural variation and an amazing range of properties and applications
  • Standards are being kept up to date and familiar British Standards are being replaced by unfamiliar Euro Norms
  • It is relatively easy to get out of date and get specifications wrong
  • TRADA employs one man full time to address the many calls from Architects with failed timber weatherboarding


  • There are too numerous problems to describe here now (but I will keep coming back to add more)
  • Despite NBS Guidance enabling competent specification of Timber Weatherboarding
    • Users of NBS still get it wrong from time to time
  • Poor detailing, poor specification, poor choice of timber, substitution and poor workmanship and maintenance all contribute to failures.


  • BRE Campus’ Innovation Village, one demonstration building had failed timber weatherboarding where two adjacent boards expanded then burst out of the cladding surface.
  • The T&G joints had been installed upside down creating a channel to collect water, the boards absorbed water and expanded until failure.  That demonstration building is gone now.


  • If an architect chooses to do ‘radical timber weatherboarding’ and the detailing is questioned by the tradesman the Architect should learn to listen and not insist on his details be followed without question.


  • Don’t be radical for the sake of being radical; learn to be competent instead
  • Subscribe to NBS and use H21 Timber Weatherboarding Specification read and use the guidance notes carefully
  • Purchase the TRADA Book on Timber weatherboarding, read it and follow its guidance
  • Consider using the GBE H21 Timber Weatherboarding (Outline), read it and follow the guidance
  • Use askTRADA website
  • Seek TRADA advice/consultancy
  • Purchase the more technical Napier University Book on Timber Weatherboarding, read it and follow its guidance
  • If you want to do open joint rain screen weatherboarding, consider using the following specification and editing to suit your project GBS H21 Timber Weatherboarding Rain screen (RSWS)
  • Listen to the tradesman

GBE Outlines

GBS Specification


  • Durable species without preservative treatment
  • Locally sourced durable softwood or hardwood
  • Unseasoned, untreated, unfinished oak can be as cheap as treated softwood
  • Architect to design joinery to minimise waste, be aware of position in log, sections sizes and lengths, before deciding use and joint patterns of hardwoods
  • FSC approved timber sources, timber board products and timber based materials See G20, Z10


  • Architects are prone to choose their favourite species of timber; availability of the species varies in the supply chain, due to numerous issues including:
    • Long growing time
    • Seasonal harvesting
    • Drying/Seasoning time
    • Availability, stock levels and delivery times
    • Matching quantity available to size of project
    • Reclassification of scarcity and endangerment, etc.
    • FSC timber or other Certification scheme requirements
  • Some favourite species have the wrong characteristics for some application
  • Most species unfinished and exposed to the sun will fade to silver/grey so the original colour becomes irrelevant
  • There is a growing preference for timber species to be specified by performance characteristics and colour range
    • enabling legal and sustainable timber to be sourced with chain of custody more readily
    • offering a few examples of species available with characteristics that match the requirements
    • leaving the final species choice to the Architect by samples


  • Timber miles shipping tropical hardwoods to the construction site


  • Preservative treatment by choosing species for durability to match the application
  • Treated softwood if a durable softwood is available
  • Treated softwood if a durable hardwood is available
  • Transporting durable hardwoods cross the globe if a local durable hardwood will do the job
  • Indonesian Hardwood
    • Recently most has been illegally logged, destroys forests, biodiversity, communities and habitation
    • Only use FSC certified
  • Preservative treated perishable softwood. See G20, Z10 and Z12
  • PVC substitute for timber cladding See Z50
  • Coated aluminium substitute for timber landing
  • Virgin Plastics See Z50


  • Avoid the use of preservatives which need touching up on site wherever the timber is cut or drilled on site


  • Avoid the use of preservatives, they are hazardous if released to the environment (air, land or water) or to nature


