Chapter 7 of 8 · 10 min read
Materials and thermal movement
Choose gutter materials that suit the roof above them, avoid harmful run-off and contact between metals, and calculate thermal movement so long gutters have expansion joints where they need them.
A gutter that is big enough can still fail through the metal it is made of. Two problems account for most of these failures. The first is chemistry: water that runs off one metal and onto another can carry dissolved metal that attacks the second, and two metals in contact in the wet can corrode one of them quickly. The second is physics: a long strip of metal in the sun grows and shrinks by centimetres between a winter night and a summer afternoon, and if nothing allows for that movement, the gutter buckles, tears its own joints or pulls out its fixings. Both problems are invisible on the drawing and obvious a few years later.
This chapter covers the materials used for roof drainage, the rules about run-off and contact between metals, how much a gutter moves with temperature, and where to put expansion joints. It works out the movement in W-40’s 40 m gutters and the gap to set in the box gutter’s expansion joint. The failure case is a long box gutter on another building, riveted and sealed from end to end, that tore itself apart in its first hot summer.
The materials
Australian rainwater goods are made mostly from a handful of materials.
Metallic-coated steel, steel strip coated with zinc or with an aluminium-zinc alloy, either bare or prepainted, is the most common material for gutters, flashings and downpipes. It is strong, easily folded and relatively cheap. Its durability comes from the coating, which corrodes slowly in place of the steel, so anything that attacks the coating shortens its life.
Aluminium is light and resists corrosion well in many environments, including near the coast. It is softer than steel, so gutters need thicker material or more support, and it moves almost twice as much with temperature.
Copper and zinc are traditional architectural metals, long-lived and attractive as they weather, and costly. Stainless steel is used for box gutters in demanding locations and for fixings. UPVC is common for downpipes and some gutters on houses.
The ABCB Housing Provisions require metal gutters, downpipes and flashings on houses to be made in accordance with AS/NZS 2179.1, and UPVC components in accordance with AS 1273. They also prohibit lead on roofs that form part of a drinking water catchment, which matters wherever rainwater is collected in a tank for use.
Run-off from one metal onto another
Rainwater running over one metal picks up small amounts of it. When that water then runs over a less noble metal, it can deposit the more noble metal’s ions on the surface, setting up tiny corrosion cells that attack the less noble metal or its coating. The classic case is copper upstream of zinc or zinc-coated steel: water from a copper roof, flashing or pipe running into a zinc-coated gutter can pit the coating through quickly.
The practical rule is about the direction of flow. Water may run from a less noble metal onto a more noble one without this effect, but not the other way. Makers of coated steel roofing publish compatibility guidance listing which materials may drain onto their products and which may not, and it is the guidance a designer should follow for the products used. Lead flashings, copper pipes from air-conditioning units and copper-containing treated timber above a gutter are the usual offenders on existing buildings.
Metals in contact
Where two different metals touch in the presence of water, they form a galvanic cell, and the less noble one corrodes faster than it would alone. The size of the effect depends on the metals, the moisture and the relative areas: a small piece of less noble metal in contact with a large area of a more noble one corrodes fastest. The Galvanic pairs and sealing chapter of Sheet Metal Joints by the Numbers works through this in detail for sheet metal joints.
For roof drainage, it means fixings and brackets should be of a compatible material, or isolated from the gutter with a non-conductive washer or tape where they cannot be. Area matters: a small stainless steel screw in a large aluminium gutter is a far milder pairing than a small aluminium rivet in a large stainless sheet, although the maker’s guidance should still be checked. Copper or brass fittings in contact with coated steel should be avoided.
- Do: Gutters, box gutter, rainheads and flashings all in the same prepainted metallic-coated steel as the roofone family of materials
- Do: Fixings and rivets compatible with the gutter, per the maker's guidancecoated or stainless steel as specified
- Do: No copper, brass or lead upstream of any gutter or on the roofair-conditioning condensate lines in plastic, not copper
- Do: Sealant suited to the metal and the movementneutral-cure, compatible with the coating
- Ask: Plant platform added later, with its own flashingsto be checked when it is designed
- Avoid: Mixing metals to use up offcuts or stockthe usual source of incompatible repairs
How much a gutter moves
Metals expand as they warm and contract as they cool, by an amount proportional to their length, the temperature change and a property of the material called the coefficient of linear expansion. For steel it is about 12 millionths of its length for each degree Celsius; for aluminium about 23 millionths; for copper about 16 to 17; for zinc about 30 to 35; and for PVC between about 54 and 110, depending on the formulation.
A gutter’s temperature range is much wider than the air’s. A dark gutter in full summer sun can be far hotter than the air, and a clear winter night can take it below the air temperature. This book uses a metal temperature range of −5 °C to 75 °C, an 80-degree range, as a teaching assumption for W-40’s mid-grey prepainted steel. Lighter colours run cooler and narrow the range; darker colours widen it.
- Coefficient for steel
- 12 × 10⁻⁶ per °C
- Length of one run
- 20,000 mm
- Temperature range
- 80 °C
- change = 12 × 10⁻⁶ × 20,000 × 80 = 19.2 mm over the full range
- a whole 40 m gutter without a joint would move 38.4 mm
Nearly 40 mm in a steel gutter, and over 70 mm in aluminium, is far more than any riveted lap or sealant bead can absorb. The movement must go somewhere, and the designer decides where.
