I learned many years ago to make it clear to the customer that any damage to roof tiles was entirely at their risk. The conversation usually went something like:
"OK Mrs.............. I've had a look around and everything is straightforward with the exception of the windows in the roof. The only way we can access them is by walking on the tiles, but as tiles are fragile I'm afraid we can't accept any responsibility in the event any of them get broken. If you want us to clean those windows we will, but only on the clear understanding that it is at your risk"
In almost all cases the customer accepted this, if they didn't we either omitted the windows from the job or declined the job altogether.
If you have to walk on roof tiles to access windows then a conversation similar to the above is absolutely essential before you do. By going ahead without their agreement you are leaving yourself wide open to a claim for damage.