  • Use unfinished untreated timber in preference to treated, painted, stained, oiled, waxed timber, any of which may off-gas VOCs to the environment
  • The smell of fresh paint is often welcomed but it invariably is not making your health better
  • Recent (2011) changes to EU Directives saw a significant reduction in VOC releases from paint to the atmosphere but this does not mean they are all gone.
  • Low-VOC can also mean more/different chemicals to limit VOC releases to atmosphere
  • There are many manufacturers who have always made VOC-free natural plant-based or mineral-based paints


  • Being able to see, feel, hear and even smell timber is very welcome by most occupants, visitors and passers-by
  • There is growing evidence that natural materials in the built environment can enhance their sense of wellbeing



Precautionary principle:

  • Avoid using preservative treatments to avoid risk of loss to the environment and nature


Resource Efficiency:

  • Architect to design joinery to minimise waste: be aware of position in log, sections sizes and lengths, before deciding use and joint patterns of softwood and hardwoods
  • Choose and reserve the log with the right amount of figuring (more or less), for your project,
    • don’t condemn the installation with too much or too little figuring
    • Avoid sending finished materials to landfill

Industry/Sector Initiatives:





  • Offer ‘excess to requirements’ materials to exchange websites


  • Recycling of timber usually means reducing to wood chips or particles and adding a synthetic binder to make chipboard or particleboard, with or without other salts for fire or preservative treatments
  • Recycling of timber can also mean reducing to wood flour and adding recycled plastics to make a conglomerate wood-plastic
  • C2C or Cradle to cradle principles advise against mixing technical (plastics) and natural (wood) since separating them at the end of life is currently economically nearly impossible and are likely to end up in landfill, with a shorter than plastic life and a longer than timber life


  • Consider heat recovery if burning untreated and unfinished timber on site
  • Use damaged untreated pallets to construct on-site composting bins to compost landscape waste


In Use Issues:

Maintenance issue:

Visible face fixed fasteners can enable:

  • Ease of build ability
  • Out of sequence building or completion
  • Ease of replacement
  • Ease of demount-ability
  • Ease of reclaim ability
  • Ease of reusability


GBE Systems

Waste issues:

Waste Issues:

  • Waste Statistics: 30 m tonnes (33% of 90 m. tonnes/year) of waste is offcuts
  • (not all timber, not all H21)

Waste Classification:

  • Untreated timber: Active (Biodegradable)
  • Treated timber: Inert but Hazardous

Hazardous waste:

  • Preservative treated timber is hazardous waste
  • Do not burn on site, nor permit it

Deleterious Substances:

  • Waste preservative treatment
  • Waste paints & stains

Waste statistics:

  • Waste Statistics: 30 m tonnes (33% of 90 m. tonnes/year) of waste is offcuts
  • (not all timber, not all H21)

Waste minimisation:

  • Architect to design joinery to minimise waste
  • be aware of position in log, sections sizes and lengths
  • before deciding use and joint patterns of hardwoods
  • Avoid water staining of timber due to stacking outside in inclement weather

Waste management:

  • Offer ‘excess to requirements’ to material exchange websites

End of Life:

End of Life options:


Visible face fixed fasteners can enable:

  • Ease of build ability
  • Out of sequence building or completion
  • Ease of replacement
  • Ease of demount-ability
  • Ease of reclaimability
  • Ease of reusability



GBE Jargon Buster


  • Timber

Initial, Abbreviations & Acronyms

  • HW
  • SW
  • H21
  • CCA
  • OS
  • Z20
  • Z21


Words & Phrases

  • Durability
  • Hardwood (HW)
  • Indigenous
  • Organic Solvent (OS)
  • Pressure equalisation
  • Rainscreen Cladding
  • Softwood (SW)
  • Temporate
  • Timber Weatherboarding
  • Tropical

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Refurbishment: Decent Homes




  • Refurbishment
  • Housing
  • Non-Domestic
  • GreenDeal


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  • Elemental


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Case Studies:

GBE Case Studies





GBE Evidence Based Case


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Product Data Sheets


By other:


  • Proposal for a timber merchant to create Embodied and Sequestered Carbon calculator

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© GBE GBC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
21st January 2016 – 14th June 2021

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