Image: elZinc

Picture: elZinc

Zinc is an excellent roofing and cladding material which will give long and almost maintenance-free service if installed properly. Proper installation means not only the correct fixing of the zinc itself, but also the correct design and installation of the supporting structure.

This article, prepared with our partners at elZinc®, looks at some options for the supporting structure for zinc roofing, which traditionally provides a continuous support for the roofing material.

What is a Zinc Roof Build up?

The supporting structure for a zinc roof is made up in a series of different layers on site, known as the ‘build up’. Depending on the design of the roof, it can include, from outside to inside:

  1. Zinc;
  2. Underlay;
  3. Substrate (providing direct support for the Zinc);
  4. Air layer (ventilated/cold roofs only);
  5. Insulation;
  6. Vapour control layer (VCL) or vapour barrier, and
  7. The Principal load bearing structure.

Modern methods of construction can combine some of these elements, such as the substrate, insulation and VCL. Examples of this are given below.

A word about Underlays

Structural Underlay for Zinc RoofUnderlays sit between the zinc and its supporting substrate and can act as a separating layer, slip layer or protection for the substrate during construction.

Structural Underlays consist of an air-permeable, fibrous mat, lifting the zinc slightly (~8mm) off the substrate surface.

Combined with a breather membrane, a structural underlay:

  • Can act as a means of draining condensate from the underside of the zinc,
  • Can even out any slight unevenness in the substrate, and
  • is often recommended by elZinc® as they have proven beneficial to the longevity of zinc roofs.

Structural underlays are recommended in particular on warm roofs.

Cold Roof or Warm Roof? Ventilated or Unventilated?

The two designs most commonly used with elZinc® are ventilated roof and unventilated roof, also known respectively as “cold roof” and “warm roof”.

Choosing which design is the most appropriate for a particular project depends on many factors such as roof form, available height, cost, and aesthetics. This is best discussed on a project by project basis with our technical team.

However, here is a rule of thumb guide to get you started.

Ventilated Zinc Roofs

Ventilated (cold) zinc roofs work best with:

  • A decent pitch,
  • Simple geometry,
  • Adequately dimensioned air gap.

They are not so appropriate for:

  • Low pitched roofs (unless good cross ventilation can be provided, which limits the rafter length),
  • Roofs where the required height of the air layer is problematic,
  • Geometrically complicated roofs (where it is difficult to achieve enough air movement through the layer),
  • Where ridge details are required to be as discreet as possible,
  • Other factors mean the cost would be prohibitive.

Unventilated Zinc Roofs

Unventilated (Warm) zinc roofs are often used when

  • The pitch of the roof needs to be low
  • The roof has complex geometry
  • A slimmer roof construction is required.

However, a warm roof construction is more sensitive to the construction process itself. For example, warm zinc roofs should be avoided for installations over humid substrates which trap moisture. Warm zinc roofs can be particularly susceptible to improper installation of the vapour barrier which allows moisture migrating through the roof to condense (in cold weather) on the rear face of the zinc. As a result, good design, installation and site practices are essential.

Warm zinc roof construction can use timber substrates, but the need to reduce cold bridging in many countries has led to the use of rigid and composite insulation boards or sandwich panels as the direct substrate under the zinc.

Example Zinc Roof Build Ups

The following examples are just some of the many possibilities that exist. For advice on which build up would be most appropriate for your project, get in touch.

Ventilated Zinc Roof Build Up (Cold Roofs)

These designs introduce an air layer under the substrate which draws warmed, moist air out from under the zinc. This layer also helps to dissipate heat in the summer months, keeping the building cooler.

Air inlets and outlets are created at the eaves and ridge of the roof, using perforated elZinc® material as an insect mesh. The net area required depends on roof pitch and is given opposite.

If a structural underlay is required, any draining membrane installed below it should be a breather membrane, since any condensate will evaporate down through it and the substrate into the air layer, where it is drawn out from the roof via the outlet.

Zinc Roof (Ventilated) on a house in Bedfordshire

This elZinc® Rainbow Red Zinc Clad House in Blunham, Bedfordshire uses ventilated cladding and roofing with the ventilation entering at the bottom of the cladding and exiting over a shallow ridge vent.

Ventilated Roof on Sheathing

Zinc Roof Buildup Ventilated on Sheathing

  1. elZinc® standing seam tray
  2. Standing seam clip
  3. Underlay (structural underlay + breather)
  4. Substrate
  5. Ventilation path/air gap
  6. Air layer batten (height = air layer thickness)
  7. Breather membrane over insulation, sd<0.04m)
  8. Insulation
  9. Wooden Rafter
  10. Vapour Control Layer (VCL) with sealed laps
  11. Ceiling finish

Ventilated Roof on Softwood Board

Zinc Ventilated Roof on Softwood Board

  1. elZinc® standing seam tray
  2. Standing seam clip
  3. Underlay (structural underlay + breather)
  4. Substrate
  5. Ventilation path/air gap
  6. Air layer batten (height = air layer thickness)
  7. Breather membrane over insulation, sd<0.04m)
  8. Insulation
  9. Wooden Rafter
  10. Vapour Control Layer (VCL) with sealed laps
  11. Ceiling finish

Unventilated Zinc Roof Build Up (Warm Roofs)

These designs incorporate a high performance, and when required, self-sealing vapour barrier on the warm side of the insulation.

