splicing wire 240 volts

Not safe to splice? Maybe not if the break is under the floor. But then you'd have to break up the floor anyway to put in a splice box. If you are sure that the break is in the wall, where a box can reasonably be installed, I don't see why not.. You will need to get the specifications (current, conductor temperature rating, conductor type) and select an appropriate splice or terminal type. If its safe to terminate this wire in a box in the wall (at the thermostat), it should be safe to splice it there.
Contact the manufacturer for advice. Perhaps the people that told you it was unsafe misunderstood exactly where you were planning on installing the splice and assumed the break was in the floor.

I spliced a de icing system for a loading dock under duress once. Lasted 5-6 months and then crapped out about 2 feet way from where we dug up the concrete. I flatly refused to deal with it again unless the customer was interested in a new system. They paid another contractor to fix it again. We had a bad ice storm and they were delivering a new MRI by truck. Truck could not stop because of the ice, it had broke again. Ruined the MRI.
Electric heating system in a shower. Hummm not anywhere near my family or loved ones. I will not even swim in a pool with the light on. But then again I think my life is worth more than 8 bucks, the cost of a gfci. The NEC says spices have to be accessible. That will be pretty hard to do under a marble floor.
Your mileage will vary.

And I got news for you: Don't think a GFCI will save your life, either. I wired a whirlpool tub for a customer with a 20 amp circuit, #12 NMB, and used a GFCI receptacle. The tub motor was cord connected. The customer called me several weeks later and said she was getting a weird feeling when she got in the tub. I went to check it out, and saw where lightning had run in on the house service and arced across the #12 NMB, and had shorted the hot wire to the equipment grounding conductor. It was shorted on the "load side" of the equipment grounding conductor, therefore would not trip breaker again. Well, the ground was now hot from where the wire had arced, and she had 120 volts on the ground. Apparently the tub and plumbing fixtures were not grounded. I was measuring 120 from the water in the tub to a good ground. Damn good thing the tub was not grounded, but then again, if it had been, the breaker would have kept tripping. Later...Blue Crown

What is the "load side" of the equipment grounding conductor? Can you draw the circuit - I'm having a hard time visualizing what you are talking about.
> Well, the > ground was now hot from where the wire had arced, and she had 120 > volts on the ground. Apparently the tub and plumbing fixtures were not > grounded. I was measuring 120 from the water in the tub to a good > ground. Damn good thing the tub was not grounded, but then again, if > it had been, the breaker would have kept tripping. Later...Blue Crown

thanks for everyone's reply. I realize my question was not clear. The thermostat is on the wall. this is the same wall with shower plumbing. The wire from thermostat ran into the concrete slab under the syn. marble floor. The shower head leaked inside the wall, the wire covering corroded and the wire broke. yes it's on GFCI. The insurance restoration guy says we cannot splice wire and the whole floor needs to be taken out, layer of concrete with heat cable removed and new floor heating system installed. This would require the entire shower to be dismantled. Can we splice the wire running between existing system in concrete slab to the existing wire coming ouf of thermostat? Do we need to move thermostat to another wall where there is no plumbing? thanks in advance. Abby

Is this an insurance job? If it is let them pay to do it right. I thought you were splicing under ground. If the back of the wall is in a dry location you COULD install a junction box that would contain the splices. It will look really ugly, having a blank plate on the wall down low IMO. Can you post a picture?

[snip]
It sounds like the inspector may suspect that the wire under the slab may have sustained some damage. Water may have wicked inside the cable to locations under the shower floor.
Replace it.

thanks everyone. it is covered by insurance - we were hoping to avoid the long process of removing, laying new heat cables, and re-installing shower walls. But I agree - there may be water under the floor, and in fact there may even be dampness behind the other walls. We will have new heat cable system installed and put thermostat on a wall that doesn't have plumbing. My husband is thinking of adding one of those sensors that alerts you when there is water leaking . . . thanks for taking time to help out Abby

Since this is an insurance job, I would let them do the job correctly and to code, making whatever changes that make it reliable in the future. Make sure to hire contractors that are licensed, bonded and insured. And a little grey hair doesn't hurt either.....(experience).... Good luck with your project....Ross :>)