Seen from a distance, a corrugated sheet looks like a simple product. Only when two cross-sections are compared do the differences become apparent. The spacing of the crests, the wave height or the design of the side edge may differ. Fibre cement sheets are therefore selected on the basis of the drawing and the documentation. The term asbestos-free fibre cement states the type of material but does not describe its geometry. That geometry determines how the sheets are joined, which accessories are used and how the surface area can be used. The pitch is the distance between corresponding points on successive waves, for example between crests. The width of the recess itself should not be measured and the result treated as this dimension. In a technical drawing the pitch has its own dimension line.

Two grey sheets can look almost identical and yet fail to form a correct joint. When buying for a roof repair, this difference matters a great deal. Fibre cement sheets must be compared by section, working dimensions and fixing rules. Asbestos-free eternit is a general name. It does not mean a single edge standard or a common set of accessories. Compatibility is determined by details that are easy to overlook in a photo. The most misleading factors are similar colour and a similar number of ridges. The perspective of the photograph further changes the proportions. A narrow corrugation photographed from the side can resemble a wider one. That is why a discussion of interchangeability should begin with the designation and drawing, not with a photo taken from the yard.

Where successive rows meet, the sheets overlap in two directions. If every corner is left full despite the required trimming, a build-up of material occurs. Fibre cement sheets may then fail to sit properly and remain under stress. Preparing the corners is not cosmetic. It is part of the roofing method. Asbestos-free fibre cement must be ordered with a specified version of edge treatment, and the installation plan should show which corners are to remain intact. At a normal side joint, one sheet overlaps the other. A similar situation occurs between the lower and upper rows. At the intersection of these joints, more layers of material build up. Without the solution provided in the system, the upper sheet may be lifted.

A longer sheet does not mean the purlins can be spaced further apart. This is one of the more common mental shortcuts when pricing a roof. Fibre cement sheets work within a defined arrangement of supports and fixings. Their length must match the laps, but the permissible spacing also depends on the product's properties and the loads. Asbestos-free fibre cement is best chosen together with the design of the substructure. That way, saving on format does not weaken the roof slope. A support must carry the load from the sheet and provide the correct fixing position. It is not enough for it to be somewhere under the sheet. Its position must be tied to the edges, laps and rows of holes.