Zinc is a lively metal. It grows when the sun hits it and shrinks again on a cold night. A standing seam roof that allows for this will look crisp and stay watertight for a very long time. One that doesn’t can buckle, tick loudly, tear at a junction or pull its fixings loose. Here’s how movement is handled on a standing seam zinc roof, and why it matters so much on the long extensions and new builds we roof across Surrey.

We’re The Specialist Roofing Company, a family firm in Old Windsor fitting zinc since 2007. We’re approved installers for VMZINC, RHEINZINK and PREFA.

How much does zinc actually move?

VMZINC gives the thermal expansion of titanium zinc as 0.022 mm per metre for every degree Celsius, along the rolling direction. RHEINZINK gives the same figure, expressed as 2.2 mm per metre over a 100 °C change.

That sounds tiny until you do the sums. A roof surface can be well below freezing on a winter night and far hotter than the air on a still summer afternoon, especially in a dark pre-weathered finish. As an illustration only, a 10-metre tray that sees a 70 °C swing over the year moves about 10 × 0.022 × 70 = 15 mm. That’s 1.5 cm of movement that has to go somewhere, every year, for the life of the roof.

Why Surrey roofs put this to the test

  • Long rear extensions. Many Surrey houses sit on deep plots, so single-storey kitchen extensions often have long, uninterrupted trays running from the house wall to the garden.
  • South-facing roofs in dark finishes. Anthracite and dark grey zinc are popular, and in full sun they get hotter than lighter finishes.
  • Pool houses, studios and garages. Low-pitch outbuildings in open gardens catch a lot of sun.
  • Junctions with the old house. A new zinc roof often meets an existing brick wall, a parapet or a tiled roof, all of which move differently or not at all.

How a standing seam roof is designed to move

Fixed zone and sliding clips

Standing seam trays aren’t screwed through the surface. They’re held down by clips folded into the seams. In one zone of each tray, the clips are fixed, which anchors the tray and stops it creeping downhill. Everywhere else, sliding clips hold the tray firmly against wind but let it slide lengthways as it expands and contracts. Where the fixed zone goes depends on the pitch and tray length, and the manufacturer’s rules set it out. We follow them for every roof.

Tray length

The longer the tray, the more it moves. Manufacturers limit how long a tray can be with standard clips. Beyond that, a roof needs longer-travel clips, a step in the roof or a cross joint designed to take the movement. On a long Surrey extension, this is decided at the setting-out stage, before any zinc is ordered.

Free ends at the top and bottom

At the eaves and the ridge or wall abutment, the tray has to be able to move. That means folded details that hook over a drip or tuck under a flashing, never soldered or fixed rigidly at both ends. A tray fixed tight at both ends has nowhere to go and will buckle or tear.

Penetrations and rooflights

Rooflights, flues and soil pipes interrupt a tray, which makes them the most likely places for movement to cause trouble. We plan seam positions around them and use details that let the zinc slide past the upstand. Our article on detailing rooflights in VMZINC standing seam goes into this in more depth.

The build-up underneath

Movement and ventilation go hand in hand. The zinc needs a suitable, smooth deck and the right underlay so it can slide freely, and moisture must never be trapped underneath. Read why ventilation matters beneath VMZINC standing seam roofing.

Signs a roof wasn’t designed to move

If you’ve inherited a metal roof and see any of these, it’s worth a survey:

  • trays bulging or rippling between the seams, beyond the slight “oil-canning” that’s normal on wide trays
  • splits or tears at the top or bottom of a tray, or around a rooflight
  • loud cracking or ticking as the sun comes on and off the roof
  • flashings pulled away from the wall

We survey and repair existing zinc, copper and lead roofs as well as fitting new ones.

Why it pays to use an approved installer

Movement rules come from the zinc manufacturers. As approved VMZINC and RHEINZINK installers, we work to them on every roof. Every job is done by our own team, never subcontracted, and it’s covered by our 20-year guarantee.

Planning a standing seam zinc roof in Surrey?

Whether it’s a kitchen extension in Woking, a pool house in Weybridge or a new house near Guildford, we’ll set out the roof, the seams and the movement details before anything is ordered. Call 01753 867288 or send us your drawings. See also zinc roofing in Surrey and standing seam roofing.