If you buy through links on this site, I may earn an affiliate commission.
Strap boss fitted on soil pipe

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

Last updated on January 31st, 2024

Fitting a strap boss to a soil pipe is often needed to connect a waste pipe or condensate pipe to a soil stack pipe.

It doesn’t matter if the soil pipe is cast iron or plastic, this process is exactly the same and can be drilled with the same hole saw and drill.

I have been a full-time plumber fitting waste pipes and strap bosses for over a decade.

Here I’ll show you how to fit a strap boss to a soil pipe based on my experiences over the years.

Hole drilled in cast iron soil pipe

Tools and Materials

The tools and materials needed to fit a strap boss:

(with links to gear on Amazon)



How to Fit a Strap Boss Steps

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

Drilling 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

54mm hole drilled in soil pipe

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

Strap boss with silicone

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

Boss fitted with water pump pliers

Open up the strap and fit it over the pipe and press the clip together with 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

Waste pipe fitted to soil pipe

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.


Fitting a strap-on boss to a soil stack pipe is fairly easy if you know how to use a drill as this is the hardest part.

Although it takes longer to drill through a cast iron pipe than a plastic pipe, drilling a cast iron pipe is easier as you can’t really go wrong with it.

With a plastic pipe you have to be more careful as it's a lot more fragile and can be damaged by the hole saw in the drill.

Make sure you get a strap boss with the right size adapter, the same size as the waste pipe that’s going into it.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


What size hole do you drill for a strap boss?

The most common size hole you need to drill for most strap bosses is 54mm.

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 typically 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).


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
Share this post

2 thoughts on “How to Fit a Strap Boss to a Soil Pipe: Step-by-Step”

  1. Jonathon Bouchard

    Will 110mm boss fit onto slightly larger assbestos cement soil pipe ?

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top