Design with the Zinc

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.

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

Layers in a traditional build up

Layers in a traditional ventilated build up

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

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

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.

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.

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.

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