Core Hole Size Guide: Choosing the Right Opening for Pipes, Ducts, and Drains

Don't core the hole the same size as the pipe. The pipe size is the wrong starting point: the hole must also allow for its sleeve, any fall, sealing space and—where required—regulatory clearance. This guide covers common extractor, waste, soil, flue, duct and drain jobs; the size chart below gives practical starting points.

Why the Pipe Size Is the Wrong Starting Point

A pipe-sized hole leaves no room to adjust the pipe or seal around it. For example, a 110mm soil pipe through a wall will usually need a 127mm core, while a 110mm drain passing through a foundation needs much more clearance.

We’re often called to fix holes drilled to the exact pipe diameter. Re-coring can be done, but it takes extra setup and can leave more making good. See our diamond core drilling service in Bristol for details of the method and materials we work with.

Core Hole Size Chart

These are practical starting sizes, not substitutes for a product manual or Building Regulations. Check the actual outside diameter and installation requirements before drilling.

Core sizes vary by equipment and supplier. Common UK sizes include 52, 62, 72, 82, 92, 107, 112, 117, 127, 132, 152, 162, 172, 202 and 212mm.

Why the Hole Needs to Be Bigger

Nominal Sizes Aren't Real Sizes

A “100mm” duct is a nominal description, not a guarantee that every part measures exactly 100mm. A fan spigot or sleeve may measure 100–104mm across, and flexible or plastic ducting can be slightly out of round.

Measure the widest part that must pass through the wall, including any sleeve lip or connector. If the hole is cut to the label size, the duct may catch or the sleeve may not fit.

Room for Fall and Alignment

An extractor duct should usually fall slightly towards the outside so condensation does not run back towards the fan. Soil and waste pipes also need their designed gradient; a pipe that cannot be set to the right angle can drain poorly or retain water.

For example, a 100mm duct in a 107mm hole has only a few millimetres of clearance all round. That may be enough for a straight, short connection, but a tight opening can prevent a small adjustment to the duct angle. Allow for the route and fittings before drilling.

Space to Seal It Properly

A narrow annular gap makes it difficult to apply mastic, foam or a mortar collar evenly. A 100mm duct in a 100mm hole has no sealing gap at all; even a small difference in alignment can leave the duct rubbing against the masonry.

Allow enough clearance for the specified sealant or sleeve detail, commonly around 5–10mm for ducts and sleeves. Use a suitable external seal where weatherproofing is needed, and do not use foam or mastic as a substitute for a required fire-stopping system.

Drains Through Foundations and Part H

A drain passing through a wall or foundation needs room to accommodate movement, rather than being tightly locked into the masonry. Building Regulations Approved Document H describes the relevant drainage provisions, including clearance around pipes or the use of rocker pipes in suitable arrangements. Check the detail for the specific installation and get advice where needed.

As a practical starting point, allow roughly 50mm clearance all round where that detail is required. For a pipe with a 110mm outside diameter, that points to an opening around 210mm across, so a 212mm core may be suitable; rocker pipes may be used as an alternative arrangement. Read the GOV.UK guidance for Approved Document H and confirm the specification before work starts.

Re-Drilling Costs More Than Getting It Right

Making a hole 10–20mm wider is possible, but the core drill needs to be guided or set up to follow the existing opening. That can take longer and create more slurry and making good than choosing the right core size first time.

LAP’s guide gives a £150 minimum visit, with additional holes typically £25–£150 depending on size and material. A slightly larger hole may need a little more making good, but that is often quicker than a return visit to enlarge a hole. See our Bristol diamond drilling cost guide for the factors that affect a quote.

When You Should Match the Manufacturer's Size Exactly

Follow the manufacturer’s stated opening size whenever the instructions specify one. Boiler flue systems are a clear example: manuals give the required opening and clearances for the particular flue, and sizes can vary. Do not infer a flue hole from “60/100” alone or alter the specified opening; have a Gas Safe registered engineer handle boiler flue work. Check the Gas Safe Register to find or verify an engineer.

Fan and MVHR kits may come with a wall sleeve sized for a particular core. Measure the sleeve and follow the kit instructions rather than assuming that the duct’s nominal diameter gives the correct opening.

Small 32mm and 40mm waste pipes are more forgiving: a 52mm core suits many installations. Still check the actual fittings, route and seal detail before drilling, especially if the pipe has to pass through a sleeve or a finished wall.

