The majority of extractor fan installation mistakes come down to one of two errors: drilling the hole too small (the spigot won't pass through) or drilling it too large (the grille can't cover the gap). Neither is a quick fix once the core is cut, particularly through 270mm or 340mm cavity walls where re-drilling means significantly more damage to the surrounding masonry.
This guide gives the correct core drill diameter for every standard UK domestic extractor fan size, covers how to account for flexi-duct and rigid spigot systems, and explains what changes when you are drilling through an insulated cavity wall rather than a solid brick or block skin.
Standard UK Extractor Fan Sizes and Required Core Drill Diameters
UK domestic extractor fans are rated by their spigot diameter — the circular outlet where the duct attaches. There are three standard sizes in common use:
| Fan Spigot Size | Typical Application | Recommended Core Drill Size |
|---|---|---|
| 100mm (4 inch) | Bathroom, WC, small kitchen | 107mm or 110mm |
| 125mm (5 inch) | Kitchen, utility room, larger bathroom | 133mm or 136mm |
| 150mm (6 inch) | Commercial kitchen extract, large utility | 157mm or 160mm |
The rule is to add approximately 7–10mm to the spigot diameter. This gives clearance for a rigid duct sleeve, allows the duct to run through without binding, and leaves enough tolerance to adjust angle slightly if the drill wanders during the cut. Do not drill to the exact spigot size — the duct will jam, and on anything other than a perfectly square cut you will end up grinding away the masonry by hand.
100mm Fan (4-Inch) — the Most Common UK Installation
The majority of UK domestic bathroom and WC extractor fans use a 100mm spigot. The correct core drill size is 107mm. This is also the standard boiler flue size for condensing boilers — making 107mm diamond core bits the single most versatile size in a UK plumber's or electrician's kit. See the boiler flue core drill size guide for the overlap between these two applications.
A 107mm core bit leaves approximately 3.5mm of clearance on each side of a 100mm duct, which is sufficient for a rigid white PVC duct sleeve. If you are using flexible aluminium flexi-duct rather than rigid PVC, 107mm is tight — go to 110mm to allow the corrugated outer profile to pass through cleanly.
125mm Fan (5-Inch) — Kitchen and Utility Room Installations
125mm fans are the standard choice for kitchen extract where cooking smells and moisture require a higher airflow rate. The correct core drill size is 133mm. Some installers use 136mm, particularly for systems that will use rigid 125mm spigot-to-duct connectors with a rubber seal, which adds a few millimetres to the outer diameter when compressed.
Kitchen extract through external walls often runs through a wider wall section than bathroom installations — it is common to encounter 340mm cavity walls with partial-fill insulation in modern builds. At this depth, a standard 150mm core barrel is the minimum — use a 200mm extension rod for anything deeper. See the UK core drill bit sizes guide for extension rod compatibility by barrel size.
150mm Fan (6-Inch)
150mm spigot fans are almost exclusively commercial or semi-commercial: restaurant kitchen extract, large workshop ventilation, plant rooms. Installations using 150mm duct need a 160mm core. At this diameter, a dedicated core drill motor on a stand is the only practical option — SDS adaptors cannot reliably handle cores above 132mm in masonry and should not be used at 160mm even with appropriate torque. See the SDS adaptor size limit guide for the detail on this.
Cavity Walls: What Changes When There Are Two Skins
A standard UK cavity wall consists of an outer leaf (typically 102mm facing brick), a cavity (50–100mm, often partially filled with insulation), and an inner leaf (typically 100mm block). The total depth from face to face is between 270mm and 330mm for a standard cavity wall, and up to 380mm for a high-insulation modern build.
When drilling through a cavity wall for an extractor fan, the core bit must pass through both skins plus the cavity. The key issues are:
- Insulation in the cavity: Partial-fill rigid insulation boards (typical in post-2003 construction) can cause the core barrel to deflect as it enters the cavity. Drive through the insulation steadily — do not allow the bit to oscillate sideways as it crosses the void.
- Alignment between the two skins: Drill from outside to inside where possible. Starting from the external face and breaking through into the internal room gives a cleaner finish on the visible internal side. If the external face is facing brick, consider drilling to just break through the outer skin, then completing the cut from inside to reduce spalling on the brick face.
- Damp-proof course continuity: Any penetration through a cavity wall for extract ductwork must be sleeved, and the sleeve must be sealed at both skins to prevent interstitial moisture build-up. Use a proprietary wall sleeve kit (available from BES, Screwfix, or specialist HVAC suppliers) rather than relying on silicone alone.
Solid Wall Installations
Solid brick walls in older UK properties (pre-1920s construction) present a different challenge. A 230mm solid wall is straightforward with a standard core barrel, but many Victorian and Edwardian properties have 340–460mm solid brick walls. At these depths, extension rods are essential. The brick wall core drilling guide covers depth extension technique and how to maintain alignment over long cuts.
Solid walls also tend to have a much higher proportion of hard engineering brick or mixed-hardness material. Check what you are drilling before choosing a diamond core drill bit bond hardness — a medium-bond bit that works on standard facing brick will glaze quickly if the wall contains hard clinker bricks or has been repointed with dense modern mortar.
Fitting the Fan After Drilling
Once the core is cut:
- Remove the core slug — twist and pull with the extractor tool or long-nose pliers if it has not already dropped.
- Clear the hole of dust and fragments.
- Fit the wall sleeve through from the outside, ensuring the louvred grille end seats flush against the external face.
- Insert the fan housing from the inside and connect to the sleeve spigot.
- Seal both ends with a suitable external mastic — not standard white bathroom silicone on the external face. Use a weatherproof, paintable mastic for external masonry.
For fans with an integral backdraught shutter, verify the shutter flap can open fully within the sleeve length. A 100mm fan fitted with a 107mm core and standard sleeve has approximately 3.5mm clearance — enough for most shutters, but measure the shutter diameter before buying the bit if in doubt.
