Research Output

Moisture conditions in external timber cladding: field trials and their design implications

  This thesis describes the development of technical guidance on timber facade design.
The study involved a state-of-the-art review; an exposure trial of external cladding
made from Sitka spruce (Picea sitchensis) and the production of construction details
and associated information. It was undertaken because timber is an increasingly
common cladding material in the UK, being used on low-rise residential buildings and
for medium-rise and non-domestic buildings. The risks have, therefore, increased but
this is not reflected in published guidance. Sitka spruce was used due to its
availability in the UK and its similarity to Norway spruce (P. abies) which is widely
used for cladding in Scandinavia.
The exposure trial indicated that the moisture content range in timber facades is wider
than accepted. The minimum moisture content of around 10% appears to be similar
for all types of timber cladding and all species. The maximum appears to vary
between species according to their fibre saturation point and is influenced by
construction detailing and workmanship. A preliminary model of these interactions is
From a theoretical standpoint, the moisture conditions observed in the trial mean that
the (commonly quoted) mean moisture content is all but irrelevant. The mode is a
more representative statistic as in most cases the data are skewed towards the fibre
saturation point for the species concerned. Most detailing combinations had a
moisture content near to the fibre saturation point throughout the winter. Sitka spruce
is, therefore, only suitable as external cladding in the UK if preservative treated.
Around 40 construction details were produced. They integrate, for the first time, all of
the performance requirements applicable to low- and medium-rise timber facades in
the UK. The work’s key benefit is that the guidance arising from this study
rationalises and improves facade design. Further research is, however, needed to
validate the moisture content model and extend it to other timber species.

  • Type:


  • Date:

    31 May 2011

  • Publication Status:


  • Library of Congress:

    TH Building construction

  • Dewey Decimal Classification:

    698 Detail finishing


Davies, I. Moisture conditions in external timber cladding: field trials and their design implications. (Thesis)



Timber facades; Sitka spruce (Picea sitchensis); timber cladding; moisture; technical guidance;

Monthly Views:

Available Documents