Main features

Zinc is an excellent roofing 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.

In traditional roofing this structure provides a continuous support for the zinc, whereas in engineered façades, metal railing and point fixing systems are employed.  The information provided in this section can be used as a guide to the basics of zinc roof construction.

designing with zinc recortada

Traditional roofing

The building envelope is built up in a series of different layers on site.  Depending on the design of the roof, it can include, from outside to inside:

  • elZinc® cladding
  • Underlay
  • Substrate
  • Air layer (ventilated roofs and walls only)
  • Insulation
  • Battens or metal rails and wall brackets
  • Vapour control layer / vapour barrier
  • Principal load bearing structure

 satolas francia 2013 rojo 09 recortada

Layers in a traditional ventilated build up

vented roof with battens 3

  1. elZinc cladding
  2. Underlay
  3. Substrate
  4. Battens / rails
  5. Air layer
  6. Breather
  7. Insulation
  8. Structure
  9. Vapour control layer
  10. Ceiling

Layers in a typical unventilated build up

warm roof 3

  1. elZinc cladding
  2. Underlay
  3. Insulation (acts also as substrate)
  4. High performance vapour barrier
  5. Decking
  6. Void for services
  7. Structure
  8. Ceiling

Underlays generally

An underlay is installed directly under the zinc.  The underlay should be:

  • Stable between -20ºC and +80ºC
  • Not stick to the zinc
  • Stable for up to 3 months outside in the sun.
  • Conform to EN 13859:1 and 2

Its functions can include:

  • Acting as a separating layer
  • Acting as a slip layer
  • Substrate protection during construction
  • Draining condensate from the underside of the zinc

Structural underlays

Structural underlays are commonly made of an air-permeable, fibrous mat, and are installed directly over the substrate or over a membrane.  This mat lifts the zinc off the membrane or substrate below, providing a thin air layer (≈8mm) which allows air to get to the underside of the zinc, drying out any condensed water vapour and preventing possible underside corrosion problems.  If the mat is combined with a breather membrane (some types are available loosely bonded to a breather membrane) or other type of waterproofing membrane, it also facilitates drainage of condensate (or indeed any rainwater that penetrated the zinc in heavy weather) to the gutter.

Structural underlays are recommended by elZinc® on all roof types as they have proved beneficial to the longevity of zinc roofs.  They also:

  • Reduce friction between the zinc and the membrane
  • Reduce rain drumming noise by up to 8dB.
  • Even out slight unevenness in the substrate

Higher standing seam clips must be used to compensate for the height of the mat.  It is also important that operatives use a piece of plywood or similarly stiff board when kneeling on the zinc to spread the load and prevent the zinc from denting.  The same applies to any other point loads.

Breather membranes

Breather membranes that are used on elZinc® roofs and façades should have a maximum Sd value of 0.04m.  Sd is the symbol for the Equivalent Air Layer Thickness, in m.  The smaller the Equivalent Thickness, the less the resistance to moisture vapour transmission.

Peel and stick self-adhering waterproofing membranes

Peel and stick self-adhering waterproofing membranes with rubberized asphalt adhesive are commonly used on warm roof applications over metal, plywood, OSB, etc. substrates. The underlayment must self-seal around punctures such as nails, screws and staples and have a non-slip surface. Asphalt membranes are ideal for warm roof underlayment and are waterproof, protecting the roof from the weather until the zinc is installed.  This can be especially important in countries with cold winters (such as Canada) where zinc work stops during the winter months, and in general in markets where commonly some time elapses between substrate completion and zinc work commencement.

Substrates and supporting material generally

The substrate provides the structural support for the zinc, and generally the standing seam or flat lock clips are fixed to it.  It should provide a minimum clip pull-out value of 560N.  Surfaces that are single plane in geometry are simple to construct, curved surfaces can require a multi-layer approach of curvable thinner sheathing.  Double curved geometry is best achieved by layers of softwood boarding.

