The shadow gap is the secret to fitting shutters!

12mm clearance is deducted from your width and height measurements to ensure the frame fits within the recess, this provides a 6mm clearance gap or ‘shadow gap’ around the shutter frame.

The shadow acts like an infill and creates a neat finish between the wall frame:

This is important as normally recesses are not square or straight.

It is neater to leave a natural shadow than it is to fill the gap, generally filler will shrink and will not match the frame colour or wall colour, hence the natural shadow looks and provides a professional finish.