Slight deformation in a catnic lintel

Rob broom

Rob broom

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Hi there.

new to the forum and finding it really useful, so thanks in advance to anyone that reads this and replies.

firstky, I have no previous experience of installing these lintels!

I just wanted to check if a minor amount of deformation in a catnic lintel is to be expected, or whether they will always be dead level/perfectly true and straight when fully loaded?

I have a couple on my new build that have bowed a tiny bit in the middle. I’m only talking about 1mm, 2mm at a push!

They are CG 90/100 as specified by my structural engineer.

dense block outer leaf
Tgermolite inner leaf
100mm cavity
Construction is a 1 &1/2 story (second story is a room in roof)

the span of windows is around 1600mm
Bedded over 150mm at each end as per guidelines.
All level and perfect when installed, but as I have now added 8 block courses above, and also all of the roof rafters too, I’m staring to worry if the lintels are ok or if it’s time to question there suitability?

I saw a thread that mentioned a calculation of span/300=deflection.

that means I can allow 1600/300=5.33mm


So I wondered if all catnic lintels will naturally bend a little when they are installed, or if I’ve got issues!?

Does the calculation mean thatvyhe lintel will be expected to flex by 5.3mm, or is that the calculated limit allowed?

Bear in mind, I’ve still got more load to add to it yet, when I’ve fitted the upstairs floor , fixtures, fittings, en-suite shower/toilet and put the tiles on the roof.

Any clarification would be great fully received!

or do I need to get the accrows and strong boys out and put in a stronger lintel?

cheers
 
All lintels - without exception - deflect whatever load you put on them. The only question is how much deflection is acceptable?
If you're only getting 1-2mm, I'd say that's fine
.
Catnic lintels have the outstand ledge at the front to carry the outer skin, so this is usually more susceptible to deflection than the inner skin, where most of the metal is. Once you've loaded it up when the job is complete, it won't deflect anymore and the lintel certainly won't collapse.

Catnic give an acceptable ratio of load on outer skin / load on inner skin and you might possibly have exceded that a little with having dense block on the outer skin, but still - it's not going anywhere.

As long as the average man in the street can't see it when looking at the extension, it's not a problem.
But your eye will always be drawn to it, because you know it's there! :)
 
You should always prop a Catnic or any tray lintel in the centre before laying anything on it. IIRC you should also lay three course of brick (or one block) and let that go off before continuing. Then they hardly deflect.

As you have already built on it, it wont get any worse structurally, but will just look like that now.
 
Sounds like that’s been my rookie mistake then?!

I didn’t prop it in the middle, and possibly did two courses too soon!

Rookie mistake - hands up!! Won’t happen next time!

Just spoke to my BCO, who is due to visit me tomorrow to check my roof is coming along nicely, and he said the same, that a certain amount of deflection will be expected!

Fortunately, I will be rendering the house when it’s all done, so the little bit of deflection will never be seen. But I’m not the sort of bloke who wants to cover it up and hide it, I’d rather be sensible and sort it out now before it bites me in the future!

figers crossed it will be fine!

cheers for the responses guys!
 
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