Made-to-Measure Chimney Cap: How to Measure, Choose, and Order Yours

A chimney cap, also called a chimney cowl or chimney terminal, is the element that crowns the masonry surround at the flue outlet. Its function is to improve the flue's draft, keep out rain, wind, and animals, and prevent smoke backflow into the building. When the surround falls outside standard market sizes, the only correct technical solution is to order a made-to-measure cap.

Most chimney surrounds don't match catalog sizes. Dimensions vary depending on the number of flues, wall thickness, and construction type. In these cases, a standard cap doesn't close properly: it leaves gaps that let in water and dirt, or simply doesn't fit. What's more, if you're after a specific color to blend with the roof, only made-to-measure fabrication guarantees the exact RAL on every face.

You only need two measurements: the exterior width and length of the masonry surround, in millimeters. If the surround is irregular and opposite faces aren't equal, always note the larger one for each dimension. There's no need to subtract anything: the manufacturer applies the technical clearance (typically 3 mm per face) so the cap fits without forcing. For circular surrounds, indicate the flue's interior diameter.

Made-to-measure chimney caps are typically manufactured in polyester-painted aluminum (6063-T5 alloy), resistant to corrosion, UV rays, and continuous temperatures up to 80°C. For more aggressive environments, such as coastal areas or pellet fireplaces with acidic condensates, the alternative is AISI 316L stainless steel, more resistant to chemical corrosion, though also heavier and more expensive. Both materials can be manufactured in any size and any RAL color.

The static cap harnesses the wind to generate suction that naturally improves draft, with no electrical consumption or moving parts. It's the most common option for residential homes with conventional draft flues. The mechanical cap incorporates a forced-extraction motor, recommended when the flue has poor natural draft (due to insufficient height, unfavorable orientation, or a location with little wind) or when the required airflow exceeds what static extraction can provide. Both types are manufactured to measure in any dimension.

VENTUM® is Industrias Mas's range of chimney caps and terminals, manufactured since 1968 in Tàrrega (Lleida) under ISO 9001 certification. They're made to measure in polyester-painted aluminum or AISI 316L stainless steel, in static and mechanical versions, for flues from 100 to 500 mm in diameter or masonry surrounds of any rectangular dimension. Made-to-measure orders include all fixing accessories and are quoted within 24 business hours of receiving measurements. For more on types and materials, see our complete guide to chimney caps.