Timber: problematic work

Doing justice to joists

26 February 2019

Modifications to timber floor joists must be made carefully to ensure they maintain their load-bearing strength. This, the second in our series on common site issues, looks at how to identify problematic drilling and notching, and how to remedy poor-quality work

Floor joists are sized to suit the spans in a building, so as to ensure they are sufficient to cover the relevant distance and support the required loads. It is therefore paramount that the integrity of the joist is maintained. This may seem obvious, but often the installation of a joist or service penetrations compromise its strength.

Typical examples include excessive notching or drilling of the joist, which reduces its integrity and may even require its replacement. It is therefore an important skill for a building control surveyor to be able to identify when a joist has been disproportionally modified.

A standard timber joist section is typically considered to be in flexion through the top chord and in tension through the bottom chord, and it therefore has a neutral axis running through its centre. Thus, drilling small holes through the joist to accommodate water pipes, for example, is typically acceptable, on the proviso that these holes are made through the neutral axis and will not compromise the strength of the joist.

Doing justice to joists figure one

Figure 1: Limits for notching of floor joists and end-trimming to fit steel hangers

Another form of modification is notching, a standard term used for effectively cutting a section from the top or bottom of the joist: sometimes for ease of fitting into a joist hanger, sometimes for passing services through it.

Again, this can be detrimental to the joist’s strength, for example if it is done too close to the bearing or too deep. Where a joist is notched excessively and too close to the point of bearing, this weakens it and can cause it to fail at that junction.

There are accepted tolerances for notching and drilling floor joists; where these are exceeded, either the joist should be replaced, or it may be possible for a structural engineer to prove its continuing functionality with calculations.

Generally, notching should not exceed 0.125 of the joist depth and a maximum width of 35mm. The notch should only be made within a permitted horizontal zone – between 0.07 and 0.25 of the joist span from either side – so as not to be too close to the bearing point or the centre. Notches should only be permitted along the top or bottom of the joist, and not both.

Doing justice to joists figure two

Figure 2: Limits for drilling floor joists and end-trimming to fit steel hangers

Holes should only be drilled through the central, neutral axis of the joist, the maximum diameter being 0.25 of joist depth. Again, the hole should only be drilled within an acceptable horizontal zone, between 0.25 and 0.4 of the joist span, so as not to compromise its bearing or centre points. If multiple holes are drilled, they should be spaced apart by at least three times the length of the hole’s diameter to ensure they are not too close together.

Figure 1 illustrates the notching limits on a timber floor joist as detailed above, while Figure 2 in turn shows where to drill timber floor joists. Reference to these diagrams is useful when assessing whether a detail noted on site is acceptable, because the limitations are clearly and simply displayed.

Typical examples of joist strength being compromised by notching or drilling can be found in domestic situations where the mechanical and engineering or drainage services haven’t been coordinated with the structural engineer’s joist plans. This may then require excessive modification of the joists to accommodate services. Water pipes are usually easy to accommodate given their small diameter, providing the contractor is competent working with timber and doesn’t drill the holes too close together.

However larger holes, such as those needed for drainage or ventilation pipework, are often riskier, with pipe diameters sometimes being in excess of 100mm.

Assessment of floor joist specification starts in the plan-checking process, focusing on the proposed span. The images above offer examples of poor-quality work that can typically be picked up during site inspections. Identifying and reviewing such problems can easily count as evidence towards the following competencies:

  • Building control inspections: carrying out site inspections ensuring building work meets relevant performance requirements, and having the ability to observe, assess and take action against contraventions on site
  • Building pathology: having an understanding of defects analysis and being able to explain building fabric failure
  • Construction technology and environmental services: understanding the design and construction process and knowledge of construction solutions to solve problems
  • Works progress and quality management: knowledge of construction technology techniques and their relevance on site, good-quality work being vital in ensuring long-term functionality.

While carrying out the inspection, the surveyor should be able to identify the types of defect discussed above, and provide appropriate measurements and photographs in site reports. The detail should be reviewed appropriately using relevant guidance such as BS 8103–3: 2009 to calculate whether the notch, drilling or relevant detail is acceptable.

From there, the building control surveyor would be able to take appropriate action, such as advising on remedial measures or the need for further review to be carried out by a structural engineer. Where a candidate is identifying and resolving such site defects it should be possible to record their experience under at least Level 2 generally, and potentially Level 3, for the Building control inspections competency.

Amy Allen is a director at Assent Building Control

Further information