This article was written by Danny Davis, Principal Technical Officer for the CIPHE.

 

Introduction

When installing plumbing and heating systems within buildings, notching and drilling of joists is often required, to enable pipework to be routed ‘out of sight’, and to allow for pipe routes to be selected that ensure that hot and cold water reaches terminal fittings within the time and temperature limits given in the guidance to the Water Regulations and other supporting documents (BS 6700, L8 etc), and to allow efficient routing of central heating and gas supply pipework.

When drilling or notching structural timbers it is crucial that the holes/notches are made in a way that does not compromise the integrity of the structure. On ‘new build’ sites and large developments, any installer intending to notch/drill holes in structural timbers must first seek permission from the architect, structural engineer and/or supervising officer for the building. Notches and holes should be as small as necessary to receive pipes, whilst allowing for expansion and contraction. Notches should be U-shaped and formed by parallel cuts to previously bored holes.

Care should also be taken to ensure that all work is completed in a workmanlike manner, not just where drilling and notching is concerned, but also where cutting and replacing floorboards are concerned.

Requirements

The requirements for notches and holes in simply supported floor and roof joists were previously given in approved Document A (Structure) of the Building Regulations, however, since 2004, although Approved Document A (ADA) still contains requirements to ensure that the structural integrity of buildings is not compromised, ADA no longer contains specific requirements for drilling and notching joists (many Approved Documents contain extracts from the Regulations, but no longer give specific requirements for individual installations and reference other documents such as British Standards accordingly). The information contained within this article is based upon the information that was previously within Approved Document A, however, these guidelines are still featured in the Scottish Building Standards and can also be found in many industry best practice documents. In addition many Building Control Departments will still have the appropriate details relating to the requirements for drilling/notching joists.

Notches and holes in simply supported floor and roof joists should be within the following limits:

• Notches should be cut no deeper than 1/8 of the depth of the joist. They should not be cut closer to the support than 0.07 times the span, nor further away than 0.25 (1/4) of the span.
• No notches or holes should be cut in roof rafters, other than at supports where the rafter may be birdsmouthed to a depth not exceeding 0.33 times the rafter depth.
• Where straight lengths of half-hard copper are required to be run in floors they can be laid in notches. If laid in joist clips with integral metal plates, the risk of damage caused by nailing accidents should be eliminated.

This method of laying pipes in joists can prevent weakening of the joists, as the rectangular shape of the joist clip can be used as a template to prevent excessively deep notches. Although unseen when the installation is complete, joist clips improve the overall quality of the installation. They do this by helping to align the tube and permit movement caused by expansion within pipework. This will help to prevent clicking noises and the water hammer that can arise due to badly aligned pipework.

It is essential that structural members are not weakened by indiscriminate notching and boring. In this respect both the size and positioning must be in accordance with figure 1. (below)

Figure 1. Requirements for notching traditional joists.

 

Drilled holes should be no greater in diameter than 1/4 of the depth of the joist. They should be drilled on a neutral axis and should not be less than 3 diameters apart, measured from centre to centre. Drilled holes should be located in the area between 0.25 and 0.4 times the span of the joist from the support. For illustration see figure 2 (below).

 

Figure 2. Requirements for drilling holes through traditional joists.

 

Engineering Joists

Buildings that incorporate timber ‘I’ joists (Engineering Joists) allow for more flexible drilling of holes for pipework. ‘I’ joists incorporate 38mm knockouts (dependant upon the manufacturer), and also allow for larger holes to be drilled through the joists. The same rules apply when drilling through ‘I’ joists as for drilling traditional joists where horizontal measurements between holes are concerned.

However, as the strength of ‘I’ joists is in the flange (See diagram), these joists are not suitable for notching, as notching the flange will damage the structure of the joist which will have a detrimental affect on its strength.

The size and positioning of holes drilled through ‘I’ Joists must be in accordance with Figure 3 (below).

Figure 3. Requirements for drilling holes through Engineering (‘I’) Joists.

 

Installation Practices

Traditionally pipes have been laid in notches in joists because the rigidity of the pipe does not easily allow for any other installation method. This means that the pipework must be installed before the floorboards are laid and fixed.

When working on site, installing pipework in notches has a number of disadvantages, as there is an increased risk of injury to the installer as he/she will have to work on open joists. Also pipework ‘tails’ for radiators etc. are often disturbed by other trades before and during the laying of floor-boards, meaning that pipework will need to be altered, which ultimately means more time spent on site and the use of additional materials.

The ability to drill holes through joists and ‘cable’ pipework offers the advantages of both enhanced safety and reduced time spent on site as the installer can work from below a pre-laid floor. The specific advantages of this are:

• The installation of pipework can be carried out at later stages in building developments meaning that installers can work in more ‘pleasant’ surroundings i.e. water-proofed buildings.
• Installers will not be exposed to the risks associated with; falling from heights, injuries to knees caused by kneeling on joists and tools and other equipment falling from above.
• Installations may not need to be broken down into stages, as installers will not have to return to connect radiators and other fixtures after floors have been laid.
• Pipework ‘tails’ can be installed without the worry of them being disturbed whilst the floor is being laid.
• The possibility of pipes being damaged by nails that are used to fix flooring will be significantly decreased.

Examples:

1. Notching Traditional Joists. For a traditional joist measuring 150mm deep and 50mm wide         spanning 3.0m;

Minimum distance from support:

3000mm(3m) x 0.07=210mm

Maximum distance from support:

3000mm x 0.25=750mm

Maximum depth of notch:

150mm/8=18.75mm

Therefore notching may occur between 210mm and 750mm from the support wall, and notches should be no deeper than 18.75mm.

 

2. Drilling holes through traditional joists. For a traditional joist measuring 150mm deep and 50mm wide spanning 3.0m;

Minimum distance from support:

3000mm(3m) x 0.25=750mm

Maximum distance from support:

3000mm x 0.4=1200mm

Maximum hole diameter:

150mm/4=37.5mm

Therefore holes can be drilled at the neutral axis between 750mm and 1200mm from the support wall. The maximum hole diameter should not exceed 37.5mm and holes should not be located less than 3 diameters apart (centre to centre). So 37.5 mm holes should be separated by 112.5mm (centre to centre).