Chimney and Exhaust Hole Cutting Done Properly

Chimney hole cutting is the process of core drilling a precisely sized, correctly sloped circular opening through an external wall so a kitchen chimney's exhaust duct can vent outside cleanly. The two things people get wrong most often are cutting the hole too small or too tight for the duct, and cutting it dead level or sloping inward, both of which cause rework, rattling ducts or rainwater dripping back inside.

For a standard kitchen chimney, the duct pipe is usually 150mm (6 inch) in diameter, so the core hole should be cut around 175-200mm to leave clearance for the duct sleeve and sealant, not an exact fit. The hole should slope slightly downward toward the outside, roughly a 5-10 degree fall, so any condensation or driven rain runs outward rather than back into the kitchen.

This guide covers sizing, positioning, slope, sealing and the difference between a chimney exhaust hole and a general kitchen exhaust fan opening, both of which we cut regularly across kitchens in Gurugram from independent floors to high-rise apartments.

Sizing the hole to the duct

Chimney duct pipes commonly come in 150mm and occasionally 100mm sizes for smaller units, but always confirm the exact duct diameter from the chimney's installation manual before drilling, since a mismatch means either a loose rattling fit or an oversized gap that is hard to seal weatherproof. A margin of roughly 20-25mm around the duct diameter gives room for an insulating sleeve or foam backing rod plus sealant.

Where to position the hole

Position matters as much as size. The hole should sit close to the chimney unit to minimise horizontal duct run, which improves suction efficiency, but not so close to a window or ventilator that exhaust air recirculates back indoors. It should also clear structural columns, avoid running directly above a neighbour's window in apartment blocks, which is both a courtesy and sometimes a society rule, and stay clear of any visible plumbing or electrical conduit; we always scan with a detector first, the same process described in our reinforced concrete cutting guide.

• Keep at least 300-400mm clearance from adjacent windows or ventilators where possible.
• Position the hole at the height matching the chimney's rear outlet to avoid long duct bends.
• Avoid cutting directly through a beam; shift a few inches if reinforcement scanning shows one.
• Check apartment society rules on exterior wall penetrations before cutting in high-rises.
• Confirm the exhaust does not vent directly onto a neighbour's balcony or window.

Getting the slope right

A core drill naturally cuts a straight cylindrical hole, so achieving slope means angling the drill rig itself before starting, not adjusting mid-cut. We set the drill stand at the target angle using a level and template, confirm it against the duct's expected path, and cut through in one continuous pass wherever wall thickness allows. Getting this angle right the first time avoids a second attempt, since redrilling an already-sloped hole straight is messy and weakens the surrounding wall section.

A level chimney hole looks neater on the wall, but a sloped one is the difference between a dry kitchen wall and a damp patch showing up after the first proper monsoon shower.

Sealing the opening properly

Once the duct is through, the gap between duct and wall needs three things: a backing material like foam rod or mineral wool to fill bulk gaps, a flexible weatherproof sealant on the outside face rated for exterior exposure, and an internal cover plate or grommet for a finished look. External sealant should be a polyurethane or silicone-based product rated for movement and UV exposure, reapplied every few years as it can crack. Our monsoon waterproofing guide covers this sealing detail in more depth for all wall penetrations, not just chimney holes.

Chimney vents versus general exhaust fan holes

A kitchen exhaust fan hole is usually a smaller, unducted opening, often 200-300mm square or round, cut directly behind a wall-mounted fan with a simple flap or grille, and it does not need the same duct clearance calculation. Bathroom exhaust fan holes are similar. Chimney ducts, by contrast, often run some distance before exiting the wall and need the slope and sleeve detail described above.

Typical costs

Our process for a chimney hole job

1. 1Confirm chimney model and duct diameter from the client or installer.
2. 2Inspect the wall, scan for rebar and services, and mark position.
3. 3Set the drill rig to the correct outward slope angle.
4. 4Core drill the opening in one continuous wet-cut pass.
5. 5Fit the duct, insert backing material, and apply exterior sealant.
6. 6Fit the internal cover plate and clean up all slurry and debris.

Book your chimney hole cutting

Whether you are installing a new kitchen chimney in a Sohna Road apartment or replacing an old exhaust setup on Golf Course Road, tell us the chimney brand and duct size when booking so we bring the right core bit and sleeve on the first visit. Browse our services or see finished kitchen openings in projects.