Saddle washers are not required with Zenon rooflights.  The recommendation on all GRP trapezoidal profiles is to fix in the trough of the sheet, using a self-drilling, self-tapping fastener with a minimum 29mm diameter washer.  Saddle washers are only required where an oversized hole should be drilled prior to installation in products such as polycarbonate where there are large differences between the contraction and expansion of the materials in use.