Technical Articles
Drilling & Venting
Published 17 December 2018

Good product design depends on there being:

Channels for the access and drainage of molten zinc
Channels for the escape of gases from internal compartments (venting)

Any processes which aid the access and drainage of molten zinc will improve the quality of the coating and reduce costs. Molten zinc which is able to flow freely to all surfaces of a fabrication will support this, particularly where there are hollow sections or internal areas, the galvanising of internal surfaces eliminates any danger of hidden corrosion during service. All tubes & hollow sections that are to be galvanised must have adequate vent and drain holes. This is to allow the free movement of pickling fluids and molten zinc through the products, to ensure a complete galvanised coating. It is also to prevent the air inside sealed hollow sections super heating and causing explosions. A further reason for putting vent and drain holes is to prevent entrapped air pockets that will not be galvanised.

For The size of Vent & Drain holes please refer to the table below:

Size of vent and drain holes
Diameter or width of hollow section Minimum diameter of Vent Holes
Less than 25 mm 10 mm
26-50 mm 12 mm
51-100 mm 16 mm
101-150 mm 20 mm
Above 150 mm Consult Clark-Drain


Please Note: The above table shows the minimum size of holes that should be used, but the larger the vent holes the better the finished job will be.

For the positioning of vent and drain holes call 01733 765318 or submit an enquiry:


E.g. Barrier vent details

Barrier vent details












Why Choosing the Right Drainage and Access Specification and Design is Paramount

Choosing your drainage and access products When choosing full article >

Specification - The key to effective cost management

Avoiding under-specification on UK driveways is key to full article >