How to Work Out Your Core Size

1. Measure the outside diameter. Measure the pipe, duct, sleeve or complete insulated line set that must pass through the opening—not just the nominal size printed on the packaging.
2. Check instructions and regulations. Use the flue manual for a boiler, the fan or MVHR kit instructions for supplied sleeves, and Approved Document H for drains below ground. Confirm the route and any fire-stopping or structural requirements.
3. Add the needed clearance. As a starting point, allow about 5–10mm around ducts and sleeves, roughly 15–20mm around a soil pipe through a wall, and about 50mm all round for a foundation drain where that detail is required.
4. Choose the next suitable standard core size. Common UK sizes include 52, 62, 72, 82, 92, 107, 112, 117, 127, 132, 152, 162, 172, 202 and 212mm. Confirm availability and suitability for the material and opening.
5. Check the wall before drilling. Brick, block and concrete drill differently. A cavity wall needs a sleeve through the cavity while keeping the cavity clear; check for services, lintels, reinforcement and any structural restrictions.
6. Send LAP the details before booking. A photo of both sides, wall thickness, intended pipe or duct size and access notes help us confirm the opening. LAP Diamond Drilling provides free quotes from photos and uses wet diamond core drilling with dust and slurry controls. We’re DSA members, with CSCS, IPAF and PASMA credentials, and have completed more than 120 jobs, from single extractor holes to commercial M&E work at Bristol Airport and on a Skanska project.

Hole Sizes by Job

Bathroom Extractor Fan

Typical core: 107–112mm for a 100mm duct. Measure the fan spigot or supplied wall sleeve, then plan a slight fall to the outside so condensation can drain away from the fan.

Kitchen Cooker Hood

Typical core: 152–162mm for a nominal 150mm duct. Check the hood instructions and duct fittings; bends and reducers can restrict airflow even when the wall hole is correctly sized.

Soil Pipe for a New Bathroom

Typical core: 127mm for a 110mm soil pipe through a wall. Check the pipe fittings and sleeve, and allow enough room to set the pipe’s route and seal around it.

Boiler Swap

Typical core: The exact size specified by the boiler and flue manuals. Do not assume a standard opening from the nominal flue description; arrange the work through a Gas Safe engineer.

Air-Conditioning Lines

Typical core: Often 82–92mm for a bundle measuring roughly 60–80mm across. Measure the insulated pipes, control cable and sleeve together, and seal the opening to suit the wall and installation.

Drain Through a Foundation

Typical core: Around 212mm for a 110mm drain where roughly 50mm clearance is required all round, or use the specified rocker-pipe arrangement. Confirm the drainage detail against Approved Document H before drilling.

Concrete Wall with Reinforcement

Typical core: Choose the size required by the service and its sleeve, then locate reinforcement before cutting. Reinforcement affects the drilling method and time; read our practical guide to core drilling through reinforced concrete and agree positions for structural openings with the relevant professional.

Brick, block and concrete each call for a suitable core bit and setup. Wet diamond drilling controls dust and is the HSE-recommended approach for concrete coring; protect the work area and plan how to capture slurry, particularly indoors.

Core Hole Size FAQs

What size hole do I need for a 100mm extractor?

A 107–112mm core is a common starting point for a 100mm extractor duct. Measure the duct or sleeve itself and allow for its route, seal and slight fall to the outside.

What size hole do I need for a 110mm soil pipe?

A 127mm core is commonly used for a 110mm soil pipe through a wall. Check the pipe’s actual outside diameter, fittings and sleeve before drilling.

Can you make a core hole bigger?

Yes, a core hole can usually be enlarged, but the drill must be guided or set up to follow the existing opening. It takes longer and can mean extra making good, so checking the required clearance first is usually the better option.

Do cavity walls need a sleeve?

A service passing through a cavity wall generally needs a suitable sleeve through the cavity, with the cavity kept clear rather than filled by the pipe or general-purpose foam. Follow the service and wall construction requirements, including any fire-stopping detail.

What size hole do I need for a boiler flue?

Use the exact opening size and clearances in the boiler and flue manuals. Flue requirements are system-specific, so have a Gas Safe engineer confirm and carry out the work.

How much does a core hole cost?

LAP’s minimum visit is £150, with additional holes typically £25–£150 depending on the hole size and material. Access, wall thickness, setup and slurry control can also affect the quote; photos and measurements help us price the job.

Is wet or dry drilling better indoors?

Wet diamond core drilling controls dust effectively, but it produces slurry that needs to be contained and cleaned up. The right setup depends on the material and site; plan protection for the room and anything below the drilling point.

Don't core the hole to the pipe size: allow for the sleeve, route, seal and any required clearance. Send LAP Diamond Drilling a photo, wall thickness and measurements for a free quote through our contact page. We’ll help confirm a suitable core size before drilling.