How do I replace a section of guttering?

Matching the profile, cutting and joining plastic gutter, and setting the new section to the existing fall.

Difficulty: intermediate
Time: 3 hr
Read: 4 min
Safety: warning

Short answer

Identify the profile and brand first — half-round, square and ogee are not interchangeable, and brands differ. Support the run, unclip the damaged length, cut the new piece with a fine saw, and clip it in with the expansion gap the joint markings show.

Modern plastic gutter is a clip-together system that is easy to work on and very unforgiving of two things: mixing brands, and forgetting that plastic expands. Both cause leaks at the joints within a year.

Safety

This is ladder work at eaves height and it is where most DIY falls happen. Use a ladder at the correct angle on firm level ground, tied or footed, extending at least 1m above the eaves, and never overreach — move the ladder instead. A ladder stand-off keeps the ladder off the gutter you are working on. If the eaves are above single-storey height, or the roof is steep, or you are working alone, pay someone with a tower or a scaffold. Never stand on a gutter or a fascia.

What you'll need

Replacement gutter of the matching profile and brand
Union brackets and seals as needed
A fine-toothed saw and a file
A ladder with a stand-off, or a tower
Silicone lubricant or washing-up liquid (optional)

Setting the fall of a gutter, and the joint that has to move

Step by step

Identify the profile before buying anything.Half-round, square, deep-flow and ogee all look similar from the ground and none of them fit each other. Take a photo and a measurement of the width, and ideally a short offcut, to the merchant.
Set up the access properly.Ladder at about 75 degrees, footed or tied, with a stand-off so it bears on the wall rather than the gutter. Move it rather than reaching. Everything else in this job is easy; this is the part that hurts people.
Support the section before releasing it.A full length of gutter with water and debris in it is heavy and will drop. Empty it, and have a second person or a prop under the far end.
Release the clips and unions.Most systems have a lip that levers out of the bracket, and union brackets that release with a screwdriver in a slot. Work along rather than pulling — the plastic snaps when cold.
Check the brackets and the fall while it is open.Brackets at 800-1000mm centres, closer near a joint, and a consistent fall of around 1:600 towards the outlet — roughly 15-20mm over a 10m run. Replace any bracket that is loose, rusted or split.
Cut the new length square.A fine-toothed hacksaw or a panel saw, cut square using the gutter's own profile as a guide. File the burrs off the cut end so it does not tear the seal.
Fit with the expansion gap.Every union has an insertion depth marked inside. Push the gutter to that mark, not fully home. Plastic expands several millimetres over the length of a run in summer, and a joint pushed fully home in winter will lift the seal or pop apart.
Lubricate the seals.A smear of silicone lubricant or washing-up liquid lets the gutter slide into the union without rolling the seal out of its groove — the most common cause of a new joint leaking.
Test with a bucket of water.Pour water in at the far end and watch the whole run. You are checking that it flows to the outlet, that no joint drips, and that nothing pools.

Tips

Take an offcut of the old gutter to the merchant. Profiles vary between brands in ways that photographs do not capture.
Do the work on a mild day. Plastic gutter is brittle in cold weather and snaps when levered.
While the section is off, clear the whole run and check the downpipe. It is the easiest access you will get.

Common mistakes

Pushing the gutter fully home in the union — There is no room left for thermal expansion, so on the first hot day the run pushes the joints apart or lifts the gutter out of its brackets. Always insert to the marked line.
Mixing gutter brands with the same nominal profile — The dimensions and seal geometry differ slightly and the joint leaks. Match the brand as well as the profile.

If it doesn't work

New joint dripping

Cause: Seal rolled out of its groove during assembly — Fix: Take the joint apart, check and reseat the seal, lubricate it, and reassemble. It is almost never a faulty seal.

Gutter popping out of brackets in summer

Cause: No expansion gap at the joints — Fix: Reset the joints to the insertion marks. This gives the run somewhere to go as it expands.

Water pooling in the new section

Cause: Bracket heights wrong, or the fall reversed locally — Fix: Reset the bracket line to a string from the high end to the outlet, keeping a consistent fall.

Gutter sagging between brackets

Cause: Brackets too far apart — Fix: 800-1000mm centres, and closer either side of a union or an outlet.

Questions people ask

Can I mix gutter brands?

Not reliably. The nominal profile may match but the union dimensions and seal shapes differ, and the joint leaks. Identify the existing brand, or replace a whole run in one system.

How do I know which gutter profile I have?

Look at the end profile: half-round is a simple semicircle, square is rectangular, ogee has an S-shaped front face, deep-flow is a deeper half-round. Measure the width across the top and take an offcut if you can.