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technical & practice

Built-up membranes:

Layers of roof covering, usually bituminous impregnated rolls, laid in several staggered layers.

Single ply:

Waterproof layer laid in a single sheet as opposed to built-up membranes in several layers.Single ply usually refers to polymeric materials.

Polymeric sheet:

Man-made organic compounds (derivation:

polymers) formed by combining long chains of component molecules.


Polymeric materials that are susceptible to organic solvents and are temperature sensitive.

Soften at high temperatures and harden at low temperatures, although this is reversible.

Includes PVC (polyvinyl chloride), CPE (chlorinated polyethylene) and chlorosulfonated polyethylene (CSM) Elastomeric:

Polymeric materials that tend to be more resistant to chemicals and solvents. It is generally more elastic, and therefore more resistant to thermal shock, with better thermal recovery.

Includes EPDM (ethylene propylene diene terpolymer [monomer]) and butyl rubber.


Not compatible with bituminous materials.

Depending on the plasticiser, it is susceptible to softening in warm weather and becoming brittle in low temperatures.Thermal or solvent welded.


High-density polyethylene containing chloride.Flexible without the need for a plasticiser, is flame-resistant and compatible with bituminous products.Thermal or solvent welded.


Highly resistant to chemical attack and maintains colouration well.Difficult material to seal and joint.


A copolymer (comprising two different molecules in the component chain) of ethylene and propylene with an unsaturated diene monomer.Can be vulcanised and has good weathering capabilities, resistant to UV and ozone.

Butyl rubber:

Copolymer of isobutylene with isoprene or butadiene.Good weathering and durability.

Flexible polyolefin:

Polypropylene or polyethylene combined with flexible co-monomers to form a synthetic reactor blend designated as an 'alloy'.Normally thicker or stiffer than conventional single ply sheets they have excellent weathering and resistance factors.Excellent puncture resistance, chemical compatibility and high flexibility and recovery.

Adhesive fixed:

Partial bonding was common in bituminous material fixed to timber decks by bonding a layer to the substrate via a loose laid perforated layer or by spot bonding.This allowed the substrate to move. In fully or partly adhered systems, the membrane and adhesive must be compatible with the insulation in warm roof conditions.

Loose-laid Common in ballasted systems, the edge wind uplift must be catered for by restraints and fixings.

Mechanically fastened Edges of membranes fixed by screws and fixing plates, overlapped by follow-on layer sufficient to cover the fixing points.Overlapping edge would be welded or adhesive fixed to the layer below to ensure a watertight seal.

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