Guttering Guide: Sizes, Materials, Falls & Common Problems

Summary

Guttering is fundamental to protecting a building's fabric. Poorly maintained or inadequately sized gutters overflow during heavy rain, saturating the walls below and causing damp penetration, rendered wall damage, and decorative failure. In the UK, where heavy sustained rainfall is common, getting the gutter design right is essential.

The choice of material — UPVC, aluminium, cast iron, or copper — significantly affects the appearance, longevity, durability, and maintenance requirements. UPVC is the most common material for domestic guttering because it's affordable, doesn't rust or corrode, and is available everywhere. However, cast iron remains the preferred material for period properties, conservation areas, and listed buildings where UPVC is not acceptable.

Gutter sizing is determined by the roof catchment area and anticipated rainfall intensity. British Standard BS EN 12056-3 provides the calculation method for gravity drainage from roofs. In practice, most domestic installations use the standard sizes (112mm or 125mm half-round) without a formal calculation, which is acceptable for typical house sizes.

Key Facts

• Standard domestic size — 112mm half-round (most common); 114mm OG (ogee/colonial); 125mm half-round for larger roofs
• Minimum fall — 1:350 (approximately 3mm per metre); recommended 1:100 (10mm per metre) for reliable drainage
• Maximum run without outlet — 12m from the highest point to the first outlet on one side
• Rainfall intensity (UK) — Design rainfall intensity: 75mm/hour for most of the UK; 50mm/hour acceptable for sheltered inland areas (BS EN 12056-3)
• Catchment area calculation — Effective roof area = plan area + half the wall area above the gutter line (for steep roofs, only plan area)
• Overflow provision — Required on box gutters and in commercial guttering; domestic half-round gutters overflow at the low point in heavy rain
• UPVC — Most common; white, grey, or black; low maintenance; ~25-30 year lifespan before brittleness
• Cast iron — Period properties; requires painting every 5-7 years; very long life if maintained; joins with sockets and nuts/bolts
• Aluminium — Lightweight; long-lasting; powder-coated; seamless systems available; good for modern and period
• Copper — Premium; develops patina; very long life; expensive
• Downpipe sizes — 68mm round (standard domestic), 65mm square profile; large roofs may need 100mm+
• Brackets — Fascia brackets (to fascia boards) or rafter brackets (over tiles to rafter); maximum 1m spacing
• Fall direction — Mark the high point at one end of the run; fall the gutter continuously to the outlet
• Building Regulations Part H — Drainage of roofs and paved areas; requires adequate drainage from roofs

Quick Reference Table

Detailed Guidance

Calculating Gutter Size

For most domestic work, using the standard size table above is sufficient. For properties with large roof areas or complex geometry, the formal calculation from BS EN 12056-3 is:

Flow capacity required (Q): Q = (r × A) / 10,000 (litres/second)

Where:

• r = design rainfall intensity (75 mm/hour for most UK locations)
• A = effective roof area (m²)

Effective roof area (A): A = plan area + (vertical projected area × factor based on roof pitch)

• For pitches up to 30°: A = plan area + 50% of the vertical face
• For pitches above 30°: A = plan area + 100% of the vertical face

Then match Q to the hydraulic capacity of the chosen gutter size (from manufacturer's data or BS EN 12056-3 tables).

Example:

• 8m wide roof with 5m slope (plan width 8m × roof length 10m = 80m²)
• Q = (75 × 80) / 10,000 = 0.6 L/s
• 112mm half-round at 1:100 fall handles approximately 1.2 L/s — adequate

Installation: Setting Out the Fall

Step 1 — Mark the high point: Fix a bracket at the highest point of the gutter run (furthest from the outlet), at a level where the back of the gutter will sit approximately 25-50mm below the bottom edge of the roof covering (tiles/slates should overhang slightly into the gutter to direct water in, but not be resting on the gutter).

Step 2 — Mark the outlet position: Fix the outlet/downpipe bracket at the opposite end of the run. The fall from high bracket to outlet: (run length in metres × fall ratio). For a 6m run at 1:100: 6 × 10mm = 60mm fall.

Step 3 — Snap a chalk line: Snap a line between the high-point bracket and the outlet; fix remaining brackets at maximum 1m centres along this line.

Step 4 — Fit guttering: Fit the outlet first, then connect gutter sections using the manufacturer's snap-fit or bolt connectors with sealing gaskets. Do not over-tighten screwed connections — UPVC needs room for thermal expansion.

UPVC expansion: UPVC expands and contracts with temperature. Expansion joints should be provided every 6-8m on long runs; alternatively, the snap-fit connectors themselves provide a small amount of movement. Do not push gutter sections fully home into union connectors — leave 5-10mm gap at each joint for thermal movement.

Cast Iron Guttering

Cast iron is the correct material for most Victorian and Edwardian terraces, and is often required by planning conditions in conservation areas and on listed buildings.

