A rebar! A rebar!

[HTFB]

Beloved Stoker has dived into property ownership with a flat which is perfect in every way, except that since it was put up in the 60s nobody has ever fitted a boiler and it's currently heated by absurdly expensive electric heaters.

The problem is the reinforced concrete roof, through which any gas flue must pass. Only recently did the owner of another top floor flat manage to persuade the neighbours (who are collectively the freehold company) that it could be done. He has expert knowledge and for a semi-DIY job produced beautiful site plans and a thirteen-step detailed sequence of what he intended to do.

This has set the bar quite high for persuading the committee (on which he himself now sits) to permit us to copy him and get a boiler fitted. We are asked to provide plans and demonstrate in particular that the flue won't compromise the steel reinforcing bars.

Lacking any expertise at all we have found plenty of people---British Gas, for example---who will happily quote for the work but seem a bit blase about drilling out the roof.

I have been googling round in circles trying to work out who and how to ask to get plans produced, preferably with a minimum of cost (especially duplicated cost, if the gas company will be locating the rebar before drilling their hole anyway) and a maximum of speed. You can hire a rebar locator for 55 quid from HSS, but does it require any skill to use?

The Beloved Stoker is deeply despondent about it all, and feels this is an unreasonable and even impossible demand that's going to delay her vital central heating---when a hole is just a hole. I'm concerned to find a solution before she just starts hacking at the roof with a pickaxe.

Any ideas?

[jogler]

Why can the flue not be horizontal rear dicharge from the boiler thus going thru an external wall & not needing to go thru the roof.
Is there no external wall?

[Tim Hall]

Is a structural engineer the way forward?

When we had some building work done that required an RSJ in the ceiling, I asked my friendly builder for details of a structural engineer. He pointed me to a small practice (one man band operation), who did a couple of simple drawings, submitted what was needed to the local authority for building regs approval, and most importantly, had a few million in the way of Professional Indemnity cover.

[gordon taylor]

I'm with jogler on this. Most gas boilers are fitted in downstairs kitchens and have their flues going through the wall. Are you sure it has to go through the roof?

[HTFB]

It has to go through the roof. Problems include conservation area status, fairly massive structural concrete beams around the edge of the roof, and asbestos panel (of the sort that is quite happy as long as you don't do anything silly like drilling holes in it) in the outer cladding of the building where the beam isn't. The flats downstairs don't have the concrete beam and have been smugly gas-fired for ever.

A structural engineer might indeed be the right professional to engage, if we have to spend the extra hundreds on one. Thanks, Tim---I was scratching around for the right job title to search for.

[rafletcher]

It has to go through the roof. Problems include conservation area status, fairly massive structural concrete beams around the edge of the roof, and asbestos panel (of the sort that is quite happy as long as you don't do anything silly like drilling holes in it) in the outer cladding of the building where the beam isn't. The flats downstairs don't have the concrete beam and have been smugly gas-fired for ever.

A structural engineer might indeed be the right professional to engage, if we have to spend the extra hundreds on one. Thanks, Tim---I was scratching around for the right job title to search for.

So where do the flues from downstairs exit the building? As to holes in the roof, are there any other penetrations in it - after all the other guy has set a precendent by proving a hole of a certain size does not compromise the structure.

[inc]

Any ideas?

An electric boiler with a wet system may be a viable  alternative  Electric Boilers | Electric Boiler | Cheap Electric Boilers

[matthew]

A rebar locator will provide you with the location of the bars (spacing), depth of the bars (cover) and an indication of the size.

Ideally once you know the cover you want to limit the hole you drill such that you maintain the same distance between your hole and the steel as the cover. This is because while the cover is partly structural it is also the corrosion protection to the steel.

The guy who has already done it should be able to provide most of the information but I would recommend a conversation with a Structural engineer who should be able to provide a specification for the restrictions on the hole which you will need to specify in the contract with whoever does the work. Possibly consider asking the engineer to have a watching brief on the drilling.

[HTFB]

A rebar locator will provide you with the location of the bars (spacing), depth of the bars (cover) and an indication of the size.

Ideally once you know the cover you want to limit the hole you drill such that you maintain the same distance between your hole and the steel as the cover. This is because while the cover is partly structural it is also the corrosion protection to the steel.

The guy who has already done it should be able to provide most of the information but I would recommend a conversation with a Structural engineer who should be able to provide a specification for the restrictions on the hole which you will need to specify in the contract with whoever does the work. Possibly consider asking the engineer to have a watching brief on the drilling.

We do have the diagrams prepared by the other bloke. Apparently it's 8mm reinforcing at 200mm centres, with 50mm of cover: that makes it impossible to leave 50mm between the hole and the bars if the hole is 100mm diameter. His diagram allows 20mm distance.

The proposed location of the boiler has enough flexibility, in the worst case (that the bars are central over the ideal location) to leave 13mm of distance for a straight-run flue, which might be shaving it a bit fine. But there are other degrees of freedom---mounting on a block sidewall rather than the very back of the utilities cupboard, and so on---so it's scarcely an impossible problem.

The all-in-one installation quotes from, eg, British Gas, tend to treat this sort of problem as a matter of routine solution on the day of the works, and not one for careful specification in the contract. Is this unpardonably lax, or do the freeholders have a point?

[rafletcher]

In truth of course, cutting a rebar will have a negligible effect on the strength of the roof. Safety factors will take care of that.  I'd guess the bigger issue is keeping moisture out if a bar is cut, to prevent corrosion. And sealing on a flat roof can be problematic.

[HTFB]

That's more or less what the structural engineer I just rang said. He was pretty reluctant to spend our money on producing a report when the only content, finding the rebar would just have to be repeated on the day.

How to persuade people that they're asking for silly things?

[Jacomus]

In truth of course, cutting a rebar will have a negligible effect on the strength of the roof. Safety factors will take care of that.  I'd guess the bigger issue is keeping moisture out if a bar is cut, to prevent corrosion. And sealing on a flat roof can be problematic.

What if it is post tensioned though? Cutting through one of those would be Very Bad IIRC.

[rafletcher]

In truth of course, cutting a rebar will have a negligible effect on the strength of the roof. Safety factors will take care of that.  I'd guess the bigger issue is keeping moisture out if a bar is cut, to prevent corrosion. And sealing on a flat roof can be problematic.

What if it is post tensioned though? Cutting through one of those would be Very Bad IIRC.

It's a cast concrete roof.  Post tensioning IIRC is used more for bridging structures and is usually cable?  Not that common, and I think post-dates the construction under consideration anyway.