Fibre saturation point FSP
Fibre Saturation Point (FSP) relates to the moisture content (MC) of timber. Saturation Point (SP) may also be used with reference to other materials such as soil, but in both cases it refers to the point at which pores or cells are 'full' with water.
The FSP of timber will vary between species but it is normally accepted as averaging around 30%MC (moisture content), that being the percentage ratio of the wet wood to the theoretical weight of the dry wood. This level of MC is significant because it is considered to be the point of reasonable movement ie when wood that is significantly above this MC level dries its physical and mechanical properties change, which can lead to deformation.
Wood is hygroscopic so moisture levels can change in relation to the localised environment. As wood dries it shrinks, and as it absorbs moisture up it expands, the severity of which depends on the amount of moisture lost or gained. When wood is at the FSP of 30% further drying is less likely to cause significant deformation and it is therefore considered more stable or predictable in terms of being worked with.
The point at which timber stabilises in balance with the moisture of its own environment (which is normally below the FSP) is referred to as Equilibrium Moisture Content (EMC). This figure will vary depending on the species but more importantly with the moisture and temperature of the local environment, this hygroscopic characteristic of timber can be beneficial to internal environments when exposed internally. Fore more information see: Equilibrium Moisture Content.
[edit] Related articles on Designing Buildings
Featured articles and news
Boiler Upgrade Scheme and certifications consultation
Summary of government consultation which closes 11 June 2025.
Deputy editor of AT, Tim Fraser, discusses the newly formed society with its current chair, Chris Halligan MCIAT.
Barratt Lo-E passivhaus standard homes planned enmasse
With an initial 728 Lo-E homes across two sites and many more planned for the future.
Government urged to uphold Warm Homes commitment
ECA and industry bodies write to Government concerning its 13.2 billion Warm Homes manifesto commitment.
Places of Worship in Britain and Ireland, 1929-1990. Book review.
The emancipation of women in art.
CIOB Construction Manager of the Year 2025
Just one of the winners at the CIOB Awards 2025.
Call for independent National Grenfell oversight mechanism
MHCLG share findings of Building Safety Inquiry in letter to Secretary of State and Minister for Building Safety.
The Architectural Technology Awards
AT Awards now open for this the sixth decade of CIAT.
50th Golden anniversary ECA Edmundson awards
Deadline for submissions Friday 30 May 2025.
The benefits of precast, off-site foundation systems
Top ten benefits of this notable innovation.
Encouraging individuals to take action saving water at home, work, and in their communities.
Takes a community to support mental health and wellbeing
The why of becoming a Mental Health Instructor explained.
Mental health awareness week 13-18 May
The theme is communities, they can provide a sense of belonging, safety, support in hard times, and a sense purpose.
Mental health support on the rise but workers still struggling
CIOB Understanding Mental Health in the Built Environment 2025 shows.
Design and construction material libraries
Material, sample, product or detail libraries a key component of any architectural design practice.
Construction Products Reform Green Paper and Consultation
Still time to respond as consultation closes on 21 May 2025.
Resilient façade systems for smog reduction in Shanghai
A technical approach using computer simulation and analysis of solar radiation, wind patterns, and ventilation.