Open gap boarding


This is formed from solid softwood boards of zinc compatible species fixed to wooden supports and should:

  • Be fixed parallel to eaves, with a 2-3mm gap between boards, using countersunk screws or nails driven to just below the surface.
  • Use rough sawn boards 80 to 140mm wide, conforming to EN12775-2, with a moisture content of 18% or less.
  • Be treated with zinc-compatible wood preservatives.
  • Should have a max. difference of 2mm (+/-1mm) in height, both between boards and over the width of each board.

Board thickness depends on rafter spacing, but is generally around 22-24mm.


Plywood, OSB or particle board sheathing is fixed using countersunk screws or nails to wooden or metal supports, driven just below the surface.  The boards should fixed as shown in the drawing and:

  • Have a 2-3mm gap between them.
  • Be protected from the rain before the elzinc® trays are installed.
  • Be supported at ≈600mm centres.

Sheathing board thickness is generally around 18 to 24mm, and the boards themselves are 1200 – 1250mm wide by 2400 – 2500mm long.

Plywood should be specified as structural plywood according to EN 636:2003, class 2.  This is suitable for external protected applications.

Orientated strand board (OSB) should be specified as complying with EN 300:2006 OSB/3 load bearing boards for use in humid conditions

Particle boards should be specified as complying to EN 312:2010 class P5: load-bearing boards for use in humid conditions.

Insulating boards

Factory bonded insulation boards made of plywood and rigid insulation are used on warm roof designs.  They avoid cold bridges and are quick to install, and provide a plywood or OSB deck for the elZinc® trays to be fitted to.

Rigid insulation

Rigid insulation is used on warm roofs and obviates the need for a timber substrate.  The elZinc® trays are fixed through it to a deck below using a special clip.  The insulation should be able to:

  • Remain stable at temperatures of up to 100ºC
  • Resist the compressive loads of foot traffic and kneeling operatives such that the elZinc® trays are not deformed.

Sandwich panels

Metal skinned rigid insulation panels can be used as a substrate in warm roof construction.  Their outer skin should be thick enough to ensure minimum clip pull-out values of 560N.  They have good spanning properties making them useful on steel structures, and they eliminate cold bridges. The inner skin acts as a vapour barrier, and so the joints between the panels must be effectively sealed.

SIPs (Structural insulating panels)

Normally used on warm roofs over laminated wooden structures which take advantage of their spanning capabilities.  The exterior wooden skin should be made of suitable grades of plywood, OSB or particle board and a minimum of 18mm thick.

Consult SIP’s manufactures for advice regarding vapour control.

Profiled metal decking

Installed to support traditional façades where its non-combustibility allows it to be used on taller buildings where fire regulations prohibit the use of wooden boards or sheathing.  It is always installed ‘façade side outwards’ to give the zinc cladding the most support, and perpendicular to the longitudinal seams of the cladding.  Its fixing must allow for thermal expansion and contraction.  It should be thick enough to ensure a minimum clip pull-out value of 560N.

Substrate supporting materials

Metal profiles

Made of either extruded aluminium profiles or galvanised sheet, they are recommended to have a fixing face of 60mm.  When supporting sheathing, they are set at centres of around 600 to 625mm, tying in with commercially available sheathing board widths and lengths.

Wooden lathes

Used more commonly to support soft wood open gap boarding, they should be regularised and appropriately treated with preservatives.

Wooden battens and blocks for zincwork

Wooden battens, blocks etc. should be treated with zinc-compatible preservatives, and have a moisture content less than 22% at the moment of zinc installation.

Thermal design – roofing

Two designs are commonly used with elZinc® – the ventilated roof and the unventilated roof, also known respectively as the 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 department.   However, as a guide the following general comments are made below.

Ventilated (cold) 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 drive to get the air moving through the layer).
  • Where ridge details are required to be as discreet as possible.
  • Where the cost is prohibitive

Unventilated (Warm) designs are more sensitive to the construction process itself:

  • Installation over humid substrates which traps moisture.
  • 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.

