Installing Pipework and Wiring Under Floors

Installing Pipework and Wiring

Installing Pipework and Wiring Under Floors Every house has some pipework and wiring, the majority of which runs in floor spaces. When replacing pipes or wiring it is often possible to follow the existing runs but things are slightly more complicated for additional services. Although this website does not describe how to connect up the wiring and plumb in the pipes, knowing how to cut and drill your floor joists to make a neat, safe job of accommodating these services is an important aspect of this type of work.

Running new pipework and electric wiring can be a time-consuming and frustrating job even at the best of times. It involves a lot of preparation before any work can be commenced since the floor covering and boards will have to be removed to gain access to the floor space where the pipes are housed. With all this disruption it therefore makes sense, if possible, to combine the installation of pipes and wiring with other work on the floor. If you are installing brand new joists, cutting any notches or drilling any holes before installation is much easier.


Safety Advice

It cannot be over emphasized that cutting notches and drilling holes through joists can seriously affect their structural, strength. To minimize this problem, when you have to cut or drill through joists, make certain that no two holes are closer than about 150mm (6in) apart or splitting between holes could, in a worst case scenario, cause the joist to fail.

Tools for the Job:

  • tape measure & pencil
  • chalk line or straightedge
  • square
  • handsaw
  • hammer
  • wood chisel
  • mallet
  • cordless drill/driver
  • brace & bit


Cutting Notches for Pipes

Unlike wiring, pipework has to be run in straight lines. Having first removed floorboards, to ensure all the notches are cut in a straight line, either lay a straightedge across the tops of the joists or snap a chalk line to give you a guideline. Make any cut-outs as close to the wall as possible where compression loads are least.

Before re-laying the floorboards you might want to fit metal protectors over pipes. These are small metal clips that cover the tops of pipes where they pass through joists, preventing nails from inadvertently being driven in them.

  1. To work out the width of the notch needed for the pipework to slot in, measure the diameter of the pipe — which for copper tubing is normally either 15 or 22mm (5/8 or 3/4in) — and then add on 2mm (1/8in) to allow for expansion and movement, though make sure the notch is not too wide. Using the chalk line or straightedge as a reference, mark pencil lines across the top of the joist at this distance apart. Using a square, transfer these measurements to either side of the joist. Mark across the lines to the same depth as the width measurements. Make vertical cuts with a handsaw along the lines on top of the joist down to the depth guideline.
  2. Use a chisel and mallet and chop out the waste timber between the cuts, gradually working down to the depth guideline. Lay a section of the pipe in the notch to check that you have removed enough timber. If the notch is tight ease the sides a little. Place a straightedge across the top of the joist with the pipe in position to ensure that when the floor is replaced it will not be resting on the pipe.


Safety Advice

Never cut too much out from the of joists when cutting notches as this could compromise the strength of the floor If you are unsure as to whether or not it is sat to cut anything out of a section of floor, contact an architect or structural engineer.


Tips of the Trade

If you have lots of notches to cut that are the same size it may pay to make a template from a scrap of plywood. Cut a piece to match the required depth and width of cut, then align it with the top surface of the joist and chalk line and draw around it.


Drilling for Cables

Wiring is flexible and as such need not run as straight as pipework, so rather than cutting notches it is often easier to drill holes instead. A right angle drill is useful for accessing between joists.

It is still a good idea to run cables in a straight line though as this makes for a neater job. Drill on the centreline of the joist where stresses are least great and do not make the hole any larger than is absolutely necessary.


Tips of the Trade

Spade bits are cheap and ideal for drilling through joists but there is often sufficient room to get the bit and drill between two joists. Overcome, this problem by cutting away most of the  shank of the drill bit. As long as you leave about 35mm (13/8in) on the length of the bit, you will be able to fit most drills between joists and still have enough length on the hit to drill through the width of the joists.


Using a Brace and Bit

If you do not have an electric drill then it is perfectly feasible to use a brace and bit to accomplish the same task. Special varieties are designed for fitting in the space between joists. Work from one side of the joist applying steady pressure to the drill as it cuts through the wood. As soon as the tip of the drill emerges through the surface of the timber, withdraw the bit and drill from the other side. This produces a far neater hole with no breakout, which would otherwise snag on the cables making them difficult to feed through.


Other Considerations

Where pipes and cables run parallel to joists, use clips and brackets to hold them in position. Be careful not to damage the services and never run cables close to heating pipes where the heat could damage the insulation.

Where water pipes run below ground floor level wrap them with neoprene insulation to prevent them freezing.

Start at one end, slipping on the insulation as you go. Hold it in position with either plastic insulation tape or plastic cable ties.

When you replace the floorboards it is a good idea to mark the position of cables and pipes on the top surface with a marker pen. This will make future maintenance easier as, should you need to gain access to any pipes or cables, you will know to remove only the boards directly above them.


Safety Advice

Never attach cables to gas or water pipes. If the cable were ever to short out it could make all the plumbing in the house live or start a fire.


Inspection Hatch

If access is likely to be needed to a junction box, isolating valve or light fitting in the future, consider fitting an inspection hatch. This could simply be a small cut-out section of floorboard directly above the fitting, which can be removed without lifting a large section of flooring. Alternatively, consider using a ‘rout-a-bout’ to cut a hole, which is especially useful in ply and chipboard floors.

13. December 2010 by admin
Categories: DIY Home, Flooring | Tags: , | Comments Off on Installing Pipework and Wiring Under Floors

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