Strap-On Boss: Adding a Waste Branch to an Existing Soil or Waste Pipe

You've built the soil stack and you need to add a basin waste connection. Cutting out a section of installed pipe and fitting a branch tee is one option: disruptive, time-consuming, and involving two extra joints. A strap-on boss is faster. Drill a hole in the pipe, clamp the boss over it, push in the waste pipe. Three steps, one new fitting, no joints to cut and remake. Understanding when to use a strap-on boss (and how to fit it correctly) is one of those practical details your plumber handles without thinking about it. When you're specifying or overseeing the work, knowing what's happening and why matters.

What it is

A strap-on boss is a saddle fitting that bonds or clamps to the outside of an existing soil or waste pipe, creating a new branch inlet through a hole cut in the pipe wall. The fitting consists of:

• A saddle body: a shaped housing that sits around the outside of the pipe, with a boss inlet (typically a 40mm or 50mm spigot) for the connecting waste pipe
• A sealing method: either a rubber ring that creates a watertight seal between the saddle and the cut hole, or a solvent-weld bond on solvent-weld pipe
• A fixing method: either a strap or clamp (on strap-on types), or a bonded solvent-weld collar (on solvent-weld types)

The finished result looks like a small additional socket emerging from the side of the soil or waste pipe, accepting a branch connection at the angle moulded into the boss body.

Boss adaptors are made for several pipe sizes. The most common applications in domestic extension work are:

• 110mm soil pipe (grey or black uPVC): for connecting basin, bath, sink, or shower waste pipes to the main soil stack
• 50mm waste pipe: for connecting smaller branches into a 50mm intermediate waste run
• 40mm waste pipe: for smaller branch connections on 40mm runs

Strap-on boss versus cutting in a swept tee

When you need a new branch connection, you have a choice: add a strap-on boss to the existing pipe, or cut out a section and fit a proper swept tee branch. The choice depends on whether the pipe is already installed and whether noise and drainage performance matter.

A swept tee is a factory-moulded fitting where the branch entry sweeps into the main pipe run at an angle (typically 45 degrees on the branch, with a curved internal radius). The swept entry guides waste smoothly around the junction and into the main flow without turbulence. This reduces noise, reduces the risk of solids getting stranded at the junction, and is considered better practice for any branch connection that will see regular use.

A strap-on boss creates a straight entry through the pipe wall. The boss opening protrudes from the side of the pipe without any swept radius inside the main pipe bore. This is functionally adequate for low-flow branches (basin, shower) but creates more turbulence at the junction than a swept fitting. On a busy stack with multiple connections, this turbulence can contribute to noise and to partial blockage over time.

The rule is clear: WC (toilet) connections to a 110mm soil stack always require a proper swept branch tee, never a boss adaptor. The flow volumes and solids from a WC require a swept entry. A boss adaptor creates a straight hole that cannot handle WC discharge safely or compliantly.

For basin, bath, shower, and sink waste pipes (which produce water only, no solids), a strap-on boss on the 110mm soil stack or on a 50mm waste pipe is fully acceptable and is the standard fitting used by most plumbers where the pipe is already installed.

Solvent-weld versus rubber-ring (push-fit)

Boss fittings come in two connection methods, and they are not interchangeable across products. You need the right type for the pipe you're connecting to.

Rubber-ring strap-on boss: The saddle body seats against the pipe with a rubber ring seal that fills the gap between the curved saddle face and the round pipe wall. The rubber seals around the cut hole. Fixing is by a strap or screw clamp that holds the saddle tight against the pipe. The joint is watertight when correctly tightened. This type suits push-fit (ring-seal) soil pipe systems and is removable if needed.

Solvent-weld strap-on boss: The saddle body bonds chemically to the pipe wall using solvent cement. The inside face of the saddle is primed and cemented to the prepared pipe surface around the hole. Once bonded, the joint is permanent. This type is used on solvent-weld pipe systems and creates a stronger, more rigid joint. The trade-off is that removing it requires cutting the pipe.

Check which type of pipe is installed before buying a boss. Ring-seal and solvent-weld pipe have the same outside diameter for 110mm and are visually similar. The distinction matters only when it comes to the joint method. If in doubt, ask your plumber. Fitting a rubber-ring boss to a solvent-weld pipe (or attempting to solvent-weld a boss to a push-fit pipe) won't create a proper seal.

