Gutter Downpipe Sizes Explained: 65mm vs 68mm vs 80mm
The downpipe size you need depends on your gutter profile and the volume of water it must carry. For most UK homes with standard guttering (112 mm half-round or 114 mm square-line), a 68 mm round downpipe is the correct choice — it handles approximately 1.0 litre per second. Mini gutters use a smaller 50 mm or 65 mm downpipe. Deep-flow and hi-cap gutters with higher capacities require 80 mm round downpipes or 65 × 65 mm / 80 × 80 mm square downpipes to avoid bottlenecking the flow.
The downpipe is the part of the rainwater system people think about least but that causes the most problems when undersized. A gutter can hold a reservoir of water, but the downpipe is the bottleneck — if it cannot drain the gutter fast enough, water backs up and overflows. Getting the right downpipe size is as important as choosing the right gutter profile.
UK Downpipe Sizes at a Glance
| Downpipe Size | Shape | Flow Capacity (approx.) | Paired With |
|---|---|---|---|
| 50 mm | Round | 0.4 l/s | Mini gutter (76 mm) |
| 65 mm | Round | 0.7 l/s | Mini gutter, small half-round |
| 68 mm | Round | 1.0 l/s | Standard half-round, ogee, square-line |
| 80 mm | Round | 1.5 l/s | Deep-flow, hi-cap |
| 65 × 65 mm | Square | 1.0 l/s | Square-line systems |
| 80 × 80 mm | Square | 1.8 l/s | Industrial, commercial |
These are the standard sizes available from UK PVC-U manufacturers including Kalsi Plastics. The flow capacity figures assume the downpipe is running freely without blockages, and the bottom outlet (shoe or gully connection) does not restrict the flow.
How Downpipe Size Relates to Gutter Size
The golden rule: the downpipe must be able to drain the gutter as fast as it fills. If the gutter flow capacity exceeds the downpipe’s throughput, the gutter will overflow regardless of how large it is.
| Gutter Profile | Gutter Capacity | Minimum Downpipe | Recommended Downpipe |
|---|---|---|---|
| Mini (76 mm) | 0.5 l/s | 50 mm round | 50–65 mm round |
| Half round (112 mm) | 0.9 l/s | 65 mm round | 68 mm round |
| Square line (114 mm) | 1.1 l/s | 68 mm round | 68 mm round or 65 × 65 mm square |
| Ogee (116 mm) | 1.1 l/s | 68 mm round | 68 mm round |
| Deep flow | 1.7–2.5 l/s | 68 mm round (2 outlets) | 80 mm round |
| Hi-cap | 2.0–2.5 l/s | 80 mm round | 80 mm round |
| Industrial | 2.5+ l/s | 80 mm round | 80 × 80 mm square |
Critical Point
A single 68 mm round downpipe tops out at about 1.0 l/s. If your gutter delivers more than this — as most deep-flow and hi-cap systems do — you have two options:
- Upgrade to 80 mm downpipe — single outlet handles up to 1.5 l/s
- Use two 68 mm downpipes — split the gutter run with two outlets, effectively doubling throughput
Option 1 is usually simpler and neater. Option 2 may be necessary on very long gutter runs where a single outlet at one end would require an extreme fall.
Round vs Square Downpipes
Both shapes are available for most gutter systems. The choice is partly functional, partly aesthetic.
Round Downpipes
- Most common in the UK for domestic installations
- Available in 50 mm, 65 mm, 68 mm, and 80 mm
- Efficient shape — circular cross-section gives maximum flow for minimum material
- Smooth interior — fewer points for debris to snag
- Universal appearance — suits all property types
Square Downpipes
- Sit flush against the wall — neater appearance from the side
- Available in 65 × 65 mm and 80 × 80 mm
- Slightly higher flow capacity per equivalent size (more cross-sectional area)
- Match well with square-line gutter profiles
- Can be less conspicuous on flat-fronted properties
| Comparison | 68 mm Round | 65 × 65 mm Square |
|---|---|---|
| Cross-sectional area | 3,632 mm² | 4,225 mm² |
| Flow capacity | ~1.0 l/s | ~1.0 l/s (similar due to corner friction) |
| Wall projection | 68 mm | 65 mm |
| Visual impact | Standard | Low-profile, flush |
| Fittings availability | Widest range | Good range |
Swan Necks and Offsets
The downpipe rarely runs straight from gutter to ground. The gutter sits on the fascia board, which is 40–60 mm forward of the wall face. A swan neck (two offset bends and a connecting piece) bridges this gap.
Standard Offset Dimensions
| Component | Typical Measurement |
|---|---|
| Fascia projection from wall | 40–60 mm |
| Offset bend angle | 112.5° (67.5° from vertical) |
| Connecting pipe between bends | 150–300 mm |
| Total vertical loss from swan neck | 200–350 mm |
Key point: The swan neck introduces resistance. At every bend, water slows down. Two tight bends in succession (as in a swan neck) reduce the effective throughput of the downpipe by roughly 10–15%. This is usually not a problem for standard domestic systems, but for high-capacity installations, keep offsets to a minimum and use gentle-angle bends where possible.
