How to measure a downpipe: Will the marten guard fit?

Choosing a suitable marten guard starts with a clear measurement. For a downpipe, the outside diameter at the intended fitting position matters. Looking at the pipe or relying on an old building description may not give you that measurement. This guide helps you choose an accessible section, convert circumference into diameter and record the details needed for your product choice.

Choose a straight section of pipe

Measure at a straight section that you can reach safely. A socket, bend or pipe clip can be wider than the pipe itself. A measurement there therefore does not describe the section on which you intend to position the collar. Do not remove clips or other components to take the measurement.

First identify the intended position and take a photograph from a little distance. Include the wall, clips and adjacent components. Mark the measuring point on a simple sketch. This helps prevent a measurement from a different section being used later when ordering.

Also check the cross-sectional shape. Calculating diameter from circumference works for a circular pipe. An oval, rectangular or distorted pipe cannot be described as a circular diameter using this method. If the shape is unclear, a question accompanied by a photograph and measurements is more useful than a rounded estimate.

Measure outside circumference and convert it

A flexible tape measure is useful when you cannot measure straight across the pipe. Run it once around the outside, flush against the surface and at right angles to the pipe axis. It should neither sit diagonally nor pass over a clip. Read the circumference and record the unit. Repeat at the same point to detect a slanted measurement.

For a circular pipe: outside diameter = outside circumference ÷ π. You can use approximately 3.1416 for π in a simple calculation. For example, a circumference of 314 mm gives a diameter of roughly 100 mm. A circumference of 31.4 cm is the same measurement; convert it to 314 mm before comparison if helpful.

Calculation examples for round pipes, not confirmation of fit

Measured circumference	Calculated diameter, approximately
283 mm	90 mm
314 mm	100 mm
346 mm	110 mm

These examples illustrate the conversion. They do not mean that the same guard suits all three pipes. Record the original circumference as well, and do not round a different result towards the product size you want. You should not automatically treat a DN 100 designation as a measured outside diameter of 100 mm either.

Record the space around the pipe

The pipe measurement is one part of the decision. Also note the distance to the wall, the location of the nearest clips and any offsets. Include shutters, cables or other components immediately nearby in your sketch. An elaborate drawing is unnecessary: a photograph with three or four labelled distances is already useful.

A guard on one pipe also concerns that access route only. An adjacent tree, another pipe or a connection to the building is not assessed by that choice. The guide to martens around the house explains how to consider possible access routes. Clarify the situation before planning work around an occupied roof space.

Compare your measurements with the specified version

The Martenbrown downpipe marten guard is intended for a diameter of 100 mm. The documented version is made from hot-dip galvanised steel, comes preassembled and fastens to the pipe with a wing screw. That specification does not approve different measurements. Clarify any deviation or limited fitting space using the contact page before choosing.

1. Select a straight section that you can reach safely.
2. Check its circular shape and measure circumference twice.
3. Record circumference, unit and calculated outside diameter.
4. Note wall clearance and neighbouring components.
5. Compare with the specific product information and resolve open questions.

Frequently asked questions

Where should I measure if the pipe socket is wide?

At the straight section where you plan to fit the collar. The wider socket is a different measuring point and can give a different result.

Does this version also fit approximately 110 mm?

The Martenbrown version is specified for 100 mm. A circumference calculation does not establish suitability for other diameters.

What should I send with an enquiry?

The circumference or outside diameter with its unit, a photograph of the measuring point and the distances to the wall and clips. That makes the situation identifiable.

Check the marten guard for 100 mm downpipes and keep your measurement notes ready for the decision.