How to Fit a Strap Boss to a Soil Pipe: Step-by-Step

By Steven Reid · Last updated on August 23rd, 2026 · 2 Comments

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Fitting a strap boss is a straightforward way to connect a waste pipe or boiler condensate pipe to a soil stack without cutting out and replacing a complete section.

The process is similar on plastic and cast-iron soil pipes, although cast iron takes longer to drill. The most important parts are cutting the correct size hole, cleaning the surface properly and sealing the boss to prevent leaks.

I have been a full-time plumber for over 15 years and fitted many strap bosses to plastic and cast-iron soil pipes. Here I’ll show you my step-by-step method.

Tools and materials

The tools and materials needed to fit a strap boss: (with links to gear on Amazon)

Tools

• Combi drill
• 54mm bi-metal hole saw
• Arbour and pilot bit
• Silicone gun
• Water pump pliers (not necessary but gets it tighter)

Materials

• Strap boss
• Silicone

Steps to fit a strap boss

Here is my step-by-step process for fitting a strap boss to a plastic or cast iron soil pipe:

1. Drill a hole in the soil pipe

Using a hole saw and combi drill, drill a hole in the soil pipe, it’ll need to be 54mm for most bosses. You should go slow if it’s a plastic pipe you’re drilling as it can be damaged easily.

2. Clean the hole

Clean all the plastic bits off the edges in the hole you’ve just drilled, and clean any dirt and grease off the pipe. The cleaner it is, the better for the silicone sticking to, and for the water to flow as smoothly as possible.

3. Put some silicone on the boss

Add a bed of silicone around the strap boss where it sits on the soil pipe. It takes practice to get the right amount but it’s better to err on the side of too much than not enough. You can clean any excess silicone away when you’re finished.

4. Fit the boss to the soil pipe

Open up the strap, fit it over the pipe and press the clip together with your hands or water pump pliers. Or, you might need to screw in the screw with a screwdriver depending on which type of strap boss you have.

5. Clean excess silicone off

Use some toilet roll and clean off any excess silicone from the soil pipe, as much as possible. You can then fit the waste pipe into the boss using a rubber connector, you might need to use some silicone grease to get it in as they can be tight and tricky to fit.

Connecting a boiler condensate pipe to a soil pipe

A strap boss can be used to connect a boiler condensate pipe directly to a soil pipe. An internal gravity connection is normally the most reliable option because it keeps the condensate pipe away from freezing outdoor temperatures.

The pipe must fall continuously from the boiler towards the soil pipe so the wastewater can drain freely. Use the correct boss adaptor or rubber reducer, and only use plastic pipe and fittings because condensate wastewater can corrode copper and brass.

If the connection has to be made outside, keep the external pipe as short as possible and protect it properly. See my guide on how to stop your boiler condensate pipe freezing for advice on pipe sizing, insulation, frost-protection siphons and trace heating.

Summary

Fitting a strap boss to a soil pipe is fairly straightforward if you have the correct hole saw and take your time.

Measure the boss before drilling because 54 mm is common, but sizes can vary. Plastic soil pipes are easier to drill but can be damaged if the hole saw catches. Cast iron takes longer, although the pipe is less likely to flex or split.

Clean the surface properly, use enough sealant and make sure the boss is secure before connecting the waste pipe.

FAQs

What should I do if the boiler condensate pipe is already frozen?

The pipe must be completely thawed before resetting the boiler. Pour warm water that is not freshly boiled over the frozen section, starting at the open end and working back towards the wall.

Do not use boiling water, a heat gun or a naked flame because excessive heat can damage the plastic. Follow my complete guide on how to thaw a frozen boiler condensate pipe for the full process.

What size hole do you drill for a strap boss?

The most common hole size you need to drill for strap bosses is 54mm but it can vary for different ones. There is a lip on the strap boss that needs to sit inside the drilled hole and is easy to measure if you’re not sure about the one you have.

What is a soil pipe?

A soil pipe carries soiled water from your toilet to the sewer. They can also be used to run waste pipes from baths, showers, sinks and boilers if needed.

How do you cut cast iron soil pipe?

You can cut a cast iron soil pipe a few different ways, but my go to way is to use an angle grinder. You can also use a reciprocating saw, a chain pipe cutter or even a standard hack saw will do the job (eventually).