Choosing the right size guttering is one of the most important decisions when installing or replacing a rainwater system. Too small and the gutters overflow in heavy rain, causing water damage to fascias, brickwork, and foundations. Too large and you spend more than you need to. This guide explains how to work out exactly what size guttering you need for your property.
How Guttering Size Works
When we talk about guttering "size" we mean two things: the width of the gutter channel (measured in millimetres across the top) and the flow capacity (how much water the gutter can carry per second). A wider, deeper gutter carries more water. The size you need depends on the area of roof draining into each gutter run and the rainfall intensity in your part of the UK.
The downpipe size also matters. There is no point fitting a high capacity gutter if the downpipe cannot drain the water fast enough. The gutter and downpipe must be matched as a system.
Step 1: Calculate Your Roof Area
You need to know the effective roof area that drains into each gutter run. This is not the same as the floor area of your house.
For a simple pitched roof: measure the horizontal distance from the ridge to the gutter (not the slope length), then multiply by the length of the gutter run. For example, if the distance from ridge to gutter is 4 metres and the gutter run is 8 metres long, the effective roof area is 4 x 8 = 32m2.
For a hip roof: the calculation is the same but you may have multiple gutter runs draining different sections. Calculate each section separately.
For a flat roof or low pitch: the effective area is roughly the same as the plan area of the roof.
Important: if two roof faces drain into the same gutter (for example in a valley), add both areas together. This is the total area that single gutter run must handle.
Step 2: Check Your Rainfall Intensity
The UK is not uniform when it comes to rainfall. Western Scotland, Wales, the Lake District, and parts of the South West receive significantly more rain than the South East and East Anglia. Building regulations use a design rainfall intensity of 75mm per hour for most of England and Wales, but this can be higher in western and upland areas.
If your property is in a high rainfall area, you should consider stepping up to the next gutter size even if the standard calculation says a smaller profile is sufficient. This gives you a safety margin for extreme weather events which are becoming more frequent.
Step 3: Match Roof Area to Gutter Size
Here is a general guide to which gutter profile suits which roof area. These figures assume one downpipe per gutter run at the standard UK design rainfall intensity.
Up to 40m2 roof area
76mm Miniflo — suitable for sheds, garages, porches, and small outbuildings only. Not for houses.
Up to 65m2 roof area
112mm Half Round PVC — the standard choice for most UK houses. Available from FloPlast, Osma RoundLine, and Marshall Tufflex. Also available in cast iron, cast aluminium, steel, and copper.
Up to 85m2 roof area
114mm Square Line PVC — slightly higher capacity than half round. Modern angular appearance. Available from FloPlast, Osma SquareLine, and Marshall Tufflex.
Up to 130m2 roof area
115mm Deepflow PVC — designed for larger roofs and high rainfall areas. Available from FloPlast Hi-Cap and Marshall Tufflex Universal XL. Also Osma DeepLine 113mm and Marley Alutec Evolve Deepflow in aluminium.
Up to 170m2 roof area
110mm Ogee PVC — decorative S shaped profile with good flow capacity. Also available in cast iron, cast aluminium, and copper.
Over 170m2 roof area
170mm Xtraflo or Osma SuperLine 125mm — commercial grade systems for large buildings. Use Osma RoofLine 150mm for the largest industrial applications.
Step 4: Choose the Right Downpipe Size
The downpipe must be able to drain the water collected by the gutter. Using an oversized gutter with an undersized downpipe will still cause overflows. Standard downpipe sizes are:
68mm round / 65mm square — used with standard half round, square line, deepflow, and ogee guttering up to approximately 130m2 roof area. The most common domestic downpipe size.
110mm round — used with commercial systems like Osma SuperLine and Osma RoofLine. Connects directly to 110mm underground drainage.
63mm round — used with Marley Alutec Evolve Half Round aluminium system.
76mm round — used with Marley Alutec Evolve Deepflow aluminium system.
