Standing Seam or Tray roofing is similar to trough section in that it is secret fixed, but it consists of just a single pan per sheet, compared to the two or three pans of a trough section.
Tray roofing may be designed to be self-supporting on purlins, or installed on solid sarking. With the former, sheets clip together, the latter has its side ribs clipped to the adjacent sheet.
Tray roofing designed to be fully supported by sarking is referred to in the COP as standing seam roofing. Clauses in this section headed Tray Roofing will apply to both types, clauses headed Standing Seam Roofing, including Load/Span tables, will only apply to generic standing seam roofing profiles
The profiles and fixing methods for tray roofing date back to traditional methods used to hand-fabricate metal roofs, predominantly in Europe and often from non-ferrous metal, prior to the development of roll forming technology. A by-product of this heritage is that, in addition to the secret clips fastening the main roof, flashings should also be installed on clips with minimal use of exposed rivets and fasteners.
In Europe, the installation of tray roofing is done by Spenglers, who serve a 4-year apprenticeship. We do not have the same qualification in NZ, but installers of tray roofing must have specific training and experience in the product if they are to achieve the expected high standard of workmanship
Traditionally standing seam roofs were manufactured from sheets of copper or pure zinc (sometimes even lead) and these materials are still popular being naturally durable and weathering to a natural patina. They are also very malleable which allows more variation in intersections and terminations, and they can be soldered.
More common in the New Zealand market are pre-painted steel and aluminium substrates. To increase its formability, Steel is normally supplied in Medium Strength G300 grade and Aluminium in hardness grade H34.
See 4.16 Materials for a more thorough discussion of roofing material.
Self-support tray roofing profiles resemble a single tray trough section but the ribs are generally narrower and the installation techniques are more sophisticated. Standing seam profiles generally have rib heights of 25 mm – 45 mm. Pan widths for both types vary from 300 mm to 500 mm.
Traditional standing seam shapes are angle-seamed, double-seamed, or roll cap. The angle seam is the most popular and the double seam is the least popular. The roll cap replicates old roofs with a capped joint installed over a longitudinal batten.
One of the features of standing seam roofs is that they can be formed by folding rather than roll-forming. This makes it possible to install roofs to buildings that are round or sinusoidal in plan, to have “random” pan widths, or to vary rib width discretely so that ribs are spaced equidistant from associated architectural features such as penetrations and windows.
All tray roofing, including standing seam profiles, must have clearance between adjacent pans to allow for timber shrinkage and transverse thermal expansion of the pan. The non-existence of this clearance gap can cause excessive canning or quilting.
Tray and particularly standing seam profiles lend themselves to many variations in installation details. Installers are generally specialist and highly trained and may modify or improvise a detail to suit the needs of a situation. Because of this, the demarcation between design and Installation is not clear cut.
The following information on design and installation is generally applicable to all tray roofing including self-support, apart from the sections on sarking and load tables. Sections particular to standing seam are headed as such, but there is a large amount of cross-over in many areas.
The malleable nature of the metal used to manufacture tray roofing, the wide pan, and the vertical rib, give many options for achieving weathertightness. Generally, options that work with a trapezoidal profile will equally work for tray roofing but are most likely to be installed on secret grab fasteners rather than being riveted. The elimination of primary and secondary fasteners is another attribute abetting weathertightness, as are the tight laps and the water carrying capacity of the wide pan.
The wide pan of tray roofing makes it impractical to have longitudinal flashings cover two pans, and because of the vertical rib and fastening techniques, longitudinal flashings covering a single pan are acceptable.
Perhaps the most outstanding feature of all tray roofs is the minimisation of visible fasteners. Apart from the secret fixing clips, attachment of flashings to the roof should be achieved by crimping to the cladding or other flashings or to a grab flashing wherever possible.
Clips for standing seam roofing are normally single clips, fastened to the substrate with screws or annular grooved nails. The sole of the clips should have a rebated or countersunk hole for the fastener, and rounded edges, to ensure thermal movement of the sheet does not cause damage by rubbing against sharp edges.
Wall cladding laid horizontally may need additional support and the standard bracket to resist gravity loads.
As the style of roofing replicates hand-formed products of yesteryear, and because the sheets themselves are relatively flexible, end laps are more common in tray roofs, and more acceptable, than with other profiles. In practice, staggered end laps are often used with tray roofing as an architectural design feature.
Sarking is commonly CPD (Construction Panel Directive), Stress grade 11, 15 mm ply laid on supports at 800 centres, or 17 mm ply at 900 centres. Ply should be laid with face grain at a right angle to supports.Edges of sheets not held by plastic tongue or T&G should be supported.Fasteners should not be closer than 10 mm from sheet edges.
