I had a metal roof installed, zinc colour (the soft gray type) Now, on certain sections I get like a spray of rust spots occurring. Is it something to worry about and what might the reason be? On most sections no problem, the normal gray colour. Thanks!

The reason would be oxidation of the metal. Worry? Maybe not. Or, try a light sanding and some spray paint.

Well I for one would be somewhat concerned. Steel roofing is usually coated with a thin film of zinc, called galvanising. The idea is that the zinc gradually corrodes away, preferentially to the steel (basic chemistry). Once all the zinc has corroded away, then the steel begins to rust, and sooner or later, holes will begin to appear.

If your roof is galvanised steel, then the galvanising may have been too thin (you don't say how long it has been up, and there is also the possibility it may have been sitting around in a suppliers yard, getting wet, prior to you getting it).

You could try painting the steel with a high zinc content paint, this may delay the rusting. But if it is galvanised steel, then you have a problem.

You might try using OSPHO.(if you can find it) it inhibits rust like no other product around & leaves a case around the metal. Used in boats cars undercarriage steel fences & and many other uses. I think most people opt for the standard roof sheets or Trilon(now) instead of metal roofs due to rust & noise when it rains.

Thailand uses the absolute lowest level of protection in Galvanized steel known to man. Cut a corner save a couple baht= planned obsolescence! Ospho may be hard to find as it comes from the U.S. & the cheap sob's hate to pay to have it shipped as hazardous materials + the U.S. customs mark-up & don't forget the Thai tag-up on the product as well. Boats & ships Ospho is bible material & used religiously to get the most out of the crusty buckets.

It may be cheaper doing a tear down & using a Trilon roof if it gets real bad. I don't think they intended the galvy roofs top last.

Well actually it is brand new; just been up a month or so - maybe have the Co. having a look at it. Seems weird having this specs appearing already, isn't it?

Well actually it is brand new; just been up a month or so - maybe have the Co. having a look at it. Seems weird having this specs appearing already, isn't it?

if they was cutting up on the roof or even down below then it could be metal filings which have started to rust and has nothing to so with the roof material altogether.this could be the reason for the spray effect seen.

metal sheeting should be cut with a nibbler and not a grinder but try telling them that.

From the OP's description, it is not rusting [just] at the penetrations or cut edges. So it would appear to be failure of the zinc coating - likely started well before the installation, in the supplier's yard... out in the acid rain. As the other posters stated, the coating is sacrificial and goes away molecule by molecule in the presence of moisture and/or alkalinity - coastal, polluted & tropical climates are hardest on galvanized coatings... does that sound familiar?

If you have a reputable builder, who worked from a reputable architect's set of drawings AND spec's, you may have recourse if still in your [specified] warranty period. If you ticked N/A for these, I'd just stabilize it with a high zinc content paint application and start saving for the eventual replacement. Depending on base metal gauge, galv. coating thickness, environment & paint coating, you could get five to ten years before full-on perforation of the base metal... or right on the beach, you could be fully done within six months!