Flat concrete roof rust

[DIY-Nick]

Hi

My house was built in 1950, it has a single story extension with a flat concrete roof. The roof does not leak but has always had condensation on the inside through the winter due to not being insulated.

Whilst renovating I have removed some plaster from the ceiling and found that some of the render underneath has blown, when that came down it revealed some steelwork in the roof which has rusted -see photos. Also I have revealed some more of this steel and can see that it is only the far end of the roof (furthest from the house) that has become significantly rusty -perhaps due to being the coldest area, it sustains the greatest condensation.

The steel appears to be perforated sheet, rolled into a shallow ‘u’ section, covering the entire length of the roof. I have researched but been unable to find any information on this type of roof construction. The roof appears to be structurally sound, there is no bulging, sagging, movement or cracks visible, and it is watertight.

Does anyone have experience of a flat concrete roof like this? How significant do you think this rusted steel is? Any information/advice greatly appreciated.

Thanks

Nick

 

[KenGMac]

DIY-Nick, good evening.

Could be that the "perforated metal" is / was Ex-WW2 temporary roadway used to get trucks over sand or soft ground.

It was used [generally] in out houses and other [generally] Non-Habitable spaces, as a so called Permanent Shutter for the concrete roof, it provided a simple cheap shutter for the concrete, with the added benefit of at times taking the place of the required Reinforcing in the form of steel mesh or / and reinforcing bars, which were more costly and difficult to get back in the day.

I would agree that severe internal condensation has allowed condensing water vapour to settle on then attack and cause the rusting of this perforated material.

Several issues then pop up.
1/. Is there any reinforcing mesh or bars embedded in the concrete roof? apart from the perforated metal shutter.
2/. if the answer to 1/. above is no??? then without the perforated metal shutter what else is providing support to the concrete roof that will not support itself without reinforcing metal mesh?? / bars?? / or the rusting perforated metal shuttering, that is becoming over time compromised by the corrosion and rust??
3/. Yes, you can [say] wire brush the soffit of the perforated metal, BUT?? you cannot "get at" and stop the rust on the upper side , the side facing and in contact with the Concrete, where if exposed to air corrosion will continue, if there is any appreciable moisture present in the air.

Sorry, no silver bullet "fix" but lets see what the board thinks??

Ken.

 

[oldbutnotdead]

Given its a flat roof (so no significant load on it apart from its own weight) it is unlikely to suddenly collapse. But there is a lot of corrosion there, it is possible that the roof is cracked (so allowing water to seep into the slab)...
If it was me, I'd be down at MKM or wherever pricing for timber for a replacement and planning on a day with a breaker drill and a grinder to drop the concrete out and replace (assuming it's 100-150mm thick).

 

[Notch7]

Given its a flat roof (so no significant load on it apart from its own weight) it is unlikely to suddenly collapse. But there is a lot of corrosion there, it is possible that the roof is cracked (so allowing water to seep into the slab)...
If it was me, I'd be down at MKM or wherever pricing for timber for a replacement and planning on a day with a breaker drill and a grinder to drop the concrete out and replace (assuming it's 100-150mm thick).

me too - it’s quite a small roof, I’d get rid: you could spent money trying to repair and find it still damp.

new flat time.

 

[Harry Bloomfield]

Does anyone have experience of a flat concrete roof like this? How significant do you think this rusted steel is? Any information/advice greatly appreciated.

Is the material which has collapsed down definitely concrete, or might it be just plaster? I would expect steel reinforcing the be deeper into the surface than that.

 

[KenGMac]

Harry.

The perforated material is / was a permanent shutter not meant to be removed just left in place, it also [when new] was capable of supporting the slab, now it is corroding not so much support.

Back in the day, Steel mesh and or embedded bars were far more expensive than leaving in a load of [at that cheap almost scrap] material as a permanent shutter, as to whether or not there is any "formal" embedded reinforcing, whether bar or mesh is anyone's guess.

I have seen a load of these things in all sort of places, coal sheds, sheds, Old single car Garages, old Air Raid Shelters + in a load of long since derelict MoD small sheds and small structures.

