Can I remove acid marks on my Eternit slate roof tiles? I must say, this man sounds like a bit of an idiot, splashing highly corrosive acid around on top of a roof, in clear contravention of the safety instructions, and without covering up adjacent areas. I don't imagine he washed the area down with water afterwards either. Your neighbour is legally responsible for making good the damage, and you should tell him you want a thorough independent inspection of your roof windows too, and replacement if they are found to have been affected. Well done for having taken the pictures. It is always a good idea to have a photographic record of the condition of your home - especially prior to adjacent building works being carried out. Eyes on the prise Q Recently we had a visit from a conservatory salesman, who used to work for a well-known glass company. He saw our windows, some of which were installed only three years ago, and told us that on several the beading had been placed on the outside (which apparently is easier to fit than on the inside) and which makes it easier to be prised out. Does this mean that we (and presumably many other houses) are at risk from any burglar with a sharp tool? KG, Oxford A Yes, possibly. Especially if the burglar has previously worked in the window industry. It is one of the many drawbacks of cheap PVC-U replacement windows. Externally beaded windows can be secured by fitting double-sided adhesive tape between the glazing and the frame. You could ask your installer whether this was done (if they are still in business), or engage another glazier to remove the glass from one window to check, and then to fit the tape to any windows at risk. No doubt the conservatory salesman will have had half an eye on getting this work for his own company. A damp spot mystery Q When it rains my front living room wall gets damp spots appearing from the top and travelling downwards. The wall is a solid nine inch (225mm) with the outside rendered. These spots only seem to appear at the top half of the wall. Could you please tell me the possible cause? PD, by email A With all dampness problems, the key is to locate the source of the moisture, and put a stop to it. You already know the source - which is rainwater. So next time it rains, I suggest you go outside and look upwards, to see what is happening. It is possible that your gutter might be blocked or leaking, or roof water might be dripping down behind the gutter. If neither of those is the problem, then look to see if rain is being driven against the wall, and getting through cracks in the render.