Calling on the wisdom of STW

We’ve got a problem with rotten joists under a concrete slab flat roof. Building is Victorian era so no damp protection with timber up against brickwork. 7x2s are rotten and need to come out. Joists and wall plate. Span isn’t massive: 2m from steel to brick chimney stack in one area, going out to 3m in adjoining space.

We’ve got builders lined up but I need to know how this is going to pan out so I can keep the builders honest.

Questions.

1. How do you support it while working?

All the structure is coming out and new structure going in. We’ve got Acrows making it safe for now but they leave no space for working. How do you do it?

2. There has been talk of breaking out the 120+ year old concrete slab and replacing with a modern roof structure. Seems a bit extreme. Is this needed? Makes it easier to work on? If we don’t do it are we just making the job more painful all the way through?

3. If we don’t break out the concrete slab, might we inadvertently break it as we jack in a new wall plate? What’s the worst that can happen?

Oh yeah and all this is happening five floors up.

First port of call is to pick honest tradesmen from the outset and trusting them. Its not a case of ‘keeping them honest’. There are plenty out there, usually found by referalls, testimonials and a gut instinct upon meeting them. If you happen upon an honest builder/carpenter, they will be more than happy to talk you through the process and put it to paper in the form of a written quote/contract.

As a trade, without seeing the job and knowing the ins and outs, assuming its a flat roof.

To do the job properly, I’d probably suggest breaking off the concrete by cutting it into manageable sections with a saw, propping each section as you go.

Then replacing with a warm roof build up and grp/3 layer felt/asphalt/single ply topper, if there’s adequate depth below any potential obstructions. I’d much rather approach like that than trying to wedge some fresh joists in under some ancient unknown concrete. It would most likely work out quicker and you’re right a lot less painful.

The 5 floors up thing just means you need scaff, no issue there.

I’d suggest you might want to speak to a structural engineer before making any decisions about removing anything! The chances of keeping the conc slab intact without cracking are minimal I’d say, no matter how careful your contractor is. Leading to potential problems down the line.
I’d also look to replace the roof slab as mentioned above rather than try and retain unless you’re in a listed building and might get in trouble with local authority?
I’d also re-iterate the comments above re trusting your contractor, if you don’t then employ an architect or engineer to make sure its done properly. IN fact I would seek PROFESSIONAL advice anyway, most of us offer an initial opinion before committing to any fees and the same comments as above apply about referrals etc

Slab needs to come out you can then use a good thermal insulator. I’m surprised no one has mentioned it.

(Trades not on here)

I’d be highly sceptical about a 120+ year old concrete slab 🤔

Pick builders you can trust.

Thanks for the input.

Conservation area but not a listed building. Phew.

Scaffolding is up. The builders have been in doing other works. They’re good. We’ve had a structural engineer in too. Was just feeling like I was in a conversation triangle and needed to take a breath over the weekend and understand what we’re in for. Five stages of grief and all that.

Specifically thanks for unanimously ruling out keeping the concrete. Really helpful (although I hope 120 year old concrete is generally ok as that’s what the floors are made of)

If you think 120 year old concrete is dodgy I wouldn’t go to Rome