Except for roofs for "subordinate purposes", the highest corrosion protection class III according to DIN 55928-8 is required for weathered external roof surfaces according to DIN 18807-1, trapezoidal steel profiles. 

But basically all coil coatings with a thickness of at least 25µm meet this standard requirement. 

Decades of experience show, however, that 25µm thick (that is only 2.5 hundredths of a mm !) coil coatings, whether of polyester (SP) or of polyvinylidene fluoride (PVDF), are unsuitable for roofing elements of slightly pitched roof surfaces; they are no longer state of the art. 

The serviceability of these thin coatings is questionable because external roof surfaces have to withstand much higher loads than, for example, external wall surfaces. 

Dirt deposits with acidic or other harmful substances often remain on low pitch roof surfaces. 

Cut edges at joint overlaps, or at the eaves, and scratches pose a significant risk of corrosion formation. Due to assembly work and subsequent foot traffic, damages to 25µm thick coatings are unavoidable in construction practice. 

Minor installation damage is hardly noticeable, especially in the case of light-colored coatings, due to the light-colored zinc layer underneath, and is therefore not repaired. 

Experience shows that these initially barely visible areas corrode after about 3 - 4 years. 

The cut edges at the end of the roof elements, e.g. trapezoidal profiles, are open, i.e. ungalvanized and uncoated. 

It is precisely at these points that, due to various conditions, water remains in contact with dirt accumulations for a long time and, despite the cathodic corrosion protection of the zinc coating, leads to corrosion of the steel core and ultimately to peeling of the coating from the cut surface. 

The durability of these cut surfaces can be significantly improved by treating the cut surfaces with air-drying clear varnish before or during assembly and/or by sealing the cross joint overlap in such a way that the two profiled sheets are about 5mm apart, so that water is not continuously held there by capillary action. 

In the eaves area, the lower chords of the roof elements should be bent downwards towards the gutter using special flat-bending pliers or special tools available from us. This greatly increases the roof pitch, so to speak, and prevents water from constantly remaining on the cut edges to promote corrosion. 

The risk of corrosion of such 25µm thick prepainted metal roofs decreases as the roof pitch increases. 

In our experience, 25µm coatings are only justifiable for roof pitches of more than 25° due to the lower stress caused by deposits. 

We recommend the following corrosion protection systems for flat sloping external roof surfaces:

1. Colorcoat HPS200 Ultra^®: on alloy zinc coating ZA 255

2.  55% AlZn AZ 185 Aluzink)

3. Tedlar (PVF) foil: on zinc coating Z 275 or alloy zinc coating ZA 255

4. SP powder coating: on zinc coating Z 275 or alloy zinc coating ZA 255