These wall or door mounting face fixing security grilles have 20.6mm square holes on 25.4mm centres. The outside border is 28mm wide so the ventilation area is outside dimension (and order size) less 56mm. The plate is manufactured from 12GA. (3mm) galvanised steel grey primed but other finishes are available to order including Grade 304 and Grade 316 stainless steel. The frame has countersunk mounting holes located 10mm in from the edge of the plate and to suit No. 8 x 25mm one way vandal proof security screws that can be screwed directly through the face plate into the wall or door. Other security screws are available, see page 114. Free air flow 66% free area. Other dimensions available from 152x152mm up to 762x914mm.