Deep Flow Guttering: When Standard Gutters Aren’t Enough
You need deep flow guttering when your effective roof area exceeds approximately 55 m² or when your existing standard gutters overflow during heavy rain despite being clear and correctly aligned. Deep-flow profiles carry 1.7–2.5 litres per second — roughly double the capacity of standard 112 mm half-round guttering. They are the go-to upgrade for large detached homes, properties with steep roof pitches, rear extensions that have increased the catchment area, and sites in high-rainfall regions of the UK such as western Scotland, the Lake District, and parts of Wales.
Standard domestic guttering — the 112 mm half round and 114 mm square line profiles that sit on most UK houses — handles the vast majority of roofs. But there are plenty of situations where standard is simply not enough. Rather than doubling up on downpipes or running multiple gutter lines, stepping up to a deep-flow profile is the cleaner, simpler solution.
What Makes Deep Flow Different?
Deep-flow guttering is not just a bigger version of standard gutter. The profile is specifically designed to maximise water-carrying capacity while maintaining a reasonable visual scale.
| Characteristic | Standard (112 mm Half Round) | Deep Flow |
|---|---|---|
| Nominal width | 112 mm | 115–170 mm |
| Depth | ~50 mm | 75–90 mm |
| Flow capacity | 0.9 l/s | 1.7–2.5 l/s |
| Max effective roof area (at 75 mm/h) | ~43 m² | 82–120 m² |
| Downpipe size | 68 mm round | 68 mm or 80 mm round |
| Bracket spacing | 800–1,000 mm | 600–900 mm |
The increased depth is the key. A deeper channel holds more water at any given moment, giving it time to reach the outlet without overtopping. The wider opening also catches more water from the roof edge, reducing the chance of overshoot in heavy downpours.
When You Need Deep Flow Guttering
1. Large Roof Areas
Any roof elevation with an effective area over 55 m² is pushing standard gutter profiles to their limit. Calculate using the standard formula:
Effective roof area = (W + H/2) × L
Common scenarios where the number exceeds 55 m²:
- Large detached houses with long front elevations (10 m+)
- Properties with steep roof pitches (45°+) where the height factor inflates the effective area
- Barns and barn conversions with wide spans
- Bungalows with extended rooflines
2. Extensions
A rear or side extension often drains into the existing gutter run. If the original gutter was sized for the original roof, adding 15–30 m² of extension roof can push the total effective area beyond standard capacity.
Example:
- Original roof section: 45 m² → standard half-round (0.9 l/s) was fine
- Extension adds: 20 m² → total is now 65 m²
- Required: 1.35 l/s → standard is undersized, deep flow is needed
3. High-Rainfall Areas
The standard UK design rainfall intensity is 75 mm/h. In western Scotland, the Lake District, Snowdonia, and parts of Devon and Cornwall, the effective intensity can be 100–150 mm/h. At 100 mm/h, a 50 m² roof needs 1.04 l/s — already beyond standard half-round capacity.
4. Steep Roof Pitches
A 60° roof pitch doubles the effective area compared to the horizontal footprint. Many Victorian and Edwardian properties have steep pitches (40–55°) that significantly increase the catchment area.
5. Commercial and Institutional Buildings
Schools, churches, village halls, small commercial units, and agricultural buildings often have roof areas that standard domestic profiles cannot handle. Deep flow bridges the gap between domestic and full industrial systems.
Deep Flow vs Other High-Capacity Options
If your roof area exceeds standard capacity, you have several options:
| Option | Capacity | Best For | Limitations |
|---|---|---|---|
| Add a second downpipe | Splits the flow (effective doubling) | Existing system is the right size but over-long run | Extra downpipe on the elevation, may not suit aesthetics |
| Deep flow gutter | 1.7–2.5 l/s | Large domestic roofs, extensions | Slightly larger visual profile |
| Hi-cap gutter | 2.0–2.5 l/s | Maximum domestic capacity | Larger profile, more expensive |
| Industrial gutter | 2.5+ l/s | Commercial, industrial, agricultural | Too large for most domestic properties |
For most homes, deep flow is the sweet spot — enough capacity to handle any standard domestic scenario with headroom to spare, without the visual bulk of industrial systems.