Step 5: Consider Adding a Second Downpipe
If your roof area is borderline for a particular gutter size, adding a second downpipe to the same run is often more practical and cost effective than upgrading to a larger gutter profile. A second downpipe effectively doubles the drainage capacity of the run. Place downpipes at each end of the gutter with a high point in the centre, creating a fall towards both outlets.
This is particularly useful on terraced houses and semi-detached properties where a single long gutter run drains a large roof area.
Gutter Sizing by Property Type
Sheds, garages, porches — 76mm Miniflo or 112mm half round depending on roof size.
Standard semi-detached or terraced house — 112mm half round with one downpipe per gutter run.
Larger detached house — 112mm half round with two downpipes, or 115mm deepflow with one downpipe.
Bungalow with wide roof — 115mm deepflow due to the larger roof footprint.
Period property (listed) — cast iron or cast aluminium in the profile matching the original. Size determined by conservation officer.
New build — specified by the architect based on calculated roof area. Typically half round or square line for domestic, SuperLine for commercial.
Extension — match the existing system if extending the same gutter run. If the extension increases the roof area significantly, consider upgrading to deepflow. Use a gutter adaptor if connecting a different profile to the existing system.
Common Sizing Mistakes
Ignoring roof pitch — a steep roof delivers water to the gutter faster than a shallow roof of the same plan area. If your roof is steeply pitched, consider stepping up one gutter size for safety.
Forgetting about valleys — if two roof faces drain into the same gutter via a valley, the effective area is both faces combined. This often doubles the area a single gutter run needs to handle.
Only one downpipe on a long run — a single downpipe at one end of a long gutter run means all the water travels the full length of the gutter. Adding a second downpipe halves the distance and halves the load.
Not considering future extensions — if you plan to add a conservatory, extension, or porch that will drain into the existing gutter, factor that additional roof area into your sizing calculation now.
Choosing by appearance alone — ogee and deepflow profiles look different but have very different capacities. Always check the flow rate for the profile you want, not just the appearance.
Materials Available in Each Size
Most gutter sizes are available in multiple materials:
PVC — available in all sizes from 76mm mini to 170mm Xtraflo. The most affordable and easiest to install.
Cast Iron — available in half round, ogee, deep half round, beaded, and box profiles. For listed buildings and period properties.
Aluminium — available in half round, ogee, beaded, and deepflow. Both cast (Alumasc Heritage) and extruded (Marley Alutec Evolve).
Steel — available in half round and rectangular from Lindab. Mid-range metal option.
Copper — available in half round, ogee, and box. Premium choice for prestige properties.
Guttering Size FAQ
What is the most common gutter size in the UK? 112mm half round PVC is fitted on the majority of UK homes. It suits most standard domestic properties with typical roof areas.
Is bigger guttering always better? Not necessarily. Oversized guttering costs more, looks disproportionate on smaller properties, and uses larger brackets. Choose the correct size for your roof area rather than automatically going bigger.
Can I mix gutter sizes on the same property? Yes. You can use different sizes on different elevations if the roof areas vary. For example, deepflow on the back where the roof is larger and half round on the front where it is smaller. Use a gutter adaptor if the two sizes meet at a corner.
How do I know what size I currently have? Check the markings on the inside of the gutter or on the brackets for the brand name and size. Alternatively, measure the width across the top of the gutter. 112mm is standard half round, 114mm is Marshall Tufflex half round or square line, 115mm is deepflow. If unsure, call us on 03330 151860 with a photo and we will identify it.
Do I need building regulations approval to change gutter size? No. Guttering is not normally subject to building regulations approval unless the property is listed, in which case you need conservation officer approval for any external changes.
What size guttering do I need for a conservatory? Most conservatories use 76mm Miniflo or standard 112mm half round depending on the roof area. Check the conservatory manufacturer's specification.
Need Help Choosing?
If you are unsure which size guttering is right for your property, call us on 03330 151860. Our team can help you calculate the correct size based on your roof dimensions, property type, and location. We stock every size from 76mm mini through to 150mm industrial in PVC, cast iron, aluminium, steel, and copper, all available for fast UK delivery from gutters.co.uk.