Hi-Cap Guttering: Maximum Flow for High-Rainfall Areas
The guttering that handles heavy rain best is a high-capacity (hi-cap) system, typically rated at 2.0–2.5 litres per second — more than double the capacity of standard half-round guttering. Hi-cap profiles are designed for large domestic roofs, properties in western and highland regions of the UK where rainfall intensity regularly exceeds 75 mm/h, and any situation where standard gutter profiles overflow despite being correctly installed and maintained. If you live in an area where heavy rain is the norm, or your roof drains a large effective area, hi-cap is the system that gives you genuine peace of mind.
The UK is not a uniformly wet country. Annual rainfall in parts of eastern England is around 600 mm, while western Scotland regularly sees 2,000 mm or more. A gutter system that performs perfectly in Cambridge may be hopelessly undersized in Keswick, Fort William, or Snowdonia. Hi-cap guttering exists to serve the wetter half of Britain — and the growing number of large, complex rooflines on modern housing.
What Makes Hi-Cap Different?
Hi-cap guttering combines three design elements to maximise flow:
1. Deeper Profile
The gutter is deeper than standard profiles, holding a larger volume of water at any instant. This acts as a buffer during peak rainfall — the gutter fills more before it reaches the point of overflow.
2. Wider Opening
A wider top opening catches more water from the roof edge, reducing overshoot during intense rain.
3. Larger Downpipe
Hi-cap systems pair with 80 mm downpipes rather than the standard 68 mm, preventing the downpipe from being the bottleneck.
| Feature | Standard Half Round (112 mm) | Hi-Cap (150 mm) |
|---|---|---|
| Gutter width | 112 mm | 150 mm |
| Gutter depth | ~50 mm | ~75 mm |
| Flow capacity | 0.9 l/s | 2.0–2.5 l/s |
| Max effective roof area (75 mm/h) | 43 m² | 96–120 m² |
| Standard downpipe | 68 mm round | 80 mm round |
| Bracket spacing | 800–1,000 mm | 600–800 mm |
Where Hi-Cap Guttering Is Essential
High-Rainfall Regions
The western and northern parts of the UK receive significantly more rainfall than the east:
| Region | Typical Annual Rainfall | Design Intensity |
|---|---|---|
| Eastern England | 550–700 mm | 75 mm/h |
| Midlands | 700–900 mm | 75 mm/h |
| South West | 900–1,200 mm | 75–100 mm/h |
| Wales | 1,000–2,000 mm | 100–150 mm/h |
| Lake District | 1,500–2,500 mm | 100–150 mm/h |
| Western Scotland | 1,500–3,000 mm | 100–150 mm/h |
| Northern Ireland | 900–1,400 mm | 75–100 mm/h |
Properties in the “100 mm/h and above” column need gutter systems sized to handle that intensity. A standard half-round that handles 43 m² at 75 mm/h only covers 32 m² at 100 mm/h. That puts even a modest semi-detached house at risk of overflow.
Large Detached Houses
A four-bedroom detached with a 12 m front elevation, 5 m span, and 45° pitch has an effective roof area of approximately 90 m² per elevation. Standard guttering cannot handle this in a single run.
Properties with Extensions
A rear extension that drains into the existing gutter increases the load. If the original system was borderline, the extension pushes it over the edge — literally.
Steep Roof Pitches
Every degree of additional pitch increases the effective roof area. A 60° pitch nearly doubles the effective area compared to the horizontal footprint.
Multiple Roof Sections Draining to One Gutter
Dormer windows, valley boards, and complex roof geometries can channel water from multiple surfaces into a single gutter run.
Hi-Cap vs Deep Flow vs Industrial
All three offer higher capacity than standard profiles. How do they compare?
| System | Typical Capacity | Best Application |
|---|---|---|
| Deep flow | 1.7–2.5 l/s | Large domestic, extensions, moderate-rainfall areas |
| Hi-cap | 2.0–2.5 l/s | Large domestic, high-rainfall areas, maximum domestic capacity |
| Industrial | 2.5–4.0+ l/s | Commercial, agricultural, very large roofs |
The practical difference between deep flow and hi-cap depends on the specific profiles from each manufacturer. In many ranges, hi-cap is the largest domestic-scale system available before stepping up to industrial profiles, which are functionally excellent but visually bulky on residential properties.
Kalsi Plastics’ hi-cap range is designed to deliver maximum flow capacity while maintaining a proportionate appearance on domestic properties.
