COP v3.0:other-products; standing-seam-cladding

15.4 Standing Seam Cladding 

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.

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

15.4.1 Standing Seam Materials 

Traditionally tray 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.

15.4.2 Standing Seam Profiles 

Self-support profiles resemble a single tray trough section but the ribs are generally narrower and the installation techniques more sophisticated. Pan widths for both types vary from 300 mm to 500 mm

Traditional tray roofs are fixed to solid sarking. Traditional 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.

All tray roofing 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. Wall cladding laid horizontally may need additional support, extra to the standard bracket, to resist gravity loads.

One of the features of the supported tray types 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.

 

15.4.2F Standing Seam Cladding on a Round Building.

 

 Source: UK Guide to Good Practice in Fully Supported Metal Roofing and Cladding 3rd Edition; © Federation of Traditional Metal Roofing Contractors

 

 

Some manufacturers have the facility to curve their trays in a concave or convex shape.

 

15.4.2G Roof-Wall Junction Without Needing Prickles

All tray roofs can be seamed at the junction of roof and wall, without the need for prickles.

 

15.4.3 Standing Seam Fixings 

Clips for tray roofing are normally single clips, fastened to the substrate with screws or annular grooved nails. The sole of the screw 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.

15.4.4 Standing Seam End Laps 

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.

 

 

15.4.4A Staggered End Laps

 

Source: UK Guide to Good Practice in Fully Supported Metal Roofing and Cladding 3rd Edition, © Federation of Traditional Metal Roofing Contractors

 

15.4.5 Sarking 

Sarking is commonly CPD, 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.

15.4.5A Ply Sarking Fixing Pattern

Wind ZoneEdges fix at 75 mm centresBody fix at 150 mm centres
Up to High Wind Zone60 x 2.8 nails or 8g x 40 mm screws60 x 2.8 nails or 8g x 40 mm screws.
Very High and Extra High75 x 3.15 nails or 10g x 40 mm screws75 x 3.15 nails or 10g x 40 mm screws
 

To allow for expansion, maintain 3mm gap between sheets.  At gutter line the ply should overhang the gutter line by 25 mm and remain square cut. Dormer valleys and valleys into spouting can be recessed or flat laid. 

15.4.6 Standing Seam Ventilation 

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.

 

15.4.6A Plywood Sarking Ventilation

Source: Plyco Ecoply Specification and Installation Guide

 

 

15.4.7 Standing Seam Roofing WInd Loads 

Clip spacings must be set out as per manufacturer’s data for the design wind load of the building. Because of the possibility of wind drumming in high wind conditions, pans over 400 mm in width are not recommended in Very High Wind Zones and greater.

15.4.8 Penetrations on Standing Seam Cladding 

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.

 

15.4.8B Avoiding Long Back Trays

Avoiding the use of long back trays helps achieve an aesthetically attractive solution for penetrations

 

15.4.9 Canning and Purlin Creasing 

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.  

15.4.10 Installing Standing Seam Cladding 

Firstly, 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.

Secondly, 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.

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.

 

15.4.10A Using Clipped Flashings Minimises the Need for Rivets

Using clipped flashings to minimise the use of rivets is a feature of a well-installed standing seam roof.

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.

15.4.10.1 Standing Seam Cladding Set Out 

With a wide tray profile, it is important to have matching distances from the adjacent rib 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 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.

15.4.10.2 Provision for Expansion — Standing Seam Cladding 

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.