How to Fix a Gutter That Drains Too Slowly After Cleaning

A gutter that still drains slowly after a thorough clean is telling you something else is wrong — most commonly a blocked downpipe, insufficient fall (slope), or a sagging section. The good news is that most of these issues are straightforward to diagnose and fix yourself with basic tools, though some situations do call for a professional.

Step 1: Check the Downpipe First

The most common culprit after cleaning the gutter trough itself is a blocked downpipe. Debris, compacted leaf matter, or even a bird nest can lodge inside the pipe and restrict flow completely.

• Run a garden hose at full pressure down the downpipe from the top.
• If water backs up or barely trickles out at the bottom, you have a blockage.
• Use a plumber's drain snake (available from Bunnings or Mitre 10) to break up and clear the obstruction.
• For stubborn blockages, a high-pressure water blaster directed down the pipe often does the trick.

Always flush from the top down, not the bottom up, to avoid pushing debris further in.

Step 2: Check the Gutter Fall (Slope)

Gutters need a slight fall toward the downpipe to drain properly — typically around 1:500 (about 2 mm drop per metre of run) is considered good practice in NZ. If your gutter is level or has a reverse fall, water will pond and drain slowly no matter how clean it is.

To check the fall:

1. Run your hose at one end of the gutter and watch where water flows.
2. If it pools in the middle or flows away from the downpipe, the fall is wrong.
3. Use a spirit level or string line to measure the slope along the gutter run.

Correcting the Fall

Gutter fall is adjusted by repositioning the gutter brackets (fascia brackets). This involves:

• Unscrewing the existing brackets.
• Repositioning them at the correct height to create the desired slope.
• Re-hanging the gutter section.

This is manageable DIY work on a single-storey home with a stable ladder, but take care — working at height carries real risk. WorkSafe NZ recommends always having a second person present when working on ladders.

Step 3: Look for Sagging Sections

Sagging gutters create low spots where water collects. This happens when brackets are spaced too far apart, have corroded, or have pulled away from the fascia board.

• Inspect each bracket along the run — push up gently on the gutter to see if any section lifts easily (a sign the bracket has failed).
• Replace damaged or missing brackets. In NZ, common gutter profiles include quad and half-round — make sure replacement brackets match your profile.
• If the fascia board itself is rotten, the bracket won't hold properly. Rotten fascia boards should be replaced before re-hanging the gutter.

Step 4: Inspect the Gutter Outlet and Strainer

The gutter outlet (the fitting where the gutter meets the downpipe) can become partially blocked with fine silt or debris that passed through during cleaning. Remove the outlet strainer if fitted, clean it thoroughly, and check the outlet opening is clear.

Also check that the outlet fitting is properly seated and hasn't shifted — a misaligned outlet restricts flow significantly.

Step 5: Check for Gutter Damage or Deformation

Over time, gutters can become dented, crushed, or warped — particularly aluminium and PVC types. Even a small deformation near the outlet can slow drainage noticeably. If a section is badly damaged, replacement is usually the most practical fix.

Preventing the Problem in Future

• Fit a downpipe strainer or leaf guard over the outlet to stop debris entering the downpipe.
• Clean gutters at least twice a year — autumn after leaf fall and spring after seed and blossom season are the key times in most NZ regions.
• Consider gutter mesh or guards if overhanging trees are a recurring problem. Products are available at Mitre 10 and Bunnings.

Proper gutter drainage is also important for meeting the NZ Building Code clause E1 (Surface Water), which requires buildings to manage stormwater effectively. Overflowing gutters can direct water toward foundations and cause moisture issues covered under clauses E2 (External Moisture) and E3 (Internal Moisture). For more on building performance requirements, see MBIE Building Performance.

When to Call a Professional

Call a qualified gutter cleaner or roofing tradesperson if:

• The problem is on a two-storey or higher building — working at that height is hazardous without the right equipment.
• The fascia or soffit boards are rotten and need structural repair.
• You suspect the downpipe connects to an underground drain that is blocked — this may involve drainlaying work, which is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board.
• You're unsure about the cause and don't want to risk further damage.

You can find experienced local gutter cleaners and roofing tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – NZ Building Code
• WorkSafe NZ – Working at Height Safety
• BRANZ – Weathertightness and Drainage Guidance
• Plumbers Gasfitters & Drainlayers Board – Licensed Tradespeople
• Bunnings – Gutter & Drainage Products
• Mitre 10 – Guttering & Roofing