Seized (watertank) shutoff valve

I live in a flat built around 1930 (ie probably quite old pipework). There is a communal cold-water tank in the roof, but the shutoff valve in my flat doesn't want to turn. I've already called one plumber out who refused to touch it unless the tank in the roof was drained (which would be a nightmare to organise). Clearly I don't want to break the pipework and empty all the water out into the flats below me! I suspect he just didn't want to do the work (and assume the risk), rather than it not being possible.
Any tips or techniques for loosening/un-seizing the copper tap on the valve? The pipework looks in good order (although it's probably rather old), but I want to be as careful as I can be. As an alternative, a friend suggested freezing the pipe just above the valve, but I don't know how viable that is.
Thanks in advance,
Steve
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Steve submitted this idea :

Quite right, supposing the valve broke open and your flat were flooded? You would be expecting him to perhaps pay for the damage.

Freezing would be fine assuming you could guarantee that the job could be done before it defrosts, I don't think you can.
Put a piece of wood across the top of the tank and tie the ball valve up securely with a bit of string. Then drain all the water from the tank. You should be able to drain it by simply running all of the water off, if not syphon it off with a garden hose. Now you can try to free the valve safe in the knowledge that breakage will not flood your flat and any flats below you.
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On Sun, 04 Apr 2004 11:09:51 +0100, in uk.d-i-y Harry Bloomfield

If he's any good he should be able to do it without flooding the place, and he should have liability insurance to cover any damage if he did.

Rather than freeing the valve I would fit a 1/4 turn valve, either in place of the tap or just above it. A lot better in an emergency.
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Steve wrote:

How many flats does it serve?
Do your flats not have maintenance contracts for this kind of thing?
I wouldn't touch it with the risk of it going horribly wrong; screwing up your own home is bad enough but getting insurance claims from x others sounds like an avoidable nightmare.
Tony
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Yes, I don't blame the guy - in fact I'm glad he refused to do it if he's not happy it can be done safely without draining the tank. There are 6 flats serviced by the tank in the roof (2 of which are directly below me).
I've spoken to the freehold owners, who said that they weren't willing to have the tank drained so that the work can be done - they also told me to get the pipes frozen above the valve. I'm battling with them at the moment, but it's a slow process and in the meantime if any problems were to occur with my plumbing it'd be a big problem.
I was just after a few (relatively safe) tips to free the valve so that I can avoid a lot of hassle if possible. Maybe some WD40 and a few light taps with a hammer might work.....If all else fails I'll just contact the people in the other 6 flats and try and arrange a day that I can drain the tank. Thanks for the tip about how to empty the tank.
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On 4 Apr 2004 09:05:42 -0700, snipped-for-privacy@hotmail.com (Steve) wrote:

Eh? You live above 2 tanks in the roof? ;)
PoP
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On Mon, 05 Apr 2004 06:54:25 +0100, in uk.d-i-y PoP

Yes, he's a bat!
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On Mon, 05 Apr 2004 11:06:31 GMT, snipped-for-privacy@microsoft.com (Lurch) wrote:

But his name is steve - not Michael Howard! ;)
PoP
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"Steve" wrote | I live in a flat built around 1930 (ie probably quite old | pipework). | There is a communal cold-water tank in the roof, but the | shutoff valve in my flat doesn't want to turn. | I've already called one plumber out who refused to touch it | unless the tank in the roof was drained (which would be a | nightmare to organise).
Do you have individual outlets from the tank to each flat?
If so you might be able to use a plastic bung[1] on the outlet for your flat and avoid inconveniencing anyone else.
A bung and freezing should stop the water enough to saw through the pipe and whack on a compression stopcock without flooding.
But I'd check your insurance covers you against d-i-y flooding downstairs just in case the worst happens :-)
Owain
[1] Google this group for central heating carrot potato and you will probably find a link to a kit to use when removing a radiator without draining down the circuit - in olden days a carrot was used on the tank outlet and a potato wedged over the vent pipe.
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