Cutting joists and adding strength
Discussion
You should find detailed guidance online somewhere.
The joists may be bigger than needed for the span, you can check this against standard tables.
It happens because all the joists are likely the same despite varying room sizes.
You will weaken a joist less by making a hole in the middle (half way up) than a notch in top or bottom.
The stress in a joist is less closer to where it's supported, so it's more acceptable to make a notch near the ends than in the middle.
From a structural theory point of view, you could make the floor structural out of suitable plywood glued and screwed to the joists, but I don't know what building regs say about that and it would be a sod for any future work. Each joist then becomes a T section with the ply.
There's good reason why our shower is raised on a little plinth and the drain runs through a box above the joists!
You don't want any perceptible flex in a bathroom floor.
I am not White Van Man, amateur disclaimer applies!
The joists may be bigger than needed for the span, you can check this against standard tables.
It happens because all the joists are likely the same despite varying room sizes.
You will weaken a joist less by making a hole in the middle (half way up) than a notch in top or bottom.
The stress in a joist is less closer to where it's supported, so it's more acceptable to make a notch near the ends than in the middle.
From a structural theory point of view, you could make the floor structural out of suitable plywood glued and screwed to the joists, but I don't know what building regs say about that and it would be a sod for any future work. Each joist then becomes a T section with the ply.
There's good reason why our shower is raised on a little plinth and the drain runs through a box above the joists!
You don't want any perceptible flex in a bathroom floor.
I am not White Van Man, amateur disclaimer applies!
Reasonable summary here,
https://www.diydoctor.org.uk/projects/Notching_joi...
Don't just start chopping lumps out.
https://www.diydoctor.org.uk/projects/Notching_joi...
Don't just start chopping lumps out.
shtu said:
Reasonable summary here,
https://www.diydoctor.org.uk/projects/Notching_joi...
Don't just start chopping lumps out.
Good find. 50mm dia. holes through the centre of the joists should do it depending on the joist spacing.https://www.diydoctor.org.uk/projects/Notching_joi...
Don't just start chopping lumps out.
shtu said:
Reasonable summary here,
https://www.diydoctor.org.uk/projects/Notching_joi...
Don't just start chopping lumps out.
And yet virtually every house has had the joist hacked about at some point and and are still standing!https://www.diydoctor.org.uk/projects/Notching_joi...
Don't just start chopping lumps out.
My bathroom joists had been hacked about over the years. Like most places I guess.
I changed the bathroom last year & had to do a bit more..
I read up & ‘sistered’ the joists with plywood, beforehand.
From AI:-
“ How to Sister Joists with Plywood:
Sistering joists with plywood involves attaching plywood to the side of an existing joist to reinforce it. This is often done to fix sagging or damaged joists.
Materials Needed:
• 3/4” or 1/2” plywood (exterior grade or structural-rated)
• Construction adhesive
• Wood screws or nails
• Clamps (optional but helpful)
Steps:
1. Cut the Plywood: Cut plywood strips to match the height of the joist and at least 4 feet in length (the longer, the better for strength).
2. Apply Adhesive: Use a strong construction adhesive on the side of the joist.
3. Attach the Plywood: Press the plywood against the joist and clamp it in place if possible.
4. Screw/Nail It Down: Use screws or nails spaced every 6–8 inches along the top and bottom edges of the plywood strip.
5. Repeat if Needed: For added strength, you can sister both sides of the joist”
I then screwed plywood floor, in place of the old chipboard.
Bought big sheet in B&Q.
They cut it to size for free.
Measured exactly so that the joins above joist/easy to handle/fit in the car!
I changed the bathroom last year & had to do a bit more..
I read up & ‘sistered’ the joists with plywood, beforehand.
From AI:-
“ How to Sister Joists with Plywood:
Sistering joists with plywood involves attaching plywood to the side of an existing joist to reinforce it. This is often done to fix sagging or damaged joists.
Materials Needed:
• 3/4” or 1/2” plywood (exterior grade or structural-rated)
• Construction adhesive
• Wood screws or nails
• Clamps (optional but helpful)
Steps:
1. Cut the Plywood: Cut plywood strips to match the height of the joist and at least 4 feet in length (the longer, the better for strength).
2. Apply Adhesive: Use a strong construction adhesive on the side of the joist.
3. Attach the Plywood: Press the plywood against the joist and clamp it in place if possible.
4. Screw/Nail It Down: Use screws or nails spaced every 6–8 inches along the top and bottom edges of the plywood strip.
5. Repeat if Needed: For added strength, you can sister both sides of the joist”
I then screwed plywood floor, in place of the old chipboard.
Bought big sheet in B&Q.
They cut it to size for free.
Measured exactly so that the joins above joist/easy to handle/fit in the car!
Edited by RotorRambler on Saturday 3rd May 08:16
tried clicking n that link above but couldn’t find a way to disable cookies easily !
https://www.labc.co.uk/news/how-get-it-right-notch...
That’s pretty good in terms of spacing, sizes, how near to end of joist etc
We had a new heating system installed at the end of last year….a lot of holes in a lot of joists…..
https://www.labc.co.uk/news/how-get-it-right-notch...
That’s pretty good in terms of spacing, sizes, how near to end of joist etc
We had a new heating system installed at the end of last year….a lot of holes in a lot of joists…..
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