Installation Notes
Hi-cap installs using the same method as standard PVC guttering, with these key differences:
Tighter Bracket Spacing
The heavier, deeper profile holds more water weight. Space brackets at 600–800 mm centres, not the 800–1,000 mm used for standard profiles. Use an extra bracket within 150 mm of every fitting.
80 mm Downpipes
Pair with 80 mm round downpipes to avoid bottlenecking. A single 68 mm downpipe (1.0 l/s) would be undersized for a hi-cap gutter running at 2.0+ l/s. Either use 80 mm throughout or add a second 68 mm outlet.
Fascia Board Depth
Check that your fascia board can accommodate the taller profile. A minimum 200 mm fascia is recommended. If the existing fascia is 150 mm, consider upgrading to a deeper board — Kalsi’s fascia range includes 200 mm and 225 mm options.
Expansion Allowance
Larger gutters mean slightly more thermal expansion per length. Follow the manufacturer’s expansion gap guidance strictly — typically 8–12 mm at each union joint.
Support the System
Ensure the fascia board and the rafter ends behind it are strong enough to carry the loaded weight. A 10 m run of hi-cap gutter holding 20 mm of water weighs approximately 30–35 kg. The fascia, fixings, and rafter structure must handle this plus wind loading.
Colour Options
Hi-cap systems are available in the standard PVC-U colour range:
- White
- Black
- Brown
- Grey
- Anthracite grey
Dark colours (black, anthracite grey) are particularly popular on hi-cap installations because the larger profile is slightly more visible from the ground. Dark colours reduce the visual weight and blend with the roofline more effectively than white.
Cost
Hi-cap guttering commands a premium over standard profiles, but the uplift is modest in the context of a full installation:
| Component | Standard Half Round | Hi-Cap | Difference |
|---|---|---|---|
| Gutter per metre | £2.50–4.50 | £5.00–9.00 | +£2.50–4.50/m |
| Running outlet | £3.00–5.00 | £6.00–10.00 | +£3.00–5.00 |
| Union joint | £2.50–4.00 | £5.00–8.00 | +£2.50–4.00 |
| 80 mm downpipe per metre | n/a | £3.00–5.00 | (vs £2.50–4.00 for 68 mm) |
For a typical full-house installation (24 m of gutter, 4 downpipes), the materials uplift for hi-cap over standard is approximately £100–160. Labour costs are identical. The peace of mind — no more overflowing gutters — is worth considerably more than the extra spend.
Case Study: Lake District Renovation
A stone-built detached cottage near Windermere with:
- Front elevation: 10 m
- Roof span: 6 m, 40° pitch
- Effective roof area: (6 + 5.1/2) × 10 = 85.5 m²
- Local rainfall intensity: 100 mm/h
- Required flow rate: 1.78 l/s
Standard 112 mm half-round (0.9 l/s) was hopelessly inadequate — the original system overflowed several times per week during the Lake District’s frequent heavy rain.
Solution: Kalsi hi-cap system with 80 mm downpipes at each end (2.2 l/s capacity). Result: Zero overflow events in two years since installation, including during Storm Éowyn which brought sustained 100+ mm/h rainfall.
Frequently Asked Questions
What guttering handles heavy rain?
High-capacity (hi-cap) guttering with a flow rate of 2.0–2.5 l/s handles heavy rain best for domestic properties. For commercial buildings, industrial guttering with 2.5+ l/s capacity is needed. Standard domestic guttering (0.9–1.1 l/s) is not designed for high-rainfall areas or very large roof areas.
Is hi-cap guttering worth the extra cost?
If your property has overflowing gutters despite them being clean and correctly installed, hi-cap is absolutely worth it. The materials uplift is £100–160 for a typical house — negligible compared to the cost of water damage to fascias, walls, and foundations caused by persistent overflow.
Can I fit hi-cap guttering myself?
Yes, if you are comfortable working at height and have standard DIY skills. The installation process is the same as standard gutter profiles — brackets, gutter lengths, fittings, downpipes. The main differences are tighter bracket spacing and the use of 80 mm downpipes. Always work from a secure ladder or scaffold tower.
What downpipe do I need with hi-cap guttering?
80 mm round downpipes are the standard pairing for hi-cap systems. A single 80 mm downpipe handles approximately 1.5 l/s. For gutter runs delivering more than this, use two downpipes. A 68 mm downpipe is too small for hi-cap unless you use multiple outlets.
How does hi-cap compare to having two downpipes on standard gutter?
Two downpipes on standard gutter effectively double the drainage capacity of the run (each outlet handles its share). However, this requires two downpipe locations, two drainage connections, and creates a more complex layout. A single run of hi-cap with one large downpipe is cleaner, simpler, and often achieves the same or better result with fewer components.
