Valley iron replacement: the rusted channel out, the tiles back where they were

Wherever two slopes of a roof meet at an internal angle, they form a valley, and every drop of rain that lands on either slope runs into it. A strip of metal called the valley iron sits in that angle on top of the battens or valley boards, and the tiles either side are cut on the angle and rest on it. On a hipped Melbourne roof with a couple of gables or a bay, there can be four or six valleys, and each one is carrying the runoff of two roof planes down a single channel that is only a few hundred millimetres wide.

That is why valleys fail before the rest of the roof. On homes built from the 1950s to the 1980s the valley is often galvanised steel, and it rusts from the underside first, where damp leaf litter sits against it for months. Leaves, moss and tile grit build up into a leaf dam; water pools behind the dam, climbs over the edge of the valley and runs under the tiles. Where the tiles either side were held by valley clips, the clips rust and the cut tiles slip into the channel and block it further.

Inside the house the result is distinctive: a stain on the ceiling that runs in a straight line under the valley, rather than the round patch a single cracked tile leaves. When you describe that to the office, they will know what the job probably is before anyone is on the roof.

A valley with surface rust and a leaf dam is a maintenance job. The dam is cleared, the valley is cleaned back to metal, cracked cut tiles are replaced and any slipped tiles are re-seated. That is often all a valley on a well-kept roof needs, and a regular gutter and valley clean keeps it that way. Our gutter cleaning guide covers what a proper clean includes.

A valley with perforation is not. Once the steel has rusted through, even in one spot, the rest of the channel is close behind, and a patch or a coating only hides it. Silver paint, bitumen and rubberised coatings all fail the same way: they trap moisture under the coating and the rust keeps going where you cannot see it. We do not coat a rusted valley, and we do not replace half of it, because a lap between old metal and new in the middle of a channel is a leak waiting to happen.

The signs that put a valley in the replace column are rust that has gone through the metal anywhere along its length, a valley that has lifted or buckled so water no longer runs down the centre, and a valley that was fitted too shallow or too narrow for the roof it drains. In each case the honest fix is a new valley from the ridge to the gutter, and we quote it that way.

The tiles either side of the valley are lifted and stacked on the roof, usually three or four courses wide, from the gutter up to where the valley meets the ridge or a hip. Where the valley runs into ridge capping, the bedding at the top is broken out so the old valley can be pulled clear. The old valley iron is cut free and lowered down, not dropped into the gutter.

With the channel open, the valley boards or battens underneath are checked for rot, and the sarking either side is checked for tears; both are repaired before the new metal goes on. The new valley is fitted in lengths from the top down, so every lap faces downhill, with turned edges to stop water creeping sideways and enough width to carry a Melbourne downpour. Valley clips or a valley batten hold the cut tiles clear of the channel so they cannot slip into it.

Then the tiles go back. Cut tiles that cracked when they were lifted, or were already cracked, are replaced and matched to the profile. Where the valley meets the ridge, the capping is re-bedded and re-pointed so the top of the new valley is sealed. The gutter below is cleared of the rust and grit that came out of the old valley. Before the tiles are re-laid we photograph the new valley in the open channel, so you can see the metal, the laps and the clips you are paying for and will never see again.

Colorbond steel is the usual choice for a new valley on a tile roof. It is pre-painted, long-lasting, and is matched to a colour that disappears under the tiles. Zincalume steel is the unpainted version and does the same job where colour does not matter. Lead is still used on some period and terracotta roofs where the valley has to be dressed by hand into an irregular shape, and where the original was lead. Lead is heavy and fatigues along its folds over decades, so a lead valley that has cracked is replaced in lead only where the roof genuinely calls for it.

What we do not do is mix metals. Lead draining onto a Zincalume or Colorbond valley, or a copper flashing draining onto a steel valley, sets up corrosion that eats the newer metal from the first winter. The valley, the flashings that run into it and the gutter it drains into are chosen so nothing dissimilar touches.

The tiles either side matter as much as the metal. On a concrete tile roof the cut tiles are re-laid on clips; on a terracotta roof the cuts are finer and more tiles tend to break when lifted, so the job is quoted with spares. On a slate roof the valley is a specialist job and the office will tell you if it is one we take on. Every valley replacement is carried out by licensed roof plumbers.