Finding a structural engineer to look at a concrete roof

[d000hg]

My Victorian house has a garden-room resting on top of an under-room... sort of a cellar but not completely buried. Originally the garden room was a large greenhouse but this is not original to the property, from investigating it seems the under-room was originally single-storey with a tin roof and then iron girders were added to create a framework and a reinforced concrete slab was poured... possibly on to the old tin roof to hold it in place. The roof is supported by 3 iron girders and one is badly rusted... inspecting this led to finding out the above as I was worried there might be more problems.

The roof girders are not badly rusted but the concrete is crumbling in places and the reinforced iron grids look quite corroded. It's probably 100 years old. I started off thinking just to replace the rusted upright - it has a hole in the middle - but would ideally like to tank the room and re-jig the uprights. However uncovering the roof has raised some concerns; while it seems sturdy it would be galling to convert the room and find in ten years the slab is deteriorating, and I have no way of telling what is superficial and what is a problem.

Getting a structural engineer in is the obvious answer but I'm nervous that this is going to cost a fortune. I've never used one before, so I don't know what exactly they do or how they charge. My only previous experience is when I was quoted £800 for one to come and do a visual inspection of a wall!

So any advice would be welcome. I don't need to do this right now (I put a prop to support the rusted upright) but it would be nice to know how bad it is and what my options are... or if in fact it's so over-engineered it's absolutely fine. My survey didn't throw this up but it looks a bit DIY... but then 100 years ago I don't know how they did things!

 

[oldbutnotdead]

If the concrete really is that old and the rebar is exposed then it'll be corroding like fun and spalling the concrete- thus losing much of the strength of the slab. What you pay the big money for structural engineers for is their signature on a bit of paper that says 'it'll be fine'- if it collapses then their insurance cover kicks in to cover costs from death and injury and rebuilding. Big big money.....

What size space are you talking here and how substantial is your garden room? (If we're talking shed size then a better use of the cash might be to move the garden room, reroof the bomb shelter with timber and ply then resite the garden room).