Who this guide is for
- Self-builders coordinating a first fix through a new floor
- Owners whose plumber or electrician has asked to route through an existing floor
- Anyone specifying engineered joists for the first time
- Owners planning a bathroom, kitchen or plant position on an upper floor
- Anyone who has lifted a floor and found cuts nobody recorded
A route is designed, not discovered
A service route through a floor is a design output. It is settled between the structural designer and whoever is running the services, against the member type, before the deck goes down. Discovered routes are what happens when that conversation did not take place, and the person holding the tool is then making a structural decision without the information to make it.
That is why the entries frame this as coordination rather than workmanship. The trade cutting the joist is usually doing competent work against the wrong constraint.
The depth was spoken for before anyone arrived
In a joisted floor the same dimension has to satisfy the span, leave room for services and give the ceiling something to fix to, and each party wants it for a different purpose. Deepen it for stiffness and the room below loses height; keep it shallow and pipes have to go through the members rather than past them, which turns a plumbing decision into a structural one.
The entries call this space contested for good reason. Nobody is being unreasonable; there is simply less of it than the three parties between them need.
The member type changes the answer completely
A joist is not equally strong across its depth or along its length, so where a hole may go is decided by how the member works and not by where the pipe wants to be. Engineered members change the answer completely: some have designated service openings and are ruined by a cut elsewhere, which is why the member type has to be known before the first pipe is run.
This is the single most transferable point in the whole subject. A rule of thumb learned on solid timber is not a rule at all once the floor is built from a manufactured member, and the manufacturer's information is the governing document.
- Solid timber members, where the constraint follows how the section carries load
- Engineered members with designated openings, where a cut elsewhere is a different matter entirely
- Cold-formed steel joists, which are met in light steel construction
- Trimmers and members alongside an opening, which are already carrying more than their neighbours
Who decides, and when
The entries route this to two parties together: the structural designer for the floor, and the manufacturer's information for the product. Neither alone is sufficient. A designer can say what the floor needs the member to do; the manufacturer can say what the member permits; the intersection is the route.
The timing follows from that. The decision has to be made while the drawing set is still open, because once the deck is fixed and the ceiling closed, changing a route means opening construction that was finished.
Some members are already carrying more than their neighbours
Trimming an opening in a floor redistributes load onto the members alongside it, and those members commonly carry substantially more than their neighbours as a result. A route that treats every joist as equivalent can land a notch in exactly the member that is already doing extra work.
The same applies where a stair lands. The member framing a stairwell may be receiving the stair's accumulated load as well as the floor members it already carries, and nothing about the surface above indicates which member that is.
Altering the floor can reach the walls
Where joists are built in or hung, the floor is also restraining the wall it bears on. The entries are explicit that removing or notching members to make room for services can affect both the floor and the walls that it restrains, so an alteration made for a service reason is not confined to the floor.
In a platform frame the floor is built as a storey deck that the walls above are raised from, and it transfers stability forces, which widens the consequence again.
What an existing floor obliges you to establish first
Existing floors often contain unrecorded alterations, and what is found on opening one may differ from whatever was drawn. That is a reason to establish the member type and the existing condition before planning a route, rather than a reason to assume the floor has absorbed previous work without consequence.
It is also the argument for recording what is done. A cut that nobody wrote down is a constraint the next person inherits without knowing it.
Planning a service route through a joisted floor
- 1Establish the joist type before any route is planned, including whether it is an engineered product.
- 2Obtain the manufacturer's information for that member and read what it says about openings.
- 3Ask the structural designer what this floor needs the members to do.
- 4Agree the route between the structural designer and the services designer, in writing.
- 5Fix the route while the drawing set is still open, not after the deck is down.
- 6Identify trimmers and the members alongside any opening, and keep routes clear of them.
- 7Ask whether the floor is restraining the walls it bears on, and what altering it would affect.
- 8Ask whether the route can pass between members rather than through them.
- 9Where services must cross, group them so the sealing and the structure are both resolved once.
- 10Provide access panels or planned routes for the parts most likely to need attention.
- 11Record what was cut, where and by whom, and keep it with the building.
- 12In an existing floor, establish what previous work has already been done before adding to it.
Common mistakes to avoid
- Treating a notch or a hole as a plumbing task, when where it may go is decided by how the member carries load.
- Carrying a habit from solid timber onto an engineered member, which may have designated openings and be compromised by a cut elsewhere.
- Planning services after the structure is set, which is the ordering that produces cuts nobody intended.
- Assuming all joists are equivalent, when members alongside an opening carry redistributed load.
- Altering members to make room for a duct, when the floor may also be restraining the walls it bears on.
- Leaving no access to the parts of the route most likely to need attention later.
- Failing to record a cut, which leaves the next person working from a drawing the floor no longer matches.
When to involve a professional
- Where services may pass through a joisted floor is determined by the structural designer together with the manufacturer's information for that member.
- Cutting, notching or drilling structural members is structural alteration and requires qualified advice.
- This guide states no permissible zone, position, proportion or dimension, and none can be inferred from it.
- Removing or notching members can affect both the floor and the walls that it restrains.
- Existing floors often contain unrecorded alterations, and what is found on opening one may differ from whatever was drawn.
Frequently asked questions
Questions readers ask about this topic
Where can I drill a floor joist?
This guide will not say, and neither will the reference entries. Where a hole may go is decided by how the member carries load and, for engineered products, by the manufacturer's information. That determination belongs to the structural designer for your floor.
Why does my engineered joist have holes already in it?
Some engineered members carry designated service openings as part of the product. The entries add the important half of that: such members can be compromised by a cut made elsewhere, so the presence of provided openings is not permission to make new ones.
Whose job is it to plan the route?
It is a coordination between the structural designer, who knows what the floor needs the members to do, and whoever is running the services. The manufacturer's information sets the outer limit. None of the three can settle it alone.
Can I notch a joist if it is only a small pipe?
The size of the service is not what governs. What governs is what removing material does to a member at that position along its length and across its depth, which is a structural question answered for the specific floor rather than by the diameter of the pipe.
What if the floor has already been cut by someone else?
Previous work having caused no visible problem is not evidence that further work will not. Existing floors commonly contain unrecorded alterations, and establishing what is actually there is the first step rather than a formality to be skipped.
Keep reading