Double Concrete Base for Static Chimney Cowl - Twin-Flue Installation
Double concrete base: two flues solved with a single piece
When a chimney rises with two twin ducts — one for the boiler and one for the fireplace, or two homes sharing a pillar — the usual approach is to fit two separate bases on the same top. The result is almost always the same: the two pieces end up misaligned, a joint opens up between them that has to be filled with mortar, and that joint is exactly where water starts to seep in within a couple of years. This base fixes the problem at the root: it's a single piece, 125 × 65 × 5 cm, that houses both 31 × 31 cm flues, with no central joint to seal and nothing to misalign.
✅ Key advantages
No central joint: both openings are formed in the same piece, with the exact spacing and square faces. There's nothing to set out on site or adjust two bases by eye on the roof.
Real watertightness: with no joint between the two flues, the weak point where water gets in disappears. The only joint to seal is the outer perimeter, which is much better protected.
Compatible with round and square cowls: whatever tower you need goes on top of each flue, round or square, depending on the look and the flue section.
Concrete with no internal reinforcement: acidic smoke condensates and rainwater have no steel to corrode. The piece doesn't swell, doesn't flake from corrosion and needs no maintenance.
Large load-spreading surface: at 125 × 65 cm it spreads the weight of both towers across the whole top, instead of concentrating it on two small supports.
Two finishes: White and Cement Grey, to match the brickwork or the rest of the cap.
🏗️ Recommended applications
Chimneys with a fireplace duct and a boiler duct on the same pillar.
Shared flue (shunt) between two homes in multi-family buildings.
Double ventilation and extraction outlets on flat or pitched roofs.
Renovating double chimney tops that have been leaking through the central joint for years.
New builds wanting to unify the cap for two twin ducts.
📋 Technical specifications
Piece dimensions: 125 × 65 × 5 cm.
Section of each flue: 31 × 31 cm.
Internal dimensions: 17 × 31 × 29 cm.
Unit weight: 67.3 kg.
Units per pallet: 10 (pallet weight, 689 kg).
Material: concrete, no internal reinforcement.
Colors available: White and Cement Grey.
Compatibility: round and square static cowls.
Use: outdoor, on the roof.
Packaging: single unit or pallet of 10 units.
🧰 Installation
Check that both ducts have a section of 31 × 31 cm or less and that the spacing between their centers matches the piece. Always measure from inside the flue.
Level the top of the pillar with mortar to get a continuous horizontal plane under the whole 125 × 65 cm footprint.
Lift the piece with mechanical aids or with two workers: it weighs 67.3 kg and can't be handled single-handed.
Bed the base on fresh mortar, centering both openings over the flue mouths, and check the level in both directions before it sets.
Seal only the outer support perimeter. Don't block or reduce the section of the openings.
Fit the tower for each cowl on top and let it cure for 48 hours before starting up the appliances.
📦 Available formats
125 × 65 × 5 cm in White, single unit or pallet of 10 units.
125 × 65 × 5 cm in Cement Grey, single unit or pallet of 10 units.
💡 Professional tip: before ordering the base, go up on the roof and measure the distance between the centers of the two flues, not just each one's section. That's the detail most often forgotten, and it decides whether the piece fits or has to be returned: if the ducts are farther apart than the base allows, there's no way to adjust it and you're back to two separate pieces. And don't use the installation as a chance to "join" the two flues internally thinking they'll draw better: two connected ducts steal draft from each other and end up sending smoke from one back into the other. Keep each flue independent all the way to the mouth.
