The brown air vent cover with gravity flap is made of high quality ABS Plastic. It comes with 4 screw cover caps and 4 fixings and is simple to fit.  The ducting size is 100mm (4 inches) with a non return valve. Please see dimensions below : 

Dimension: A-190mm, B-72mm, C-130mm, Ø- 100mm

Brown Gravity Flap 100mm 4" External Ducting Ventilation Cover Air Vent Grille

• KO100-30BR
• Air Vent Cover
• Reduced Back-draft
• High Quality ABS Plastic
• Gravity Flap
• Comes with 4 Fixings
• 4 Screw Cover Caps
• Simple To Fix

Dimensions:

• A-190mm, B-72mm, C-130mm,
• Length Of Flange 20mm,
• Ø - 100mm