What are these screws used in an old step ladder?

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I'm fixing a broken folding step ladder and am coming to the point where I'm reassembling it. Looking at the screws that were used, the ones that were used to secure the step planks are of a type I'm unfamilar with.
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They are pan washer heads with a thread which looks extremely ineffective. There's hardly any depth to it at all. They can almost be pushed in and out. Initially, I was going to simply replace them with a new screw of the same dimensions but with threads which will bite into the wood and provide purchase.

However, thinking about it some more and considering where they are used (only to secure the step planks) I'm wondering if they are specifically made for this purpose? I.e. perhaps they allow a very low bite which means the ladder can have a certain amount of 'give' lowering the chance of a split or break?

Or they could just have been replaced by someone in the past who just used them because they were all that they had to hand at the time! Does anyone know what they are called or knows of their correct usage?

Any advice much appreciated.
 
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they might have been hammered (or pressed) in.

though the thread is not as slow as on twist nails.

There are some carpenters who say that the correct tool to drive a screw is a hammer, and the slot is only there in case you want to take it out.

Does the slot look like it was ever used?
 
They are a variety of a twist nail, often used in roofing......Ive used them to secure asbestos sheets etc down.
Horrible things for stepladder use, I’d replace them with maybe a torx domed head screw or indeed an ordinary long wood screw.
On occasion I’ve used a length of 8mm studding below every other tread for extra security.
John :)
 
@JohnD That's interesting. By slot do you mean the holes? Checking the insides, there doesn't seem to be any ridges at all.

@Burnerman Ahh ok good to put a name to them. They did initially strike me as completely ineffective when I extracted them.
 
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One shot use only, gizmo.....for roofing purposes there’s often a plastic washer come hat that goes on the nail first.
John :)
 
There are some carpenters who say that the correct tool to drive a screw is a hammer, and the slot is only there in case you want to take it out.
Give over, John! Those of us who are carpenters refer to them as wood butchers (or handymen......)
 
Thanks for all the replies everyone. Yes, I'd say the wood was oak, it's a good point because the previous screws rusting have definitely tainted the wood. I'm minded to replace them really, these look suitable.

The only question mark is if the use of the twist nail is for any sort of structural reason? Does anyone know if that might be the case, or am I over thinking it!;)
 
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Twist nails are normally used to fix joist hangers and truss clips, etc to timber elements in building structures. AFAIK they are unsuitable for any other use. The type of screw you have is more a kind of knock in screw fixing with an integrated washer rather than a twist nail. They were probably used because it is easier and faster to knock in a nail than it is to drive in a screw
 
The originals are 40mm so I think I should keep around that size. The oak has come up really well, but is showing a few signs of splits. So I'd like to keep as much material as possible and not tempt fate going deeper and things possibly splitting.
 
For sure, any replacement screws need to be longer than the original twist nails in order for them to grip into virgin material.....I’d say 20mm or more.
Splitting is unlikely but if you aren’t sure then a small diameter pilot drill hole takes care of that.
John :)
 

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