Notching joists

[trevorbayliss]

I have placed my joists hangers on my new blockwork today. Because of doors still in place i have not been able to get a fully accurate level with the new extension floor and the current internal floor.

I wanted to be a little high so all i would need to do was notch the joists to get the correct level.

I think i will need to notch the joist about 50mm. Is there a limit to how much you can notch.

I have purposefully overspeed the joist size as i prefer to use a larger one as it gives more stability. I am using a 47 x 145mm joist at 450mm centres and the max span is 2.5m.

 

[^woody^]

Your joist will be equivalent to a 47x95 joist now. Is that enough?

 

[John D v2.0]

Your joist will be equivalent to a 47x95 joist now. Is that enough?

That would be for shear at the ends, but presumably if an SE was calculating, the deflection would not be as much?

 

[trevorbayliss]

Well for the spans of 1.6m and 1.9m i think it will be ok, but i have a span of 2.5m so it won't be enough.

I thought it was ok to notch these joists to fit. Ive cocked up here.

Tonight i have just laid one section of bricks over the joist hangers which are masonry ones. I bought 145mm ones. So could i get a bigger size joist hanger and use these instead, say one for a 175 joist. Then i would notch out only 1cm. I think the notching may only need to be 40mm, so this would reduce the joist down to a 105mm size. It is actually 150mm in size as it has be treated and so expanded a bit.

Help appreciated !

 

[^woody^]

Glue and nail about 300 mm of 9mm ply to both sides of the joists.

I can't understand why you did not transfer a level from the existing floor.

 

[^woody^]

That would be for shear at the ends, but presumably if an SE was calculating, the deflection would not be as much?

Dunno, but if it fails in one it fails as a joist

 

[trevorbayliss]

Panic over !!!

Just been out and set a joist in the first joist hanger and levelled it up with a block. The joist is 20mm below finished floor level. Which means my 20mm oak floor has room to fit in !!!

So if i am a little out i think notching the joist by 10mm will be ok given my spans. I was wanting to make it just a bit higher just so i do hit this FFL. I suppose until i rip out the door and adjust the bricks underneath it the exact level won't be determined, but i think all is ok.

It has been hard getting a good level because i have a big threshold between the extension and the kitchen floor. Have been using a spirit level over the top of it and measuring down both sides. Having that joist in place next to the threshold really made it easy to measure.

 

[^woody^]

Put something, a block say, on the floor, level off the block to a mark on the frame, or use another block on the joist on the other side of the frame.

 

[tony1851]

Well for the spans of 1.6m and 1.9m i think it will be ok, but i have a span of 2.5m so it won't be enough.

I thought it was ok to notch these joists to fit. Ive cocked up here.

Tonight i have just laid one section of bricks over the joist hangers which are masonry ones. I bought 145mm ones. So could i get a bigger size joist hanger and use these instead, say one for a 175 joist. Then i would notch out only 1cm. I think the notching may only need to be 40mm, so this would reduce the joist down to a 105mm size. It is actually 150mm in size as it has be treated and so expanded a bit.

Help appreciated !

Relax, it will be fine!
The only criterion is the shear at the ends.
Even on the 2.5m span, the shear stress is still within the limit allowed by the BS code for timber.

 

[tony1851]

That would be for shear at the ends, but presumably if an SE was calculating, the deflection would not be as much?

Notching the joist at the end would not affect the deflection.
The 3 main concerns when calculating floor joists are - in order of importance - deflection, bending stress and shear.

 

[John D v2.0]

Notching the joist at the end would not affect the deflection.

Exactly well it might be the tiniest bit more I suppose, anyway according to my SE it's usually the deflection that's the limiting factor on simple beams/joists hence the op doesn't have that issue.

Going off topic but it does make me wonder why they say don't notch close to the ends of joists. I've always thought the closer the better. Same for drilling the neutral axis, closer to the middle is better surely?

 

[tony1851]

Going off topic but it does make me wonder why they say don't notch close to the ends of joists.

The accepted notching and drilling rules are conservative, to allow for idiots overdoing things, eg when plumbers hack big chunks out for pipes.
Doing the actual figures from 1st principles allows you to be more accurate in how much you cut away or drill before the strength becomes compromised. But tradesmen won't pay to get calcs done, and rules of thumb are easy to follow.

PS, I really don't know why they say don't notch near the end. If, for example,
you notch a 200 deep joist by 50 at the end, it has far less effect than if you notch it away from the end. And the nearer the middle the notch is, the worse it gets until at the middle, a 50 notch makes it effectively a 150 deep joist.
End notching is only a problem when a timber beam is carrying a very heavy load, not a situation you find in domestic work.

 

[John D v2.0]

at the middle, a 50 notch makes it effectively a 150 deep joist.

For the purposes of deflection or bending strength?
I agree for the bending strength, but for deflection presumably the overall deflection would be better than a plain 150 joist, due to the thicker, less elastic parts taking a lot of the total amount of bending.

Or does it not exactly work like that?

 

[garyo]

I've always found the counter-intuitive nature of stress raisers fascinating - e.g. drilling a hole in glass will stop it cracking further, or in this case, a 200mm deep joist with a 50mm notch in the middle will be weaker than a simple 150mm joist due to the forces being concentrated at the centre by either side being far more rigid. Similarly, fettling engine con-rods etc...

 

[John D v2.0]

Yeah it is interesting! Whenever I open a pack of something wrapped in that thin plastic eg museli I always turn any rips in the bag back round towards the open end, then it stops the whole side of the bag opening and everything spilling. I guess that's just the stresses and it's like your hole in the glass at work.

However I think with timber the strength is rarely the issue, usually it's the elasticity/deflection. I don't know of any joists that have snapped due to excessive notching but there are a lot of bouncy floors around.