Water Butt Diverters: Choosing the Right Downpipe Fitting

Illustration of a covered rainwater butt beside a brick shed

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Table of Contents

Start with the downpipe, not the diverter

Before comparing products, measure the downpipe you intend to fit. The two details that matter most are the external size and shape, and the way the pipe is fixed to the wall. UK plastic downpipes are not all the same, and a diverter that fits a 68 mm round pipe may not fit a 65 mm square one, or a square pipe held on close-fitting brackets.

Measure the external diameter of a round downpipe, or the external width of a square one. Do not rely on the pipe looking ‘standard’. Also check the bracket type: stand-off brackets hold the pipe away from the wall, while close-fitting brackets hold it almost flush. For some diverters, this changes the fitting procedure and whether an adaptor is needed.

Two common fitting approaches

Two useful examples to compare are in-line filter diverters that replace a section of downpipe, and simpler rain traps that sit in a cut section and channel water sideways. The fitting requirements differ, so identify which type you are considering.

In-line filter diverters: the Gutter Mate example

The Gutter Mate diverter and filter is designed to accept 68 mm, 76 mm and 80 mm diameter round downpipes, and 65 mm and 75 mm square downpipes, without additional adaptors or reducers. It will also fit a 60 mm downpipe if the wall thickness is 1.5 mm or less, or a 61 mm downpipe if the wall thickness is 2 mm or less. For square downpipes fitted with close-fitting mounting brackets, the supplied square-to-round adaptor is needed; with stand-off brackets it is not.

Use the installation diagram for the exact Gutter Mate version to mark the cuts. Its reference is the maximum water level in the installed butt, and the cutting dimensions change for square downpipes with close-fitting brackets. Put the tank and stand in their final position first, then follow the complete diagram rather than copying a single measurement from another installation.

If the main outlet has to fit into a downpipe with an internal dimension smaller than 60 mm square or 65 mm round, a female-to-male reducer is needed. The product page also states that the filter should not be used on downpipes larger than 100 mm diameter, and that 40 mm and 50 mm downpipes common on greenhouses, sheds and garages require a reducer (the standard 68 mm female to 50 mm reducer is listed for either size).

Simple rain trap diverters: the Harcostar Universal example

The Harcostar Universal Rain Trap Diverter lists compatibility with round downpipes of 63 mm, 68 mm, 80 mm and 100 mm, and square downpipes of 61 mm and 65 mm. It is not suitable for cast iron downpipes. Installation involves cutting a short section from a plastic downpipe and inserting the unit; the supplied flexible hose is 0.5 m long with a 25 mm diameter, and the product description states that hoses cannot be joined together. Some water butts may require a small hole to be drilled to attach the hose.

Decisive checks before you buy

External size and shape. Confirm the downpipe’s external diameter or width against the product’s listed range. A 63 mm round pipe may fit one diverter but not another.
Bracket type. If you have a square downpipe on close-fitting brackets, check whether the product includes a square-to-round adaptor and whether the cutting measurements change. The Gutter Mate diagram gives different cut-out dimensions for these two bracket arrangements.
Internal diameter at the outlet point. For in-line diverters, the main outlet must fit inside the lower downpipe. If the internal dimension is below the product’s minimum, you need a reducer. The Gutter Mate note specifies a reducer below 60 mm square or 65 mm round internal dimension.
Wall thickness for borderline sizes. The Gutter Mate information gives wall-thickness limits for certain 60–61 mm round pipes; do not transfer those limits to the Harcostar fitting. Measure the pipe wall or check the manufacturer’s specification before assuming a fit.
Material. The Harcostar product explicitly excludes cast iron. For other materials, check the manufacturer’s guidance; cutting cast iron requires different tools and fittings.
Distance to the water butt. The supplied hose is often short. The Harcostar hose is 0.5 m and cannot be joined; the Gutter Mate includes 500 mm with additional sizes available. Measure the actual distance before buying, and allow for the height difference between the diverter outlet and the butt inlet.
Water butt inlet size. The Gutter Mate instructions require a 25 mm hole for the hose adaptor. Check your butt has a flat area suitable for drilling and that the hole size matches the supplied adaptor.

Common fitting mistakes

Cutting the downpipe before marking the maximum water level. The Gutter Mate sequence depends on that mark; if the butt’s overflow level is wrong, the diverter may sit too low or too high.
Assuming all square downpipes are the same. Close-fitting brackets change the required cut length and often require an adaptor.
Ignoring internal diameter. A pipe may have the right external size but too small an internal bore for the diverter’s main outlet, causing a poor fit or blockage.
Using a diverter on cast iron without checking. The Harcostar product is not suitable; other products may have different restrictions.
Underestimating hose length. A 0.5 m hose is often too short if the butt is not directly beside the downpipe.

Comparing the two diverter examples

Feature	Gutter Mate Diverter & Filter	Harcostar Universal Rain Trap
Round downpipe sizes listed	68 mm, 76 mm, 80 mm; 60 mm/61 mm with wall-thickness limits	63 mm, 68 mm, 80 mm, 100 mm
Square downpipe sizes listed	65 mm, 75 mm	61 mm, 65 mm
Adaptor for close-fitting square brackets	Supplied square-to-round adaptor	Not specified on the product page reviewed
Reducer needed for small downpipes	Yes, for internal dimension below 60 mm square or 65 mm round; 40 mm/50 mm pipes require reducer	Not stated; listed sizes start at 61 mm square and 63 mm round
Maximum downpipe size	Not to be used above 100 mm diameter	100 mm round listed
Cast iron suitability	Not specified on the product page reviewed	Not suitable
Supplied hose	500 mm, additional sizes available	0.5 m, cannot be joined
Built-in filter	Yes	No filter mentioned

These are compatibility examples from the linked product pages and installation instructions, not hands-on test results. Confirm the current specification for the exact version before ordering.

What to do if your downpipe is outside the listed range

If your downpipe is smaller than the product’s listed sizes, a reducer may allow the main outlet to connect, but check that the top opening can still accept the downpipe. For the Gutter Mate, 40 mm and 50 mm downpipes are mentioned as usable with a 68 mm female to 50 mm reducer. If your downpipe is larger than 80 mm, the Gutter Mate product page says a male-to-female reducer is needed to enter the top of the filter, and the filter should not be used above 100 mm. For the Harcostar, no reducer options are listed on the product page reviewed, so a different product may be needed for very small or very large pipes.

Always confirm the complete reducer arrangement with the diverter supplier before cutting. A reducer name alone does not establish a fit at both ends of the assembly. Check the current installation instructions for the exact version purchased.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

Gutter Mate Diverter & Filter in Black: Helps readers compare downpipe compatibility, adaptor needs and hose length for an in-line filter diverter. Measure downpipe external diameter, bracket type (stand-off or close-fitting), internal diameter at outlet, wall thickness for 60–61 mm pipes, and distance to water butt. Confirm whether a reducer is needed for 40–50 mm or >80 mm pipes. Search Amazon for Gutter Mate Diverter & Filter in Black.
Harcostar Universal Rain Trap Diverter: Helps readers compare a simple rain trap's listed downpipe sizes, material restriction and hose length. Compare the exact downpipe shape, diameter and material with the Harcostar compatibility list. Check the supplied hose length and permitted connection method. Search Amazon for Harcostar Universal Rain Trap Diverter.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.