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.

A compatible boss can connect a small waste branch to a soil stack where the layout permits. A connection between branch pipes of similar diameter needs the specified swept or angled junction; do not assume a straight saddle entry suits every 40mm or 50mm waste run.

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. Prepare the saddle and pipe surface and apply the compatible cement as the fitting instructions specify. Use primer only where that system requires it. 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.

Hole size belongs to the boss fitting, not just to the nominal waste inlet. The FloPlast SP319 guide specifies a 54mm hole in the soil pipe and separate adaptors for 32mm, 40mm or 50mm waste connections. Follow the exact product instructions before drilling.

Standard hole saw sizes for domestic boss fitting applications:

Boss type (inlet size)	Pipe	Typical hole saw diameter
40mm waste inlet	110mm soil pipe	54mm for FloPlast SP319; other bosses may differ
50mm waste inlet	110mm soil pipe	Use the exact boss instructions
32mm waste inlet	40mm or 50mm waste pipe	Use the exact boss 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 in the WC no connection zone. A boss must not enter the stack between the centreline of a WC branch and 200mm below it. Inside that zone, 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. Approved Document H (paragraph 1.17) calls for a WC branch to join the stack with a sweep of at least 50mm radius or at 45 degrees, which a boss cannot provide. 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 on the upper part of a horizontal pipe, ideally at the top (12 o'clock), as Approved Document H paragraph 1.18 advises.

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.

Where to buy Strap-on boss

FloPlast 110mm Soil Ring Seal Strap Boss White SP319W Trade-brand 110mm strap boss taking 32/40/50mm boss adaptors, as described on the page.

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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 (outside the zone from the WC branch centreline to 200mm below it, and at least 450mm above the foot of the stack), and confirm your plumber air-tests the full system including boss connections before anything gets boxed in.