To allow for expansion, maintain a 3mm gap between sheets. At the gutter line, the ply should be cut flush. Dormer valleys and valleys into spouting can be recessed or flat laid.
Because of their relatively narrow ribs, tray roofing generally self ventilates less than other roof profiles. This puts more responsibility on the designer to consider ventilation of the ceiling space. Ventilation is particularly important for fully supported profiles; the ply must have a gap at the apex to allow for egress of air and a gap at the bottom, or soffit vents, to allow air ingress.
Wind-induced noise can also be an issue in high-wind areas with winds above 20m/s. It can be minimised by specifying a narrower pan width and closing clip centres. Further noise reduction can be gained by putting a concave shape into the pan, which can be achieved by:
installing longitudinal stringers (e.g., a 10 mm batten) under the centre of the tray
installing strips of compressive material along the purlins.
Geotextile mat layers have the most significant effect on noise attenuation, but the added cost of such should be weighed against the expected benefits
The wide flat pan of tray roofing makes it easy to achieve secure penetration details. Proprietary rubber boot flashings can be used for small pipe penetrations, but a more aesthetically pleasing solution is to make up a flanged upstand in pre-painted steel or colour matched malleable metal, and fit a “Chinese hat” to the penetration to allow for thermal movement and weatherproofing.
Oil canning, panning, or quilting, is the term used to describe visible waviness of the pan of a metal roof.
Oil canning is one of the most controversial aspects of tray roofing. Some people accept it as an innate feature of a tray product, others want a flat tray with no visible waviness. It may not be obvious, but it is always present in tray roofing to some extent. The visibility of canning is affected as much by the lighting, line of sight, cleanliness, and gloss levels of the roof, as it is by the degree of canning present in the product.
Clients expecting no canning should be informed of the reality, particularly if the roof runs at an acute angle to one’s line of sight. Canning can also be induced by stretching the material or excessive foot traffic. The substrate must be true to plane and not convex.
The most effective ways to minimise canning in a highly visible situation is to use a profile with a narrower pan and use material with a low-gloss or textured surface.
With self-support tray roofs, excessive foot traffic will accentuate purlin lines, because of the ductility of the metal (grade G300) and the wide flat pan.
Tray roofing is a predominantly flat, secret fixed profile with one tray per sheet and vertical ribs. The roofing sheets are clip-fastened, using hidden clips. Tray cladding which is installed on sarking by traditional methods with hand-formed side lap seams is known as standing seam cladding.
With a wide tray profile, it is important to have matching distances from adjacent ribs to any major architectural details. Prior to commencing the lay, the roofer must determine what the predominant features are on a given face and set the roof out to maximise the symmetry of side flashings and matching rib lines. With fully supported profiles, sheets can be folded to varying widths to achieve symmetry across a number of architectural features.
Install the eaves grab flashing to which the sheets will be crimped and any other flashings that may be behind the cladding, for example, window jamb flashings.
Fit the netting and/or underlay. On flat roofs requiring underlay support, using twine is preferable to netting as the joins in the latter may imprint through the medium strength iron through foot traffic.
Form stop-ends in the sheets.
Then the sheets can be laid, starting from a distance from the barge to give equal cover distance to the opposing barge and other architectural features and penetrations. Fasteners should be annular grooved nails or screws, with clips at centres specified by the manufacturer to suit the product and wind zone.
Pre-painted tray roofing will usually be supplied with strippable film to give temporary protection from scratching. The film should be removed from underlaps while laying, and removed entirely before UV sets the adhesive making it difficult to remove without leaving glue residue on the sheet. Traffic across sheets should be kept to a minimum, particularly with self-supporting products.
Non-ferrous tray roofing expands at about twice the rate of ferrous metals. Supported angle seam and double seam profiles must be installed using a balance of sliding clips to allow for expansion, and fixed clips to withstand gravity loads.
The position of the fixed clips depends on the roof pitch. The width of the fixed clip portion should be sufficient to install five clips at the required spacing.
Steel based angle seam and double seam profiles up to four metres in length can be laid without sliding clips. Roll cap and self-support tray roofing have clip systems that can accommodate thermal movement, and also do not require special sliding clips.
15.4.13.4 Finishing at Ridges and Eaves of Tray Roofing
Crimp sheet at ends to eaves flashing with eaves crimping tool, and cut and fold rib to close rib ends. Remove strip film, if applicable, and for standing seam roofs lock adjacent sheets together with rib purpose made closer tool.