Where to put expansion joints
An expansion joint is a deliberate break in a gutter that lets two lengths move independently while still keeping water out. The best place for it is a high point, where the gutter falls away in both directions, because almost no water flows there. A joint at a high point needs only to keep out rain falling directly on it and the occasional splash, not to hold back flowing water.
W-40’s box gutter has its high point in the middle, so its single expansion joint is there. Each 20 m run is fixed at its outlet end, where it is sealed to the parapet opening and rainhead, and free to slide along its continuous support towards the high point. W-40’s rear eaves gutter falls to six outlets, with a high point between each pair; the designer placed one expansion joint at the high point nearest the middle, at about 20 m, again giving two runs of about 20 m.
The joint itself can take several forms: a gap between two stop ends, bridged by a loose cover flashing fixed to one side only; a flexible, UV-resistant membrane bonded across the gap; or a proprietary expansion piece. In a box gutter, the joint is usually formed as a raised saddle, so that the gutter sole on each side is turned up into a small ridge and the gap sits above the water level, under a cover.
Set the gap for the day it is installed
An expansion joint’s gap must allow for movement in both directions from the temperature on the day it is installed. If W-40’s box gutter is installed when the metal is at 25 °C, it can warm by 50 °C, to 75 °C, and cool by 30 °C, to −5 °C.
Warming by 50 °C, each 20 m run grows by 12 × 10⁻⁶ × 20,000 × 50 = 12.0 mm, so the two free ends approach each other by 24.0 mm. Cooling by 30 °C, each run shrinks by 7.2 mm, and the gap opens by 14.4 mm. The gap at installation must therefore be at least 24 mm, with some clearance, and the cover must still overlap both sides when the gap has opened by a further 14.4 mm. W-40’s joint is set with a 30 mm gap at installation, with a cover flashing that overlaps each side by at least 75 mm at the widest opening. The installer measures the metal temperature on the day with a contact thermometer and adjusts the gap if the metal is much hotter or colder than 25 °C.
Fixings that let the gutter move
Between its fixed point and its expansion joint, a gutter must be free to slide. Eaves gutter brackets that clamp the gutter tightly at every bracket fight the movement, and something gives: the bracket, the gutter’s lip or the fixing. Brackets that support the gutter while letting it slide, and a single fixed point per run, allow the movement to happen where it should.
Downpipes move too, mostly vertically. A 6 m UPVC downpipe warming by 50 °C grows by about 21 mm, using a coefficient of 70 millionths per °C. Downpipe clips should support the pipe without gripping it rigidly, with an expansion socket where the maker requires one, and the connection to the gutter outlet should allow a little vertical movement.
A failure case: the gutter that tore itself apart
This is an illustration. Another building in the owner’s group has a 36 m dark-coloured steel box gutter behind a parapet, installed during a cool winter. To make it watertight, every lap was riveted and sealed, and the gutter was fixed firmly at both ends and at intervals along its length. There was no expansion joint.
Over the first summer, the gutter’s metal temperature ranged widely. Over an 80 °C range, a 36 m steel gutter wants to change length by about 12 × 10⁻⁶ × 36,000 × 80 ≈ 34.6 mm, and installed cool, most of that range was expansion. Restrained at both ends, the gutter could not lengthen. It buckled upwards in the middle, lifting the sole off its support, and the force tore rivets out of two laps near the buckle. The sealant, which can absorb only a small movement, failed at several more.
The detection came in the first autumn storm, when water ran through the torn laps into the ceiling of the office below. The inspection found the buckled sole, the torn laps and failed sealant along the middle third of the gutter. The correction was to cut the gutter at its high point, fit a saddle expansion joint with a cover, replace the damaged lengths and re-lap the rest, releasing the intermediate fixings so the runs could slide. The lesson went into the business’s specification for all future gutters: any metal gutter longer than a set length, decided with the designer for each material, has an expansion joint at a high point, shown on the drawings.
How this applies to a small Australian business
Choose gutter materials that match the roof above them, and check the roofing maker’s compatibility guidance before allowing any other metal to drain into the gutter. Keep copper, brass and lead away from coated steel and zinc. Use compatible fixings, isolated where needed. Calculate thermal movement for every long gutter, using the material’s coefficient and a realistic metal temperature range for its colour and exposure, and put expansion joints at high points, with the gap set for the temperature on the day of installation. Let gutters and downpipes slide on their supports between fixed points. Sealant is for keeping water out of a lap, not for absorbing movement.
Exercise: an aluminium box gutter
For a coastal site, the owner considers making W-40’s box gutter in aluminium, with the same layout: an expansion joint at the middle, two 20 m runs fixed at their outlet ends, the same −5 °C to 75 °C range and installation at 25 °C. Using a coefficient of 23 × 10⁻⁶ per °C, how much does each run move over the full range? By how much does the gap close when the metal reaches 75 °C, and open at −5 °C? What gap would you set at installation?
Worked answer. Over the full 80 °C range, each run moves 23 × 10⁻⁶ × 20,000 × 80 ≈ 36.8 mm. Warming by 50 °C, each run grows by 23 × 10⁻⁶ × 20,000 × 50 = 23.0 mm, so the gap closes by 46.0 mm. Cooling by 30 °C, each run shrinks by 13.8 mm, and the gap opens by 27.6 mm. The gap at installation should be at least 46 mm plus clearance, say about 55 mm, with a cover that overlaps each side generously when the gap has opened a further 27.6 mm. Aluminium’s larger movement may also justify two expansion joints, which would mean redesigning the falls to create additional high points.