However, their effectiveness is not dependent on roof geometry and a warm roof provides a slimmer roof construction.

Ventilated (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 below.

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.

Ventilated roof designs

Traditional ventilated design on softwood boarding

vented boarded roof with sub roof numero

Vented roof over wooden sheathing

vented sheathed roof nuemros

  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, sd<0,04m
  8. Insulation
  9. Wooden rafter
  10. Vapour control layer with sealed laps
  11. Ceiling finish


  • Open gap boarding ≤ 15º                Structural underlay optional
  • Open gap boarding >15º                 None required
  • Sheathed boarding all pitches       Structural underlay + breather


Fixed perpendicular to the direction of the standing seams with a 2-3mm gap between boards.

Substrate options (in decreasing quality).

  • Open gap soft wood boarding
  • EN 636 class 2 plywood
  • EN300 class 3 OSB
  • EN312 class P5 particle board

Air gap

Ventilating layer height is varied according to its length and its pitch.

Rafter length

Rafter length >10m                 10cms

Pitch < 20º                                8cms

20º< pitch < 60º                     5cms

Pitch > 60º                                4cms

Air inlets and outlets

Net air inlet

  • 3º < pitch < 20º          1/500th of roof area
  • Pitch >20º                    1/1000th of roof area

Net air outlet

  • 3º < pitch < 20º           1/400th of roof area
  • Pitch > 20º                    1/800th of roof area


Round holes or slots, approx.. 5mm diameter /width.

Vapour control layers

These are used to limit the moisture being drawn into the air layer, and are more important nowadays since ever increasing insulation requirements weaken the air drive through the layer.  Typical Sd values should be > 100m.

Breather membrane over insulation

Recommended for roofs on softwood boarding and pitched below 30º.  Its principal function is to protect the insulation from the possibility of condensate dripping onto it in adverse climatic conditions.

Unventilated (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.

Any membranes used under the structural underlay mat are either breather membranes, peel and stick type membranes or bitumenous waterproofing type membranes, depending on climate, market and local building practices.  Contact elZinc® or you local representative for specific information.


Unadvisable warm roof build-up – runs the risk of vapour barrier damage.

Ridged insulation substrate

The zinc is supported by rigid insulation.


The insulation should be stable up to 100ºC, and not permanently depress under foot traffic or kneeling operatives.

Vapour barrier

Typical Sd values should be ≥ 800m.


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.

  1. elZinc® standing seam tray
  2. Standing seam ’warm–fast’ clip
    • 2.1 Stainless steel grip plate
    • 2.2 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.

Substrate – Foamglas insulation

All information given here is indicative.  The system itself is the insulation and the vapour barrier.  Contact the manufacturer for project advice.

Substrate – Foamglas insulation

All information given here is indicative.  The system itself is the insulation and the vapour barrier.  Contact the manufacturer for project advice.

Trapezoidal metal deck

Foamglas should be contacted to verify the validity of the trapezoidal deck, as deflection limits are imposed on it by the insulation system.

  1. elZinc® standing seam tray
  2. Standing seam clip
  3. Structural underlay
  4. Bitumenous waterproofing membrane
  5. Serrated fixing plate
  6. Hot bitumen top coat
  7. Foamglas closed cell insulation (previously dipped in hot bitumen)
  8. Hot bitumen bottom coat (from dipping insulation boards)
  9. Self adhesive layer
  10. 10.Trapezoidal roof deck

Ridged insulation substrate

The zinc is supported by rigid insulation.


To be fixed according to the manufacturers’ instructions.

Vapour barrier

Typical Sd values should be ≥ 800m. Must be suitable for laying over a trapezoidal deck

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

Carrier panel warm roof


Standing seam clips

These should be fixed using fasteners able to achieve a pull out value of 560N or greater per clip.

Carrier panel

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.  It is vitally important that all panel joints are vapour-tight, the same is true for all perimeters of the panel installation.

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


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