Installation differences from UPVC:

• Joints are made with a sealing compound (Plumbers' Mait or similar bituminous mastic) rather than rubber gaskets
• Sections bolt together; the nut and bolt should be tightened onto the mastic to compress it
• Cast iron is heavy; fixings must be adequate for the weight — typically 75mm × 10mm coach screws into the fascia
• Expansion is minimal in cast iron; joints don't need expansion gaps

Maintenance:

• Clean out leaves and debris annually
• Re-paint every 5-7 years with a good quality exterior paint; treat any rust before painting
• Replace old mastic seals if joints are leaking (rake out old mastic, clean, apply new sealing compound)
• Sand and prime bare metal before repainting; use bituminous paint on the inside of the gutter

Sourcing: Reclaim cast iron from salvage yards for replacement sections; new cast iron sections are available from specialist suppliers (Hargreaves Foundry, Brett Martin, Castrads).

Common Guttering Problems

Overflowing during heavy rain:

• Gutters may be undersized for the catchment area
• More likely: outlets or downpipes are blocked with debris
• Check fall direction; gutters sagging (failed brackets or incorrectly installed) create low spots

Persistent dripping from joints:

• UPVC: failed rubber seal in the joint connector; replace the connector or the seal
• Cast iron: failed mastic seal; rake out, clean, and reseal
• Check that the gutter is correctly overlapping at each joint (socket joints must face the right direction relative to flow)

Leaks at downpipe connections:

• Ensure the downpipe socket engages over the gutter outlet properly; use silicone or the manufacturer's sealant at the joint
• Check the downpipe for cracks (frost damage is common in cast iron)

Sagging gutters:

• Brackets have pulled out of the fascia (fascia board rot is common — replace the fascia if rotten)
• Too few brackets (maximum 1m spacing)
• Long UPVC runs without expansion provision sagging at mid-span

Water running behind the gutter:

• The back edge of the gutter should sit tight to the fascia; if the fascia is bowed or the gutter is twisted, a gap opens
• Water runs through the fascia board, into the rafters
• Refix brackets; ensure gutter back edge is flush with fascia

Downpipes and Drainage

Downpipes must discharge to a legal drainage point:

• Foul sewer — via a back-inlet gully (not direct to foul pipe; must be sealed to prevent odour)
• Separate surface water drain — direct connection acceptable
• Soakaway — minimum 5m from any building; sized for the catchment area; requires building control approval for a new soakaway in some situations

Downpipe connections:

• Offset (swan neck) sections redirect the downpipe from the gutter position to the wall face
• The offset is typically 100-150mm for standard eaves overhang
• Fix the downpipe to the wall with pipe clips at maximum 2m intervals; 1m near the top of each section

Frequently Asked Questions

Does guttering replacement need building control or planning permission?

Like-for-like replacement doesn't need planning permission or building control notification — it's maintenance. However, in conservation areas, changing the material (e.g., UPVC replacing cast iron) may require planning permission; check with the local planning authority. On listed buildings, any material change may require listed building consent.

How do I stop leaves blocking the gutters every autumn?

Gutter guards (mesh inserts or brush-type guards) are the most effective solution. Mesh guards prevent leaves entering the gutter while allowing water through. They're not perfect — fine debris still accumulates over the mesh — but they significantly reduce cleaning frequency. Position gutter guards after the autumn leaf fall and inspect/clean annually. Alternatively, if trees are the problem, the most effective long-term solution is removing or cutting back the overhanging tree (with appropriate consents).

My customer wants black UPVC to match their windows — is this a good choice?

Black UPVC looks good on modern and traditional properties and has become very popular. The main consideration is heat absorption: dark-coloured UPVC absorbs more solar radiation and can reach higher temperatures than white or grey, causing more thermal expansion. This is manageable with correct expansion provision. For conservation areas and period properties, black aluminium or black-painted cast iron is often the more authentic and planning-acceptable choice.

How do I calculate the right size soakaway for a house?

A soakaway must accommodate the runoff from the roof during a storm event. The approximate minimum volume: catchment area (m²) × design rainfall depth (mm). For a 100m² roof and a 1-in-10 year storm (approximately 30mm in 1 hour): 100 × 0.03 = 3m³ soakaway volume. In practice, a 3m³ soakaway (e.g., 1.5m diameter × 2m deep with crate fill) is appropriate for most houses. Conduct a percolation test to confirm the ground can absorb at the required rate. Building Regulations Part H provides the formal design method.

Regulations & Standards

• Building Regulations Approved Document H — Drainage of roofs and paved areas; requires adequate roof drainage; soakaway design

• BS EN 12056-3 — Gravity drainage systems inside buildings; roof drainage, layout and calculation; the design standard for gutter sizing

• BS EN 607 — Eaves gutters and fittings made of PVC-U; product standard for UPVC guttering

• BS EN 612 — Eaves gutters and rainwater pipes of metal sheet; product standard for metal guttering

• NFRC Drainage Guidance — National Federation of Roofing Contractors guidance on roof drainage

• Hargreaves Foundry Cast Iron Guttering Guide — Cast iron specification and installation guidance

• Brett Martin Rainwater Systems Guide — Sizing and installation for UPVC guttering

• pitched roof repairs — Pitched roof repair and maintenance

• flat roofing — Flat roof drainage requirements

• building control — When building control is needed for drainage work