3.3 Timber preservation (natural solid timber)

3.3.2 Durability

Timber and joinery used in the construction of homes shall either have adequate natural durability or, where treatment is undertaken, receive a satisfactory preservative treatment against fungal decay and insect attack.

The natural durability of heartwood varies between wood species. For some end uses, the natural durability of the heartwood of a particular species may provide sufficient durability to be used without preservation. BS EN 350 defines the durability of commonly available wood species and the Wood Protection Association Wood Selection Guide and BS 8417 provide guidance on natural durability and preservative treatment appropriate for a specific Use Class and Desired Service Life.

Timber component groups and preservative treatment or natural durability requirements are shown in Table 1 below (based on BS 8417), which provides information to establish the appropriate timber Durability Class or requirement for preservative treatment according to the component and conditions of use.

Preservative treatments should be suitable for the Use Class of a component; not all treatment types are suitable for all Use Classes. Guidance on treatment types and schedules can be found in Table 4 of BS 8417 and by consulting treatment manufacturers or The Wood Protection Association.

Table 2 provides information on the timber species and durability of heartwood to protect against attack from fungal decay. Sapwood is generally regarded as not durable, unless proven otherwise by test data.

Table 1: Timber component groups and preservative treatment

Notes

1. Preservative treatment of timber should be in accordance with penetration and retention values listed in Table 4 of BS 8417. It is important to note that not all preservatives are appropriate for all Use Classes and that some timber species are not sufficiently permeable to achieve the penetration levels required, at least without additional processing such as mechanical incising.

2. Where natural durability is used in lieu of preservative treatment, timber must be specified and selected as being heartwood only. Durability Classes for the heartwood of commonly used timbers are available in BS EN 350 and summarised in Table 2 below.

3. Any hardwood can be used (subject to adequate structural performance). Recommendations based on evidence that the house longhorn beetle (H.bajulus) can attack the heartwood of some softwoods of lower natural durability. Geographic areas where treatment is required are defined in Approved Document A.

4. Pitched roofs with a high condensation risk are considered to be roofs over swimming pools, pitched roofs with a fully supported weatherproofing membrane (eg single ply membrane, bituminous membranes, GRP etc) or continuous metal coverings, and any other roof system not designed in accordance with BS 5250.

5. Sole plates should be positioned above DPC. Preservatives used should be resistant to leaching.

6. The base of support posts should be at least 200mm above any adjacent horizontal surfaces (eg ground or supporting walls) supported on a free draining post base.

7. For decking and external stairs that are more than 600mm and less than 2.4m in height, deck boards, balustrades and external stairs should have a Desired Service Life of 30 years, and deck joists and posts should have a Desired Service Life of 60 years. Reference should be made to Chapter 7.1 ‘Flat roofs and balconies’, Chapter 10.2 ‘Drives, paths and landscaping’ and The Timber Decking and Cladding Association ‘Code of Practice: Raised timber deck structures on new homes’. For balconies greater than 2.4m in height, timber is not permitted for gallows brackets, posts, columns, stairs, guardrails or their support, and cantilevered or infill joists. See Clause 7.1.6 for further guidance.

8. No generic treatment specifications for a 60 year Desired Service Life in Use Class 4 are provided in either BS 8417 or the WPA Code of Practice, and so specialist advice must be sought from the preservative treatment supplier and/or manufacturer. Due to the potential difficulties in achieving the very high levels of preservative treatment penetration and retention likely required, NHBC require satisfactory assessment by an appropriate independent technical approvals authority accepted by NHBC for timber components or systems in this scenario.

9. Selected woods of natural Durability Class 1 may be able to achieve a Desired Service Life of 60 years. See BS 8417 for further details.

10. For modified timber products (such as thermal or chemical modification), guidance on the Durability Class and use of the product should be obtained from the manufacturer.

11. Timber retaining structures should not be used to provide support to homes, garages, roads, drives, car parking areas, or drainage systems. Further guidance can be found in Chapter 10.2.

Table 2: Natural durability of building timbers (heartwood only)

Notes

1. Where a timber species has a range of Durability Classes, and for end uses where durability is important, the lowest Durability Class listed for a wood species should be assumed unless specific source-related information is available.

Last updated: 2nd January 2024

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