Bára, Smábára and Trapísa: what the numbers mean and which profile suits which job.
Corrugated sheet is flat steel rolled into ribs. A flat sheet is limp, but once it is rolled into ribs it becomes stiff along its length, spans between purlins on its own and carries water down the ribs and off the roof. The height and pitch of the rib decide how stiff the sheet is, how much water it carries and how it looks on the building.
Corrugated iron reached Iceland in the 1870s. General use on roofs began after 1880 and on walls after 1890, because it was fireproof, light to ship and stood up to the weather better than timber cladding. The traditional Icelandic profile, 18 mm tall with 76 mm between crests, has dominated ever since and is still the most common roof cladding on Icelandic houses.
The first number is the rib height in millimetres, measured from the valley to the crest, and the second is the distance between two crests. Bára 18/76 is therefore 18 mm tall with 76 mm between crests. Most manufacturers use the same naming, though some write the pitch first, 76/18, so sheets from different suppliers can be compared on the name alone.
The third dimension, which is not in the name, is the thickness of the steel. All our corrugated sheet is rolled from 0.6 mm BaoAM steel, whether unpainted or painted.
Rib height 18 mm, rib pitch 76 mm
Rib height 8 mm, rib pitch 32 mm
A sheet is wider than the area it covers, because each sheet lies over the next. Overall width is the width of the sheet as it leaves the roll former. Coverage width is what each sheet adds to the covered area once it is lapped. Bára 18/76 is 1,126 mm wide and covers 1,050 mm with a one-rib lap. A roof 10 metres wide therefore takes ten sheets, not nine, and coverage width is the figure to use when counting sheets.
All corrugated sheet starts as flat steel from a coil. Rolling draws the width in as the steel folds into ribs: 1,250 mm of flat steel becomes a 1,126 mm wide Bára 18/76. The material yield in the table above is the ratio of coverage width to flat width. It explains why taller and tighter ribs cost more per square metre of cover: they simply use more steel.
On walls the side lap is one rib, 76 mm, and that is what the coverage width of 1,050 mm assumes. Roofs are lapped more, at least one and a half ribs, and two ribs where the pitch is low or the exposure severe. Each sheet then covers 974 mm, so allow for more sheets.
The Icelandic building regulation no. 112/2012 sets minimum roof pitches by the material that forms the main rain barrier, in article 10.5.4. For corrugated sheet on a felted sub-roof of boards or moisture-resistant panels the minimum is 1:4, or 14°. Standing-seam metal may go lower, 1:5 (11°) with a single lock and 1:15 (4°) with a double lock. The Rb guidance sheet (27).001 on roofs says the same and recommends at least 14° where corrugated sheet is the main water barrier over a cold roof space. On a lower pitch corrugated sheet may not be the main rain barrier. The water barrier then has to sit under the sheet, a membrane or double felt that meets the regulation on its own, and the sheet becomes a protective skin over it. The Rb sheet notes that the risk of leaks at nail holes is highest on low-pitched roofs, so the shallower the roof, the more the sub-roof detailing matters.
Where two sheets meet along the slope they need an end lap, at least 150 mm and more the shallower the roof. The best end lap is none at all: order the sheets full length. We roll sheets up to 15 metres long. Above 7 metres, transport needs special attention.
Allow 12 to 14 fasteners per square metre on roofs and 10 to 12 on walls. Roofing nails or screws with rubber washers always go through the crest of the rib, never the valley where the water runs. Fix every crest at end and side laps, and every second or third crest elsewhere depending on exposure.
Bára 18/76 is the traditional Icelandic profile and goes on roofs and walls alike. It matches older houses, and most flashings and ridges are shaped for it. The roof area calculator assumes it unless another profile is chosen.
Smábára 8/32 is lower and tighter, with 35 ribs per sheet instead of 14. It gives a finer, flatter surface and is first and foremost wall cladding, outdoors or indoors, horizontal or vertical. It is less suited to roofs, since a lower rib carries less water and spans less between purlins.
Trapísa 24/136 has taller, wider ribs with a flat pan between them. It is stiffer than the corrugated profiles, spans further between purlins and carries more water. It is common on larger buildings, industrial halls and outbuildings. Trapísa is coming soon. Contact us for details and a quote.
Rib height 24 mm, rib pitch 136 mm
All cross sections are drawn to the same scale and show 300 mm of each sheet.