Ken.

 

[JohnD]

I am no expert, but these are still popular in some countries, and their USP is that they can be used both as formwork to pour the slab, and also provide enough tensile strength to reinforce the roof after build, with no mesh or rod reinforcement required, so cheap to buy and cheap to build. they are perforated to allow enough of the mix to drip through to give good adhesion.

the underside coverage is too thin to prevent rust, so I don't believe there can be any guarantee of residual strength when rusted. I expect there is some way of calculating it.

 

[DIY-Nick]

Hi all.

Thanks for your replies.

I think replacing the entire roof would be a last resort for me. The extension is approximately 6x3m. It would be a colossal job to break through the concrete and then build a new roof structure, and a huge price tag also!

My strategy at the moment is to remove the render from the ceiling, allow the roof deck to dry out as much as possible, then insulate to prevent significant condensation in the future. For insulation I’m looking at an inverted warm roof solution. Kingspan website describes it well here:
https://www.kingspan.com/gb/en-gb/p.../articles-and-advice/what-is-an-inverted-roof

I’m also considering treating the rusted metal with a rust inhibitor of some sort.

Thoughts/opinions welcome.

 

[oldbutnotdead]

A lot depends on what other reinforcing is in the slab. Concrete is good in compression and poor in tension, if the invisible bits of that metal are also badly corroded (and there is no other reinforcement) then the slab centre could end up in tension, which would be bad. You might want to try exposing some invisible metal and see what state it is in.
Depending on access above and slab thickness, removal would not be an enormous job- messy and hard work but not much more. Yes the timber for a new roof structure will be pricey at the moment but you could be sly and increase the roof height by a couple of courses to give room for wool insulation instead of PIR- much cheaper.
End of the day it's your choice, it could be fine or it could bite you especially if you decide to sell.

 

[Mr Chibs]

How thick is this slab?

What covering (if any) is on the outside?
If it’s just the concrete, have you been up to have a butchers... what condition is it in?

Any pics?

 

[DIY-Nick]

Thickness is approximately 200mm at house end and 120mm at far end -measured awkwardly though window openings. Covered in a black waterproof coating, unsure what it is. It was recently repaired by an experienced roofer who applied a coating over the top. Pics attached.

 

[Harry Bloomfield]

Suggestion for comment.....

Do you have access to a metal detector - one might allow you to check from above, if there is metal reinforcing in there, in addition to the obvious metal. I wouldn't expect unsupported concrete, an average of 6" thick to collapse anytime soon. My own utility/pantry roof is of a similar construction and similar size, 1955 - but without any signs of the metal showing below. I have never checked, but I always assumed there was no steel reinforcing in it. It was refurbed in the early 1980's, to add insulation above, below, and PB.

 

[Mr Chibs]

Won’t a metal detector pick up the shuttering stuff at the bottom of the slab? can you turn the sensitivity down on these things??

When I poured a slab for a floor, most of my neighbours had said... ‘you know it will crack, don’t you’ after a good few days... this is what happens as it cures and the water evaporates.
Surely this would be the same for your roof?

If there was no reinforcement in there it wouldn’t be safe... 60+ years later it’s still up. Albeit in a state.

You need to find some oldies who could give you an idea of how these were constructed.

Not sure how you’d take this down, as the top courses of bricks will be dislodged etc.

 

[Harry Bloomfield]

Won’t a metal detector pick up the shuttering stuff at the bottom of the slab? can you turn the sensitivity down on these things??

I have a small hand wand and a larger type for 'sweeping', both have adjustable sensitivity.

 

[DIY-Nick]

I don’t feel overly concerned that it would fall. There are no cracks or distortion visible and it has been standing for 71 years, can’t see a reason it would suddenly collapse now. There are many other examples of the flat roof extension in the street which to my knowledge have no problems with them.

I don’t have access to a metal detector, I like the idea of a metal detector but not sure how you would determine if you’re picking up the metal shuttering or any additional reinforcement.