The effectiveness of this vapour barrier is of paramount importance to the roof, so:

  • Its installation should be carefully controlled on site
  • All joints and penetrations should be sealed
  • It should wrap around all edges of the insulation
  • It should always be installed over a structural deck

Composite insulating boards provide a timber decking that is factory-bonded to the insulation, so the installation of the zinc is executed using traditional clips and fixings.

Zinc Roof Unventilated at The Alchemist Salford

The Alchemist at Salford Quays is roofed and clad in elZinc Rainbow Gold Protect over an Admaster composite panel. The panel provides both insulation and acoustic attenuation, most appropriate for the complex geometry and the use of the building as a bar restaurant.

Warm Zinc Roof on Rigid Insulation

Rigid insulation is used in combination with special clips that enable the zinc to be fixed to the supporting decking below. The insulation should be stable up to 100°C, must be able to withstand foot traffic and not deform during the lifetime of the building. Plywood or similar can be used. Only one screw is used per clip, so pull-out values and clip centres should be checked, and deck thickness adjusted.

Vapour barrier: Typical Sd values should be ≥800m.

Unventilated Zinc Roof on Rigid Insulation

Close up of fixing in Unventilated Zinc Roof over Rigid Insulation

Key:

  1. elZinc® standing seam tray
  2. Standing seam ’warm–fast’ clip
    • Stainless steel grip plate
    • Spacer plug
  3. Structural underlay with water proof membrane
  4. Rigid insulation
  5. Self-sealing high performance vapour barrier
  6. Decking (plywood, OSB).
  7. Wooden rafter
  8. Ceiling finish.

Warm Zinc Roof on Cellular Glass Insulation

Cellular glass is mainly used in high humidity applications such as swimming pools, the cellular glass panels are 100% vapour tight which stops any vapour drive from the building.

Zinc Roof using Cellular Glass Insulation

  1. elZinc® standing seam tray
  2. Standing seam clip
  3. Structural underlay
  4. Bituminous waterproofing membrane
  5. Serrated fixing plate
  6. Hot bitumen topcoat
  7. Cellular Glass closed cell insulation
  8. Hot bitumen bottom coat
  9. Self-adhesive layer
  10. Trapezoidal roof deck

All information given here is indicative. The suitability of the trapezoidal deck will be dependent on the insulation system – talk to us about your requirements and alternative substrate options.

Unventilated Zinc Roof on Timber Insulation Board

The factory bonded insulating board is screw fixed through the vapour barrier into the decking below. The high performance vapour barrier must self- seal against these fixings, and all laps and roof penetrations must be taped and sealed. The exterior face of the insulating board should be exterior grade plywood or class 3 OSB, at least 18mm thick. The performance of the vapour barrier is fundamental in this design. Typical Sd values should be ≥ 800m and the vapour barrier must be suitable for laying over a trapezoidal deck.

Unventilated Zinc Roof on Timber Insulation Board

  1. elZinc® standing seam tray
  2. Standing seam clip
  3. Structural underlay with waterproof membrane
  4. Rigid insulation board
    • Plywood decking
    • Insulation
  5. Self-sealing high-performance vapour barrier
  6. Metal trapezoidal deck

Carrier Panel Warm roof

This approach is very popular with more complex buildings, but as is the case with all warm zinc roofs, it is vitally important that all panel joints are vapour-tight, the same is true for all perimeters of the panel installation.

Carrier panels have a thicker outside skin (≥0,7mm) to ensure pull- out values for the clips are maintained at 560N per clip or more. Installation should be according to the manufacturer’s instructions.

The standing seam clips should be fixed using fasteners able to achieve a pull-out value of 560N or greater per clip. The fixings of the carrier panel to the steel structure are normally visible from the inside.

Zinc Roof over Carrier Panel

  1. elZinc® standing seam tray
  2. Standing seam clip
  3. Structural underlay with waterproof membrane
  4. Metal sandwich carrier panel
  5. Structure

More Information

All these details and further information can be obtained by viewing the elZinc® Design Guide, Architectural Solutions in Zinc.

You can also download the details for all the various buildups by visiting the downloads section of the elZinc® Website.

Need help designing your zinc roof? SIG Zinc and Copper who supply elZinc® in the UK offer support free of charge for elZinc® and other metals projects in the UK. Talk to us about your project by dropping us a contact form or emailing us at info@sigzincandcopper.co.uk.

About the author – SIG Zinc and Copper Team

We are the team at SIG Zinc & Copper. We design and supply zinc, copper, aluminium and stainless steel roofing and cladding products all over the UK. We also publish blog posts! We're part of SIG Roofing. With over 120 branches nationwide, SIG Roofing is the largest supplier of roofing materials in the UK, providing our customers with impartial advice on the right roofing solution for their projects.

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