Installation Considerations
Deep flow guttering installs in the same way as standard profiles, with a few key differences:
Bracket Spacing
Because the gutter holds more water (and therefore weighs more when full), bracket spacing should be tighter:
- Recommended: 600–800 mm centres
- Maximum: 900 mm
- Use extra brackets either side of any fitting (angle, union, outlet)
Downpipe Sizing
Deep-flow gutters deliver water to the downpipe faster. Ensure your downpipe can handle the flow:
- A standard 68 mm round downpipe handles approximately 1.0 l/s
- If your deep flow gutter is delivering 1.7+ l/s, you may need an 80 mm downpipe or two 68 mm outlets per run
- Check the manufacturer’s recommendations for the specific profile
Fascia Board Depth
Deep flow gutters are taller than standard profiles. Ensure your fascia board is deep enough that the gutter does not project above the tile line or below the soffit. Typical fascia depths:
| Fascia Board Depth | Suitable Gutter Profiles |
|---|---|
| 150 mm (6”) | Mini gutter, standard half-round |
| 175 mm (7”) | Standard half-round, square-line, ogee |
| 200 mm (8”) | Deep flow, most profiles |
| 225 mm (9”) | Deep flow, hi-cap |
If your fascia is only 150 mm deep, you may need to upgrade it when fitting deep flow. Kalsi’s fascia board range includes 150 mm, 175 mm, 200 mm, and 225 mm options.
Thermal Expansion
Deep-flow profiles, being larger, can exhibit slightly more thermal movement than standard gutters. Always observe the manufacturer’s expansion gap requirements at every union joint and running outlet.
Cost Comparison
Deep flow is more expensive than standard profiles, but the premium is moderate:
| Component | Standard Half Round | Deep Flow |
|---|---|---|
| Gutter per metre | £2.50–4.50 | £4.00–7.00 |
| Running outlet | £3.00–5.00 | £5.00–8.00 |
| Union joint | £2.50–4.00 | £4.00–7.00 |
| Fascia bracket | £1.00–2.00 | £1.50–3.00 |
For a full house installation (approx. 24 m of gutter), the materials uplift for deep flow over standard is typically £40–80. Given that labour and scaffolding are the major costs in any gutter replacement (£1,500–3,000+), the upgrade to deep flow is excellent value for the additional capacity and peace of mind.
Colour and Style Options
Deep-flow guttering from Kalsi Plastics is available in the standard colour range:
- White
- Black
- Brown
- Grey
- Anthracite grey
The profile is available in both round-bottomed and flat-bottomed variants (depending on the system), giving you design flexibility. The deeper profile is visible from the ground but is not unattractive — many homeowners prefer the more substantial look compared to a slim half-round.
Browse the full deep-flow range to see available profiles, fittings, and colours.
Case Study: Detached House in West Yorkshire
A four-bedroom detached property with:
- Front elevation: 12 m long, 40° pitch, 4.5 m span
- Effective roof area: (4.5 + 3.8/2) × 12 = 76.8 m²
- Required flow rate at 75 mm/h: ~1.6 l/s
The original 112 mm half-round guttering (0.9 l/s capacity) overflowed every time it rained heavily. Adding a second downpipe in the middle of the run was impractical because the elevation had no suitable drain position.
Solution: Replace with deep-flow guttering (2.0 l/s capacity) with a single 80 mm downpipe at the existing outlet position.
Result: Zero overflow since installation. Total materials cost: £195. Labour: one day.
Frequently Asked Questions
When do I need deep flow guttering?
When your effective roof area exceeds 55 m², when you are in a high-rainfall area (western UK), when an extension has increased the roof catchment, or when your existing standard gutters overflow despite being clean and correctly aligned. Use the effective-area formula to calculate — if the number exceeds the capacity of standard profiles, deep flow is the answer.
Is deep flow guttering more expensive?
Slightly. Materials typically cost 40–60% more per metre than standard half-round. However, the total uplift on a full house installation is usually £40–80 in materials — modest compared to the overall project cost including labour and scaffolding.
Can I replace standard gutter with deep flow using existing brackets?
Generally no. Deep flow profiles are a different size and shape, requiring their own specific brackets. You will need to remove the existing brackets and fit new ones at the correct (tighter) spacing for deep flow.
What downpipe size do I need with deep flow guttering?
A standard 68 mm round downpipe handles about 1.0 l/s. If your deep flow gutter is delivering more than this, upgrade to an 80 mm downpipe or add a second outlet. Check the specific system’s data sheet for the recommended downpipe pairing.
Does deep flow gutter look too big on a house?
Deep flow is taller than standard profiles but not disproportionately so. On most two-storey properties, the difference is barely noticeable from the ground. On larger detached homes, the slightly more substantial profile actually complements the elevation better than a slim half-round that can look undersized.
