Pathologies – part I
01/30/2013 2 Comments
Pathologies in roofing slates
The pathologies formed in slate roofs are mainly due to the presence of potentially unstable minerals (iron sulfides, carbonates and organic matter). These minerals may become altered by the effect of environmental agents, once the slate roof is finished. The pathologies are mainly associated with oxidation and gypsification processes of the cited mineral phases.
The oxidation is generated when the iron sulfides which may contain the slate became weathered, forming iron oxides. This forms reddish rust marks on the surface of slate tiles. This is mainly an aesthetic defect, as only rarely slate tiles disintegrate due to oxidation. However, it is the main fact in volume of complaints from slate customers (Figure 01). The presence of tiny fragments of organic matter may favor the oxidation processes.
The gypsification occurs when the carbonates react with the environmental SO2, forming gypsum. In this case, gypsum has larger size than carbonates, so a swelling may occur within the slate tile, causing it to disintegrate. Despite this, the incidence that this pathology in the customer complaints is significantly lower than oxidation, maybe since it is not as striking (Figure 01).
There are also other characteristic pathologies and minor defects but also must be taken into account.
Following the criteria dictated by ICOMOS, defects and pathologies found in roofing slates can be classified into 3 groups (Table 01).
Further reading: Standard tests for the characterization of roofing slate pathologies