The importance of a moisture control plan

Production of a Construction Stage Moisture Control Plan (CSMCP) is an essential part of the process in managing risks to the timber components.

The Construction Stage Moisture Control Plan (CSMCP) needs to encompass the complete process – from timber manufacture to handover of the finished building. By making a detailed plan ahead of execution, the relevant methods, mitigations, and monitoring can be planned, critically reviewed, and refined before site works commence.

It may be beneficial for the client if some actions extend through the defect liability period. For example methods for detection, monitoring and making good.

The outline of the plan should be drawn up during the design process, while there is still opportunity to modify the design to facilitate more efficient execution.

The UK National Structural Timber Specification (NSTS) invokes the requirement for the contractor to produce a CSMCP, in collaboration with the project engineer. The NSTS also provides an overview of the general objectives and contents of the document. However, the guidance is high-level and – depending on the nature of the construction – project teams are likely to wish to add further detail and requirements in the accompanying Project Specification.

The second generation of Eurocode 5 is expected to include an additional Part 3, specifically covering the execution stage. The publicly available draft of the standard outlines highlevel principles and minimum requirements for moisture control plans. It is expected that project specifications will extend and elaborate on the requirements depending on the specific requirements of the project.

CLT and glulam, sorted into assembly lots and prewrapped for transport; © Buro Happold.

The plan should contain details of monitoring, recording, and communicating moisture-related data. There should also be contingency plans that can be implemented if the plan is found to be failing to meet the required moisture performance. The use of simple checklists and flow-charts is recommended to aid site operatives in implementing the plans and recording actions taken.

The requirement for a moisture control plan must be explicitly stated in the project specifications forming part of the Employers’ Requirements. The objectives, subjects to be covered, and any project particular parameters should be stated.

Decisions made during design can have a significant effect on the efficiency of the execution. Likewise, decisions on the methods of execution must be informed by the nature of the design. In both stages, some form of project-specific risk analysis process should be consciously undertaken.

Risk analysis process
Construction-stage moisture control plans need to be tailored to the specific construction. A key part of the process is to analyse the moisture risks for that particular scheme.

Objectives and acceptability criteria
A first stage is to identify the general objectives for the timber regarding the moisture effects. A key question is whether any elements will be visually exposed. For these, the objective is likely to be that no staining is acceptable at handover. For visually encapsulated elements staining may be acceptable.

Moisture-related dimensional movement of the timber is an almost inevitable consequence and very large forces can be associated if it is restrained. It can, in principle, be acceptable where free movement is provided and adjacent elements are suitably detailed.

The remainder of the moisture effects are likely to be unacceptable for all structural timber. However, certain elements or assemblies may be more sensitive to moisture movement and their moisture content may need to be more closely controlled.

Acceptability and failure criteria will need to be associated with the objectives. The more specifically these can be defined the better. In most instances these will be defined as acceptable bounds of moisture content. They should refer to depths in the timber as well as positions on the element. These can then be directly measured, recorded, and reported. The acceptance criteria may also be usefully split into in-progress limits and limits at certain milestones.

For instance, it may be considered acceptable that half way through frame erection the upper lamella of a CLT floor is at 19%MC and the central lamella at 16%MC. But at the point of pouring a concrete screed, it may be required that the readings are significantly lower, due to the water that will be introduced by the pour. Likewise, a CLT roof slab may be required to be at a lower reading when the roof finishes are applied.

The risk factors particular to the scheme need to be identified to inform the mitigation and management decisions.

The factors to consider are likely to include the following…

End-grain sealer applied to CLT sill for temporary protection; © Buro Happold.

Climate, site & timing

• What are the temperature, humidity, weather, salinity, and pollution conditions like for the site?

• Is the site subject to flooding?

• How long will the timber take to erect?

• During which season will execution take place?

• Are there other requirements such as phased installation of fire protection, cladding, services etc.?

Materials, assemblies, detailing and geometry

• What types of timber material are used – sawn, engineered, cassettes?

• For which elements are each material used?

• What level of DfMA will be used?

• What geometries do element interfaces have, what degree of water-traps do these present, what path will water naturally take through the structure?

• Are structural roof planes flat or pitched?

• How large or tall is the structure, are there natural phases vertically or horizontally that can be protected in turn?

• What form of wet services does the building have, where is it routed?

• How will it be checked and evidenced that the timber remains at the required moisture content at all defined milestones?

The hazards and risks should be recorded in a systematic way and illustrated via extracts from design drawings where appropriate.

This article is taken from Designing Timber issue 7

This is an extract from the forthcoming document Moisture Management During Construction, by Matt Caldwell, Associate Structural Engineer at Buro Happold.