Hole saw size: this matters

The hole you drill in the pipe must match the boss adaptor exactly. Too small and the boss won't seat over the cut edge. Too large and the rubber seal cannot span the gap, giving a leaking joint.

For a 110mm soil pipe strap-on boss (accepting 40mm waste pipe), the standard hole size is typically 43mm or 46mm depending on the brand. The boss fitting instructions always specify the correct hole saw diameter. Read the instructions before drilling. Boss adaptors come in versions for different waste pipe sizes (32mm, 40mm, 50mm), and the hole size differs between them.

Standard hole saw sizes for domestic boss fitting applications:

Boss type (inlet size)	Pipe	Typical hole saw diameter
40mm waste inlet	110mm soil pipe	43mm or 46mm (check fitting instructions)
50mm waste inlet	110mm soil pipe	50mm or 56mm (check fitting instructions)
32mm waste inlet	40mm or 50mm waste pipe	36mm (check fitting instructions)

How to fit a strap-on boss (step by step)

Common mistakes

Drilling with the wrong size hole saw. The single most common error. Always read the instructions for the specific boss fitting you've bought, because the required hole size varies by brand and inlet size. Buying a pack of boss fittings without checking the hole saw size before the drilling stage means a separate merchant trip.

Positioning the boss below the WC branch level. Boss connections must sit at least 200mm below the centreline of any WC branch connection above them on the same stack. If the boss is positioned too high (close to the WC branch), the WC flush surge can back up into the waste pipe connected at the boss inlet. This creates momentary dirty water at the downstream fixture.

Fitting with a gap in the rubber ring. When tightening a rubber-ring boss, the rubber must compress evenly around the full perimeter of the hole. If the saddle rocks (because the pipe surface isn't clean, or because there's a moulded pipe seam under the saddle), one side compresses and the other doesn't. Check that the saddle sits flat before and after tightening.

Using a boss for a WC connection. Toilet waste must not connect to a soil stack via a boss adaptor. The soil pipe regulations (and BS EN 12056-2) require a swept branch fitting for WC connections. No exceptions.

Connecting at the bottom of the pipe. On a horizontal waste pipe run, fitting the boss at the bottom of the pipe (6 o'clock position) means the boss inlet is at the lowest point of the pipe cross-section. Any sediment in the waste pipe collects at the boss inlet and can partially block it. Position the boss at the top (12 o'clock) or side (3 or 9 o'clock) of a horizontal pipe.

Prices and where to buy

A strap-on boss for 110mm soil pipe (accepting 40mm waste) costs around £12 at major UK retailers. Screwfix lists the FloPlast range at the lower end; McAlpine variants run slightly higher.

Screwfix, Toolstation, and Wickes all carry strap-on boss fittings as a standard stocked item. For trade quantities or specialist sizes, plumbing merchants (Wolseley, City Plumbing, BSS Industrial) carry the full McAlpine, FloPlast, and Polypipe ranges.

Buy the correct hole saw at the same time. A bi-metal hole saw mandrel set (£20 – £40) will include the sizes needed for boss fitting work and can be reused across other jobs. Buying a single hole saw arbor for the specific size costs £5 – £10 from Screwfix or Toolstation.

Alternatives

Where the pipe is not yet installed, a factory-moulded swept tee is always the better choice for drainage performance. The boss adaptor's advantage is specifically that it can be added to installed pipe without cutting it out. For a new installation where all the pipe work is being specified from scratch, plan the branch positions at the design stage and use swept tees as part of the primary pipe specification.

Boss pipe (a short section of 110mm pipe with factory-moulded boss connections already built in) is another option for new work. Boss pipe sections are available with one, two, or three pre-formed boss inlets at specified positions. They cost more than plain pipe but eliminate the need for cutting and fitting individual boss adaptors. For a stack that will have multiple waste connections, a boss pipe section in the right location is neater and faster.

Where you'll need this

Strap-on boss fittings appear in:

• First fix plumbing - adding branch connections for basin, bath, and shower waste pipes to the soil stack; typically installed after the soil stack is in position but before the air test and plasterboarding
• Drainage - understanding the full soil and waste system into which boss connections feed

For any extension that adds basin, bath, or shower waste connections to an existing or new soil stack, a strap-on boss at around £12 is the standard connection method. Understand where they go, know the position rules (below WC branch, at 200mm minimum), and confirm your plumber air-tests the full system including boss connections before anything gets boxed in.