Rectangular Cast Iron Downpipes

Replacing a Square Cast Iron Downpipe: A Buying Guide

Replacing a square cast iron downpipe is not always an easy process. The dimensions, fixing ears, direction of bends and distance from the wall all matter. Especially if it's an old system installed decades ago.

At Cast Iron Warehouse, we supply genuine Alumasc Apex Heritage rectangular cast iron downpipes in the traditional 100 x 75mm (4" x 3") profile, along with matching offsets, shoes, sockets and other fittings.

Here are a few recommendations we would give before buying.

Square vs Rectangular Cast Iron Downpipes: What's the Difference?

The terms square and rectangular cast iron downpipes are frequently used interchangeably when shopping for a traditional rainwater system online.

There are differences, however.

Alumasc produces square cast iron pipes in 75 x 75mm and 100 x 100mm sizes. In addition to that, the rectangular profiles include 100 x 75mm, 125 x 100mm and 150 x 100mm.

Our range of products focuses on the 100 x 75mm rectangular cast iron downpipe system, also referred to as a 4" x 3" cast iron downpipe. Although sometimes described as square, this profile is technically rectangular.

The flatter shape makes it a popular solution for Victorian and Edwardian properties, traditional townhouses and buildings with masonry decorations. It gives the elevation a crisper look than a conventional round rainwater pipe.

In case of replacement, matching the original profile is generally the most logical choice. For a new rainwater system, the pipe size should be chosen according to the drainage requirements of the building.

Understanding 100 x 75mm (4" x 3") Cast Iron Downpipe Dimensions

It's worth mentioning that 100 x 75mm is the nominal size of the pipe. This means that not all the external dimensions of the casting will be exactly 100mm by 75mm.

For instance, the technical dimensions of the P43 rectangular pipe by Alumasc give 108mm as an external front-face dimension. The socket is wider. It becomes particularly important if you are trying to measure an existing cast iron downpipe and figure out if the new fittings will connect.

We would recommend checking the following:

• The width of the pipe at its front face.
• The depth of the pipe from front to rear.
• The dimensions of the socket and fixing ears.

Measure the pipe body instead of relying on the widest part of the socket. If possible, take pictures of the pipe profile and the socket separately.

Don't assume that two old cast iron rainwater systems will connect just because they are both 4" x 3". They can have different joint dimensions.

Cast Iron Downpipes With Ears or Without Ears?

This is one of the most common distinctions you will find in our Alumasc Apex range. The eared cast iron downpipes have traditional fixing lugs cast into the pipe socket. These allow the pipe to be fixed to the wall using suitable fixings.

They are a great choice for period properties, where traditional fixing details are visible on the elevation.

The uneared cast iron downpipes have a clean socket without those projecting lugs. They require a separate fixing arrangement, such as the traditional holderbats supplied by Alumasc. It's not only about the look. The fixing method changes too.

If you are replacing an existing pipe, make sure you check how it is currently fixed. Matching the original fixing method can help maintain the character of the building and avoid unnecessary changes to the masonry.

Why Rectangular Cast Iron Downpipe Offsets Need Careful Measuring

This is something worth spending a little bit more time on before ordering. Unlike round cast iron downpipes, rectangular systems have a fixed orientation. You cannot rotate an offset freely to align a slightly misaligned gutter outlet with the pipe below.

According to the Alumasc installation guide, the gutter outlet and ground-level gully require accurate alignment when using rectangular offsets. This is important when dealing with an old building, where the gutter projects far from the wall, or where the existing drainage connection is not aligned with the outlet.

Our Apex Heritage range includes swan necks and plinth offsets to cover various projections.

Swan necks are widely used near the roofline to bring the pipe from the gutter outlet closer to the wall. Plinth offsets are helpful lower down, where the pipe needs to go around projecting stonework or brickwork.

Before choosing an offset, make sure you measure the required projection and check the fitting's technical dimensions. Don't rely on the look of the product photograph only.

