Double Standing Seam Roofing/Folded Roofing - Pitched (Gable) Roof

From the ground the roof looks simple. Whether it stays watertight for decades depends on how carefully each step is done. On a pitched gable roof the work usually goes like this:

1. Checking the deck. The continuous boarding must be flat, dry and solid. The roofer also checks the breather membrane and the ventilated gap under the boarding.
2. Separating layer. A structured underlay mat, or another layer the manufacturer recommends, goes on the boarding. It keeps the metal off the timber and drains away the moisture that condenses under the sheet.
3. Eaves edge. The eaves flashing and gutter brackets go on first, so that water runs off the roof straight into the gutter.
4. Preparing the trays. Trays are cut to the length of the roof slope. Long trays are often formed on site with a roll-forming machine, so there are no cross joints on the roof.
5. Fixing with clips. Each tray is fixed to the boarding with fixed and sliding clips hidden inside the seam. Fixed clips hold the tray in one place. Sliding clips let it expand and contract as the temperature changes.
6. Seaming. The edge of the next tray goes over the previous one, and the joint is folded with hand tools or a seaming machine. For a double standing seam it is folded twice, so the joint stays tight even under snow and standing water.
7. Details. Then come the ventilated ridge, verge trims, valleys, chimney flashing, abutments against walls and any pipes or vents through the roof.
8. Snow guards. Snow slides off smooth metal quickly, so snow guards go on the roof to protect the gutters and anyone walking below.

The metal is only the top layer. Most problems that show up after a few years start underneath it, not in the metal itself. A typical build-up for a pitched roof over a heated loft looks roughly like this, from the inside out:

• Vapour control layer on the room side, which stops warm, damp air from the house getting into the insulation
• Insulation between the rafters, and sometimes also below or above them
• Breather membrane over the rafters
• Counter battens, which create the ventilated gap
• Continuous boarding made of planks or boards, as the roofing manufacturer specifies
• Structured underlay mat and finally the standing seam roofing

Why does ventilation matter so much? Metal cools down quickly at night, and moisture condenses on its underside. If air does not move under the boarding, the timber gets damp and loses strength. The ventilated gap needs an air inlet at the eaves and an outlet at the ridge, otherwise it does not work.

The minimum roof pitch depends on the specific system, and the manufacturer states it in the technical documents. A double standing seam can handle low pitches, often just a few degrees. The flatter the roof, though, the more precise the seaming has to be and the more the details matter. Sometimes a sealing tape inside the seam is needed as well.

Three materials are used most often:

• Coated galvanised steel, the usual choice for family houses, available in a wide range of colours
• Aluminium, light and resistant to corrosion, a good fit for coastal, damp or industrial surroundings
• Titanium zinc, which develops a natural patina and lasts a long time, popular on historic and architecturally demanding buildings. It needs an experienced roofer to work it properly.

Be careful when mixing metals. Water running off copper onto zinc or galvanised steel causes corrosion. Gutters, flashings and accessories should be made of a metal that is compatible with the roofing itself.

Besides the classic seam that is folded on the roof, there is also a click system. The trays snap into each other without seaming, so installation is faster. It suits simple roofs with a steeper pitch better. For low pitches, curves and complicated shapes, the classic double standing seam is the usual choice. The same technique also works on walls if you want the roof and façade to look like one piece.

Two roofs of the same area can take very different amounts of work. The job gets harder mainly because of:

• A complicated roof shape: every valley, dormer, chimney or roof window means a detail made by hand
• A low pitch, which calls for more careful sealing of the seams
• Height and access: scaffolding, edge protection and lifting long trays onto the roof
• The condition of the deck: on a re-roofing job the boarding often needs replacing, ventilation needs adding or the roof timbers need repair
• The material: titanium zinc and aluminium are worked differently from steel

A simple gable roof, a flat and dry deck and good access for the roll-forming machine make the work easier. This is work at height that belongs to a qualified roofer with proper fall protection. It is not a job to try yourself.

Before replacing the roof covering on an existing house, check with your local building authority which rules apply to your building. A change of material or appearance may need approval, especially in a conservation area or on a listed building. Do not assume it is automatically allowed. In a block of flats, decisions about the roof are made by the building management or the owners' association.

• Metal fixed rigidly. Without sliding clips the tray cannot move freely, the surface ripples and the seams are put under strain.
• No ventilation. Moisture stays under the metal and the boarding starts to rot.
• A system that does not suit the pitch. A click system or single seam on a roof that is too flat lets water in when snow lies on it or the wind drives rain.
• Sloppy details. Leaks almost always appear at valleys, chimneys or wall abutments, not on the flat area of the roof.
• A missing separating layer where the manufacturer requires one, especially with titanium zinc.
• Forgotten snow guards, which are harder to fit afterwards.

When you take over the work, look at the roof from the ground and from the scaffolding while it is still up. Check that:

• The seams are straight, parallel and evenly folded along their whole length
• There are no visible screws, dents or scratches on the surface
• The surface does not ripple noticeably, even in sunshine
• The eaves edge leads water into the gutter and the gutter falls towards the downpipe
• The ridge is properly ventilated and the air inlet at the eaves is protected with a mesh
• Valleys, the chimney and wall abutments are flashed without gaps, and sealant has not been used where there should be metal
• Snow guards are fitted as designed
• The contractor has given you the material details, the manufacturer's warranty terms and maintenance advice