I had something like this a few years ago. In fact I could say when it was almost to the day because it was just after Diana Spencer's death.
There had been blistering heat mixed with very heavy showers. Between the showers I leaned my ladder up to get onto a flat roof as this was the only way of getting to some of the windows. I put my ladder up at in a different place from my usual favourite as there was a load of garden stuff in the way. The roof was 20 years old but looked sound and I had been up there many times before (though not on that particular bit). I took a few steps and my foot went right through the decking. Fortunately, it did not go through the plasterboard as well. I know I'm overweight but this was nothing to do with it. It looked like water had seeped under the felt - possibly due to bitumen joint melting in the heat then rain getting underneath. The chipboard (N.B. not allowed to be used on modern roofs) had turned soggy even though it looked OK from above.
Anyway, the guy next door was a decent sort and happened to have a tarpaulin so we covered the roof and weighed it down.
I left a note updating the customer and he phoned me up later. I gave him my insurance details and contacted my insurance company to inform them that they may be getting a claim.
However, although it was my foot that went through the decking, I feel that there is a lot more to this.
(1) I was up there with his implied consent because he wanted his windows cleaned and the only way to do it was by climbing onto the flat roof.
(2) As I was up there with implied consent, it is the householder's duty to ensure that the roof is sound.
(3) The flat roof was 20 years old and for chipboard and felt roofs the recommendation was to change them 10 yearly.
(4) If I was near the edge and this had caused me to fall off the roof, surely the householder (or their insurance co.) are the liable party. Just because I did not get hurt, this should not alter who is liable IMO.
Anyway, it seems that his household insurer took a similar view to mine. They actually paid for a whole new flat roof - even though they could have given a "wear and tear" argument. Even then the customer wanted his excess back from me. I did explain that the roof clearly needed replacing and if it wasn't for my mishap, he would have borne the entire cost himself within a few months or a year at most.
He must have thought about that because he said on my next visit that he was OK about me not paying the excess.
He also cancelled.
Funnily enough, fast forward nine years and I get a phone call from a lady who wants a quote on a commercial job. I go to view the job and give my quote with this vague feeling that I know her from somewhere.
Turns out she's the wife of the guy at the house.
Anyway, I got that job and still do it so I suppose that at least no-one holds a grudge anyway.
No flat roofs to get on there mind you.