Left-Hand and Right-Hand Cast Iron Downpipe Fittings

Some rectangular cast iron fittings are handed, including side offsets and shoes. It's worth checking this carefully. The fitting can be the right nominal size, but still turn in the wrong direction for your installation.

Use the manufacturer's technical drawing to determine the correct orientation. Don't assume that the left-hand and right-hand descriptions use the same viewing direction across all manufacturers. In case of a complicated pipe route, a picture of the building elevation and a simple sketch of the pipe route can save you a lot of time.

If you are not sure about the orientation, please send us those details before ordering. We will help you choose the appropriate Alumasc Apex fittings.

How Many Rectangular Cast Iron Downpipes Do I Need?

Our 100 x 75mm rectangular cast iron downpipes are available in two standard lengths:

• 1.83m (6ft)
• 0.9m (3ft), approximately

Start by measuring the vertical distance from your upper connection to the proposed outlet at ground level.

For example, two 1.83m lengths will give you 3.66m of nominal pipe length. However, it won't necessarily give you 3.66m of installed coverage, taking into consideration the length occupied by spigot and socket joints.

It's better to plan the entire route before purchasing, instead of calculating the number of pipes based only on the wall height.

Cast iron is heavy, therefore it requires proper support. According to the Alumasc installation guide, the typical wall-fixing centres are 2m, with the actual fixing arrangement depending on the building and installation.

Matching Rectangular Cast Iron Downpipes With Moulded Ogee Guttering

Rectangular cast iron downpipes can be combined with traditional Moulded Ogee cast iron guttering.

The sharp profile of the gutter works well with the straight edges of the downpipe, especially on heritage properties with masonry decorations. In case the rainwater needs to be collected from a roof valley, parapet or any other concentrated outlet, a hopper head is more suitable.

We supply matching rectangular outlet cast iron hoppers with 100 x 75mm outlets, available in traditional black and other selected finishes.

Make sure you match the hopper outlet with the pipe system you are going to order. A rectangular-shaped hopper head does not necessarily have a rectangular outlet; there are also plenty of hopper heads manufactured with round outlets.

Choosing Between Black, Primed and Custom Colours

Black is a popular choice for traditional cast iron rainwater goods, especially for older brick and stone properties.

Our painted black rectangular cast iron downpipes are a practical option if you would like to have a factory-finished system.

Primed cast iron is available for projects where the final colour will be applied on site. Please note, a factory primer is not the final protective layer. The correct paint system needs to be applied.

According to the Alumasc installation guide, exposed metal needs to be protected after cutting and any coating damage repaired during installation. It is particularly important around cut pipe ends and sockets, where water can get to the bare cast iron.

Selected custom RAL colours are available too, subject to the product and finish requirements. This can be helpful if you need to match existing architectural metalwork or meet a particular conservation specification.

Completing Your Rectangular Cast Iron Rainwater System

A complete cast iron downpipe installation can require several fittings, apart from the straight lengths.

At the top, you may need a gutter outlet, hopper head or swan neck. Lower down, there may be additional offsets, branches or fixing components. At ground level, the pipe can end with a suitable shoe over a gully or connect directly into the drainage system.

A useful detail from the Alumasc installation guide: if a pipe is connected directly into the gully, the manufacturer recommends installing an access fitting within 750mm of ground level. It allows future maintenance and blockage clearance.

For cast iron rainwater pipes, regular inspections are recommended. Make sure you keep the hoppers and outlets clean, inspect the joints and repair damaged protective finishes before corrosion appears.

And if you are working on a listed building, please check the conservation requirements before changing the existing rainwater system.

Need Help Matching an Existing Cast Iron Downpipe?

If you are looking to replace an existing square or rectangular cast iron rainwater system, you are welcome to contact Cast Iron Warehouse for advice.

The most helpful information you can provide is a few clear photographs, the approximate width and depth of the pipe, and a photo of the existing sockets fixed to the wall.

If you need offsets, please include the distance between the gutter outlet and the wall, along with the position of the drainage connection below.

We can help you choose the appropriate Alumasc Apex Heritage options, from individual replacement pipes to complete rectangular cast iron rainwater systems.