How Many Downpipes Do You Need?
The number of downpipes depends on:
- Total effective roof area draining to each gutter run
- Gutter run length (long runs need more outlets)
- Available drain positions (downpipes must connect to something at the bottom)
Rule of Thumb
| Effective Roof Area | Downpipes Required (68 mm) | Downpipes Required (80 mm) |
|---|---|---|
| Up to 45 m² | 1 | 1 |
| 45–90 m² | 2 | 1 |
| 90–135 m² | 3 | 2 |
| 135+ m² | Calculate individually | Calculate individually |
For runs longer than 10–12 m, two downpipes are recommended regardless of roof area. This halves the effective run length, reduces the fall needed, and prevents the lower end of the gutter from being overwhelmed.
Connecting Downpipes to Drainage
The bottom of the downpipe must discharge safely. Options include:
Direct Connection to Underground Drain
- Downpipe connects via a back-inlet gully or rainwater adaptor to the underground drainage system
- The preferred method for Building Regulations compliance
- Required on new builds and most extensions
Shoe Over Open Gully
- A pipe shoe (angled bottom piece) directs water into an open gully grate
- Common on existing properties
- Simple but can splash in heavy rain
Splash Block
- For sheds, garages, and outbuildings where no formal drainage exists
- A concrete or stone pad disperses the water
- Not suitable for main house downpipes near foundations
Water Butt Diverter
- Redirects water into a storage barrel for garden use
- Excess water bypasses to the drain when the butt is full
- Can be fitted to any downpipe size
Downpipe Fixing and Support
Clip Spacing
| Downpipe Size | Maximum Clip Spacing |
|---|---|
| 50 mm | 1,500 mm |
| 65 mm | 1,800 mm |
| 68 mm | 1,800 mm |
| 80 mm | 1,800 mm |
| Square (any) | 1,800 mm |
Clips must be fixed to the wall with appropriate fixings — wall plugs and screws for masonry, heavy-duty fixings for render-over-insulation. The bottom clip should be within 300 mm of the shoe or drain connection.
Pipe Joints
Downpipe lengths connect with a socket joint. Most PVC systems use push-fit connections with no adhesive — the upper pipe slides into the socket of the lower pipe. Allow a small gap (3–5 mm) for thermal expansion.
Common Mistakes with Downpipes
1. Undersized Downpipe for the Gutter
Fitting a 68 mm downpipe to a hi-cap gutter system bottlenecks the flow. Always match the downpipe to the gutter’s maximum capacity.
2. Too Few Downpipes
A single downpipe at one end of a 15 m gutter run is asking for trouble. The water at the far end has to travel the full length before reaching the outlet, causing overflow at the far end during heavy rain.
3. Blocked Shoes
The pipe shoe at the bottom collects debris — leaves, moss, litter. Check and clear it during routine maintenance.
4. Downpipe Not Connected to Drain
A downpipe discharging onto bare ground next to the foundation wall is worse than no gutter at all — it concentrates all the roof water at one point next to the building. Always connect to a drain, gully, or soakaway.
5. Unsecured Downpipe
Without clips, the downpipe swings in wind, stressing joints and pulling away from the gutter. Fix clips at the spacing specified above and check them annually.
Frequently Asked Questions
What size downpipe do I need?
For standard domestic guttering (half round, ogee, or square-line), a 68 mm round downpipe is correct. For deep-flow or hi-cap guttering, upgrade to 80 mm. For mini gutter systems, use 50 mm or 65 mm. The downpipe must be able to drain the gutter as fast as the roof fills it.
Can I use a smaller downpipe to save money?
This is false economy. An undersized downpipe creates a bottleneck — the gutter overflows even though it is correctly sized. The cost difference between a 68 mm and 80 mm downpipe is minimal (£1–2 per metre), and the consequences of undersizing can include water damage to fascias, walls, and foundations.
What is the difference between 65mm and 68mm downpipe?
The 65 mm downpipe is an older size standard, still used in some mini gutter systems and legacy installations. The 68 mm pipe is the modern standard for full-size domestic guttering. They are not interchangeable — fittings, clips, and connectors are specific to each diameter. When replacing, check which size your existing system uses before ordering.
Do I need round or square downpipes?
Both work equally well. Round is the UK standard and has the widest range of fittings. Square downpipes sit flatter against the wall and look neater from the side. Choose based on aesthetics and your gutter system — square-line gutters pair naturally with square downpipes.
How far apart should downpipe clips be?
Maximum 1,800 mm (roughly 6 feet) for standard and large downpipes. One clip near the top (just below the swan neck), one near the bottom (within 300 mm of the shoe), and intermediate clips at regular intervals. Do not rely on the push-fit joints alone to support the pipe — they are for connection, not structural support.
