How Do I Tell If My Gutter Is Pitched Correctly Toward the Downpipe?

A correctly pitched gutter should slope gently but consistently toward the downpipe so rainwater drains away completely after every shower. If water is pooling, overflowing mid-span, or you can see a sag or bow in the gutter profile, the pitch is likely off. The good news is that checking gutter fall is a straightforward DIY task — fixing it may or may not be, depending on what you find.

What Is the Correct Gutter Pitch in NZ?

The standard minimum fall (slope) for a residential gutter in New Zealand is 1:500 — that means 1 mm of drop for every 500 mm of horizontal run. In plain terms, a 5-metre gutter run should drop at least 10 mm from its high end to the downpipe outlet. Many installers use a slightly steeper fall of 1:200 to 1:300 to give a comfortable margin, especially in high-rainfall regions like the West Coast, Northland, or Wellington.

This guidance aligns with the requirements under NZ Building Code clause E1 (Surface Water), which requires that roof drainage systems effectively remove water from the building. You can find more detail on building performance requirements at MBIE Building Performance.

How to Check the Pitch Yourself

1. Visual Check After Rain

The simplest test: go outside during or just after a decent rain shower and watch the gutter. Water should be flowing steadily toward the downpipe. If you see:

• Standing water sitting in the gutter after rain has stopped
• Overflow spilling over the front edge mid-span (not at the downpipe)
• Rust staining, moss, or sediment build-up in one spot

…then the pitch is likely insufficient or the gutter has sagged.

2. The String Line Method

This is the most reliable DIY check:

1. Clear the gutter of all debris first — a blocked gutter mimics a poorly pitched one.
2. Tie a string line from the high end of the gutter (furthest from the downpipe) to the downpipe outlet end.
3. Pull it taut and use a spirit level or a line level clipped to the string to confirm the string itself is perfectly level.
4. Measure the gap between the string and the bottom of the gutter at several points along the run.
5. At the downpipe end, the gutter bottom should be lower than the string by at least 1 mm per 500 mm of run.

For example, on a 6-metre run, you'd expect the outlet end to sit at least 12 mm below the high end. If the gutter bottom is level with the string — or worse, higher — you have a fall problem.

3. The Water Test

If you can safely access the gutter, pour a bucket of water at the high end and watch how it travels. It should flow steadily and drain fully within a minute or two. Slow movement or pooling confirms insufficient fall.

Common Causes of Incorrect Gutter Pitch

Can I Fix the Pitch Myself?

For minor adjustments — such as re-setting a loose bracket or adding an extra bracket to lift a sagging section — a confident DIYer with safe ladder access can often manage this. You'll need:

• A sturdy ladder (follow WorkSafe NZ ladder safety guidelines)
• Replacement gutter brackets (available at Bunnings or Mitre 10)
• Stainless steel screws appropriate for your fascia material
• A spirit level and tape measure

However, if the fascia board is rotten, the gutter needs to be completely re-hung, or you're working at height on a two-storey or steep-roofed home, it's safer and smarter to call a professional.

Gutter Profiles and Downpipe Sizing

Pitch alone won't fix drainage if the downpipe is undersized for your roof catchment area. NZ homes commonly use 75 mm or 90 mm round downpipes. If your gutter is correctly pitched but still overflows at the downpipe during heavy rain, the downpipe or outlet may be too small for the roof area it's draining. BRANZ publishes guidance on roof drainage sizing that your tradesperson can reference.

When to Call a Professional

Call a qualified gutter installer or roofing tradesperson if:

• The fascia board behind the gutter is soft, rotten, or damaged — this may also involve a carpenter
• The entire gutter run needs re-pitching or re-hanging
• You're working at height above a single storey
• You suspect the gutter size or downpipe is undersized and needs replacing
• Water is tracking back under the roofline, which could indicate a flashing or roof issue requiring a licensed roofer

Note: gutter work itself is not restricted electrical, plumbing, or gasfitting work, so no special licence is required. However, roof plumbing (connecting downpipes to underground drainage) may involve work that requires a licensed drainlayer under the Plumbers, Gasfitters and Drainlayers Board — check with your tradesperson.

You can find experienced local gutter cleaners and installers on tradedirectory.co.nz to get the job done safely and correctly.

Sources & Further Reading

• MBIE Building Performance – Building Code Clause E1 Surface Water
• BRANZ – Roof Drainage Guidance
• WorkSafe NZ – Ladder Safety
• Bunnings – Gutter & Roofing Supplies
• Mitre 10 – Guttering Products
• Plumbers, Gasfitters and Drainlayers Board