Module 2 of 2 · 45 minutes
Structure: joists, stud walls and hanging a door
By the end of this module you will be able to
- Notch and drill a joist within the permitted zones, and explain why they exist
- Build a stud wall that is straight, plumb and takes fixings where needed
- Hang a door so it closes cleanly and stays closed
- Recognise the structural work that needs professional design
Work through it
2 interactives for this module, built on the WAJD Teach engine. Nothing moves until you ask it to, and every one has a written version if you would rather read it.
Amara A plumber came and cut a big notch in the bottom of my joist. Is that a problem?
Nadia Yes, and it is one of the most consequential things anybody does in a house without realising.
Amara Explain why the bottom is different.
Nadia A loaded joist is in compression along its top edge and tension along the bottom, with a neutral axis through the middle where there is very little stress. Notch the bottom edge and you have cut into the most stressed fibres in the whole beam, in tension.
Amara So it is a crack waiting to start.
Nadia That is exactly what it is. And it is why floors bounce and ceilings crack under bathrooms, because that is where the pipework goes.
Amara Where am I allowed to cut?
Nadia Notches in the top edge only, no deeper than an eighth of the joist depth, and only between about 0.07 and 0.25 of the span from a support. Holes on the neutral axis, no bigger than a quarter of the depth, between 0.25 and 0.4 of the span, and at least three diameters apart.
Amara And if my pipe does not fit in the permitted zone?
Nadia Then drill on the neutral axis instead of notching, or run it in the void below and accept a boxing or a ceiling, or get the joist assessed. What you do not do is cut where it is convenient and hope.
Amara Anything I should never cut at all?
Nadia Engineered joists and I joists. Completely different rules, and the manufacturer specifies exactly where you may make a hole and how big. Cut one of those in the wrong place and you have removed the flange that was doing all the work.
Amara Let us do stud walls. What do people forget?
Nadia Two things, and both bite later. Noggins, the short pieces between studs, which stiffen the wall and give you a fixing at board joints. And timber where something heavy is going to hang.
Amara A television.
Nadia A television, a radiator, a grab rail. Put a proper piece of timber in now while the wall is open, and photograph where it is before you board over it. That photograph is worth more than any stud detector you will ever buy.
Amara Anything else worth doing while it is open?
Nadia Insulate it, even internally. It costs very little and acoustic separation is the single thing people most appreciate about a stud wall afterwards.
Amara Hanging a door. Mine always bind.
Nadia Then your hinge recess is cut too deep, almost certainly. But the real problem is usually sequence.
Amara Meaning?
Nadia People fit the hardware first. Fit the door to the opening first, before anything goes on it. Even gap all round, two to three millimetres at the sides and top, bottom gap to suit the floor.
Amara Then hinges.
Nadia About a hundred and fifty millimetres from the top, two hundred and twenty five from the bottom, third hinge in the middle if it is heavy or a fire door. And cut the recess so the leaf sits exactly flush. Too deep and it binds. Too shallow and it will not close.
Amara And when it swings back open?
Nadia That is the striking plate, not the door. Either not deep enough or slightly out of alignment. People plane the door in frustration when the fault is a two minute adjustment to a piece of metal.
Amara Last question. What do I not touch?
Nadia Anything load bearing. New openings in walls. And above all, trussed rafters.
Amara People board lofts out all the time.
Nadia They do, and they cut a diagonal member out of the way to get a clear space. A truss is a complete triangulated system designed as a whole. Cut one member and you have changed how the entire roof behaves. It is probably the most consequential amateur alteration in British housing.
The written material
Notching a joist wrongly is how ceilings crack
A loaded joist is in compression along its top edge and tension along its bottom edge, with a neutral axis through the middle where there is very little stress. Everything about the permitted zones follows from that.
Notches are permitted in the top edge only, no deeper than 0.125 of the joist depth, and only between 0.07 and 0.25 of the span from a support. Holes must be on the neutral axis, no larger than 0.25 of the depth, between 0.25 and 0.4 of the span, spaced at least three diameters apart.
Notching the bottom edge is the serious one. That edge is in tension and a notch there is a deliberate crack initiator in the most stressed fibres of the beam. Plumbers and electricians do it because it is convenient and it is how floors end up bouncing and ceilings cracking.
Stud walls, and the things people forget
Set out the head and sole plate together so the studs are square, fix the sole plate to the floor and the head to the joists above, and space studs at 400 or 600mm centres depending on the board and the loading.
The two things beginners omit both matter later. Noggins, the short pieces between studs, stiffen the wall and give you a fixing at board joints. And timber noggins where a heavy item will hang, so a radiator, a television or a grab rail has something real to fix into. Put them in now and mark their positions with a photograph before boarding.
Insulate between studs even on an internal wall. It costs very little and the acoustic improvement is the single most appreciated thing about a stud wall.
Hanging a door
The sequence matters, and the usual failure is fitting the hardware before the door fits the opening.
Get the door to fit the opening first, with an even gap all round: roughly 2 to 3mm at the sides and top, and a bottom gap suited to the floor finish. Then mark the hinge positions, conventionally about 150mm from the top and 225mm from the bottom, with a third hinge in the middle for a heavy or fire door. Cut the hinge recesses so the leaf sits flush with the timber surface: too deep and the door binds against the frame, too shallow and it will not close.
Hang it on the hinges, check the swing, and only then fit the latch and the handle. A door that binds is almost always a hinge recess cut too deep, and a door that springs back open is usually a striking plate that is not deep enough or is out of alignment.
- Fit the door to the opening before any hardware goes on
- 2 to 3mm gap at sides and head, bottom gap to suit the floor
- Hinges about 150mm from the top and 225mm from the bottom
- Binding usually means a recess cut too deep; springing open usually means the striking plate
- Fire doors have specific gap, intumescent and hinge requirements and must not be improvised
Where to stop
Removing or altering anything load bearing, forming a new opening in a wall, cutting or altering a trussed rafter, and altering engineered joists all require design by a competent person and usually building control approval.
Trussed rafters in particular: they are designed as a complete triangulated system and cutting a single member can compromise the whole roof. People do it for loft storage constantly and it is one of the most consequential amateur alterations in British housing.
Knowledge check
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