Profiled metal roofing – staining

Cause

Run-off from lead flashings

Repair

  • you need to stop galvanic corrosion from taking place. This is an interaction where one metal is relatively protected while another suffers accelerated corrosion. This problem can occur when water flows over lead and then onto steel or steel coated with a zinc/aluminium alloy
  • if the steel roofing and lead flashings have been installed in recent years, call the installer back. Manufacturers of profiled steel roof claddings make it clear in their installation guides that lead is not compatible with their products. They say that flashings should be made from the same coating system as the profiled metal roofing
  • if the roofing is not new or the installer cannot be located, talk to an experienced roofer about options. The best approach is likely to be replacing the flashings with ones that do not contain lead. Another (less than ideal) approach may be to carefully lift the lead flashing, paint the top and underside of it with appropriate paint and then reinstall it
  • where galvanic corrosion over the medium-long term has stained steel roofing, removing the stains may be difficult or impossible. Once the source of the problem has been addressed, treating the stain or corrosion and then repainting the whole roof may be the best answer. Sand the corroded areas lightly to remove rust but do not use a wire brush. Prime with a zinc-rich primer, then apply at least two coats of acrylic paint designed for roofing
  • painting just over the treated stain or corroded area with touch-up paint is unlikely to be successful over the long term because this air-dried paint ages differently from the steel coating and the repainted areas are likely to become more visible over time
  • always tell someone when you plan to work at height. Follow this guidance around using ladders and scaffolding safely

Cause

Soot or particulate emissions from nearby chimneys or flues

Repair

  • wash the roof on a regular basis to remove deposits
  • if the soot or particulates come from your own wood burner, a part solution may be to run the burner more efficiently. Use only dry, seasoned wood and don’t damp the fire down when you turn in for the night
  • for a longer-term solution, consider replacing the wood burner with a more efficient burner that produces fewer particulates such as an ultra-low emission burner (ULEB), or ideally replace it with a heat pump that will remove the problem entirely
  • always tell someone when you plan to work at height. Follow this guidance around using ladders and scaffolding safely

Cause

Discharge onto the roof from a copper pipe, such as that from a hot water cylinder vent or from a pressure relief valve, or from copper piping in solar panels or air conditioning units

Repair

  • you need to stop galvanic corrosion from taking place. This is an interaction where one metal is relatively protected while another suffers accelerated corrosion. This problem can occur when water flows from a copper pipe and then onto steel or steel coated with a zinc/aluminium alloy
  • if the steel roofing and copper pipe have been installed in recent years, call the installer(s) back. Manufacturers of profiled steel roof claddings make it clear in their installation guides that copper is not compatible with their products
  • if the roofing is not new or the installer cannot be located, talk to an experienced roofer or plumber about options to stop water from copper pipes running onto the steel roof. The work involved should be done by a licensed building practitioner such as a registered plumber and not by a DIYer

Cause

Staining may be evidence of corrosion

Repair

  • treat as for corrosion. Seek advice from the roof cladding manufacturer/supplier. In some cases, treating the corrosion and repainting the whole roof may be an option. Sand the corroded areas lightly to remove rust but do not use a wire brush. Prime with a zinc-rich primer, then apply at least two coats of acrylic paint designed for roofing. Painting just the affected area with touch-up paint is unlikely to be successful over the long term because this air-dried paint ages differently and the repainted areas are likely to become more visible over time
  • in severe cases replacing the roofing is likely to be the best long-term solution
  • always tell someone when you plan to work at height. Follow this guidance around using ladders and scaffolding safely