DIY sit harness.

You can fashion a sit harness from a long tape sling. I've used the 3rd method described  In this article on one occasion to ab off, and it works. But you do need an element of faith in it, and its definitely not one to sit in for very long

What are you wanting to do?

Is if for you in which case how happy are you with whoever is nearby rescuing you? Since you are asking a mountain bike forum I would tend towards nah and suggest at the minimum ukclimbing and even then I would do a bit more work and find whoever generally certifies height work.

For someone else see above times ten. 

Who is managing the job since I would be expecting them to provide both trained people plus the equipment to do the job. 

If they're a builder working on your instructions I'd suggest they and you need to be mindful of and comply with the working at height regulations not looking at a home made harness for a contracted job no matter how small.  

Was the scaffolding specified with abseiling in mind?

You want to make a harness out of rope so a fat lad can abseil down some scaffolding to impress his mates while they should be working?

Is that right?

I'm sure this will be totally fine.

Was the scaffolding specified with abseiling in mind?

Without wanting to sound all fun-spongy ...you're having fun but the builders are on a work site and subject to a whole lot of regulations, liabilities and responsibilities that you aren't. They also shouldn't be letting you anywhere near their scaffolding. If they are getting involved in this I'd be questioning their competency to do any of their job properly

I get an email every week from the HSE with details of their investigations, prosecutions, surprise inspections and enforcement actions stemming from deaths and injuries and building trades and falls from height feature so often that they get boring to read.

I've done the workplace fatality thing. The guy was just having fun.

Yeah. No. 

You just don't dick about on scaffolding (or a  building site) 

you're having fun but the builders are on a work site and subject to a whole lot of regulations, liabilities and responsibilities that you aren't

Also under CDM regulations this isn't true. You absolutely are responsible you cannot absolve yourself of your responsibilities.

"Under the Construction (Design and Management) Regulations 2015 (CDM 2015), you are required to ensure that the work is carried out—as far as is reasonably practicable—without risks to the health or safety of workers or the public"

The bit that locks you in is "ensure" 

I'm seeing a bit of a contradiction between these two statements:

 

. I have the skills to tie one. I have the materials required..

whilst I can concoct them I have never actually used a rope harness so I don't know which is best for a larger bloke...

Any suggestions are welcome

Like the poster above, I have used option 3 in that link many times BUT it really only advisable for skinny people. Even in that photo there is horrendous 4 way loading on the karabiner and it would be even worse with a bigger bloke. Also pre-sewn slings are so thin these days and a tape knot ....just no.

The best option I found was to tie two overhand knots a bit more than 1/3 of the way from each end of a standard sling. Stick one leg through each end loop and pull it up to your crotch. Then get another sling and tie it round your waist intersecting with the middle loop of the leg sling. Piss about with the lengths to make one side of the middle loop on the leg section a bit longer so get everything into position. Voila

Worked fine for the abseils on the Cuillin ridge, and I'd use it again if needed. Should you try to decipher the dodgy description above and make one for your scaffolders?

Absolutely not. Really bad idea.

My knowledge from 20 years ago doing my Summer ML is that classic abseiling and Thompson knots both work but they are both awful awful things (especially when using a thin walking rope).

So not either of those techniques. Whole thing sounds well dodgy.

HTH 🙂

Even if I believed I had the skills there's no way I'd be doing such a thing for a third party workman, especially one that's a "unit."  If you get it wrong - or even if you get it right and it fails anyway - I'd expect you'd be going to jail.

They are (presumably) professionals Working At Height.  Why don't they have their own gear?  If it's just a bit of fun, is the scaffold designed to be loaded in such a manner?

Come on OP,  don't keep us dangling.

How is the client-tradesman hing jinks and bantz going? Hopefully you haven't been squashed under an 18 stone roofer...

Posted by: robola

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Come on OP,  don't keep us dangling.

How is the client-tradesman hing jinks and bantz going? Hopefully you haven't been squashed under an 18 stone roofer...

I think dangling is kind of the point

You can fashion a sit harness from a long tape sling. I've used the 3rd method described  In this article

That was our standard method for abseiling before the Whillans harness appeared.

there's no way I'd be doing such a thing for a third party workman, especially one that's a "unit."

Fascinating story, which you won't believe, but it is true...

When I taught climbing at York Barbican wall many years ago we had one guy shouting agitatedly to me as he was being lowered off a third of the way up a climb. His belayer held him stationary just off the deck and I wandered over.

As I watched, the legloop straps slowly pulled through the buckles on his harness. They were properly threaded but there just wasn't enough friction and his significant weight was undoing the harness.

We lowered him to the ground  swapped harnesses and carried on. I sent a note to the centre manager saying that if I saw the harness again ( a Troll one) I would cut it up

I can't recall what action they took, but clearly they didn't initially believe  me, and reckoned it was  not doubled back.

I was just so lucky that the guy fell off the climb so low down. If he had been higher up he could have been horribly injured and I would have been prosecuted as people would have understandably but incorrectly said that he wasn't using the harness right .

Ok  so maybe only fascinating fur me and the climber.....🤔

And the belayer