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Concrete Roofs


General

This section refers to general low-slope (less than 5º pitch) concrete roofs screeded to falls and crossfalls to outlets where the roofing membrane is exposed to the elements and temperature fluctuations.
The waterproofing membrane, correctly selected, installed and properly maintained, is now considered as a permanent structural element with a life expectancy equal to that of the building it is protecting.


Substrates & Surface Preparation

General

Design and detail together with proper preparation of the substrate are key elements to successful roofing. Too often Waterproofing Contractors are compelled to install the waterproofing system on inadequate or poorly prepared surfaces.
It is good practice to ensure that ponding of water does not occur. Whilst the Derbigum membranes are unaffected by standing water, it should be noted that ponding, besides being unsightly, could result in deterioration of the final painted finish if painted finish was specified.
The substrate shall be to falls and cross falls of 1:80 to outlets. If no falls and cross falls exist (not recommended) a dual layer system must be used. The substrate shall be surface dry, clean and smooth; free of voids, protrusions and contaminants. The area is then to be primed using a bituminous primer. Internal corners shall be filleted and external corners rounded. Most waterproofing materials are bonded or applied to surfaces that are installed by other trades. It is essential to the performance of the waterproofing system that these substrates be structurally sound and free from excessive cracks, holes or projections. Certain curing compounds and finishes may affect or interfere with the performance of the waterproofing material. The use of oils, waxes and other surface contaminants should be avoided or the contaminants must be removed prior to waterproofing. The waterproofing contractor should visually inspect the substrate surfaces before the application of waterproofing materials and report any deficiencies so that they may be corrected by the responsible trade.


Concrete

Floated concrete surfaces are to be finished smooth without voids and/or protrusions. It is recommended that horizontal concrete decks cure a minimum of 28 days, or as specified by the material manufacturer, to allow moisture to dissipate from the top surface (forming systems typically prevent dissipation of moisture from the underside of horizontal decks) prior to applying waterproofing materials.

Form release agents and concrete curing compounds must be compatible with the waterproofing materials being used or must be removed from the concrete surface by the responsible trade. Honeycombs, tie-wire holes and other voids in the concrete substrate must be cut out and re-pointed with a non shrinking concrete patching compound. Concrete fins or other projections should be removed to provide a smooth surface. Horizontal concrete slabs should be free from cracks, voids, depressions, ridges and concrete droppings.


Sand & Cement Screeds

A sand/cement screed is the most widely used and acceptable substrate. It provides a firm and even surface to which the Waterproofing membrane can be properly sealed. It should be free of voids and protrusions, dry and clean, free of contaminants. The substrate should be stable and inert. Falls to outlets etc. should be formed in the substrate and not in the loading layer.

Lightweight Screeds

Foamed or expanded cement, polystyrene bead screeds etc. used to create falls and crossfalls are to receive a sand/cement topping and are to be properly cured and thoroughly dry prior to installation of the waterproofing membrane. It should be smooth, free of voids and protrusions, contaminants and undulations.

Ash breeze and vermiculite are not conducive to stability and should be avoided wherever possible as they may severely impact on the long-term effectiveness of any waterproofing system.

Priming

Generally all surfaces to receive Derbigum modified-bitumen membranes are to be primed with a bitumen primer. Allow solvents in the primer to flash off prior to application. Surfaces primed too far in advance which have gathered dirt and dust must be lightly re-primed prior to application of the membrane.
Priming procedures for other systems, such as acrylics, self-adhesives, flashings etc. are referred to in each particular section and/or datasheet.


Specification

Bituminous Aluminum Paint

It is the most widely used protective coating, with the particular advantage of excellent solar reflective properties.

Specification
One layer Derbigum SP4 waterproofing membrane, with 75mm side laps and 100mm end laps, sealed to primed surface to falls and crossfalls by 'torch-fusion' finished with two coats bituminous aluminum paint. Waterproofing to be installed by an Approved Derbigum Contractor under a ten year guarantee.


Specifications Aluminium paint Diagram


Acrylic Paint

Specification
One layer Derbigum SP4 waterproofing membrane, with 75mm side laps and 100mm end laps, sealed to primed surface to falls and crossfalls by 'torch-fusion' finished withone coat of acrylic primer followed by two coats acrylic paint. Waterproofing to be installed by an Approved Derbigum Contractor under a ten year guarantee.

Specifications Acrylic paint diagram


Crushed Stone

A layer of light coloured stone chips laid loose over the Derbigum waterproofing provides
a number of advantages:

A stone-guard or band of larger stones should be positioned around the outlets so as to ensure that the smaller stone is not 'washed' down the outlets.

Specification
One layer Derbigum SP4 waterproofing membrane, with 75mm side laps and 100mm end laps, sealed to primed surface to falls and crossfalls by
“torch-fusion” including protection of 50mm thick layer clean 19-25mm crushed stone on Interdek separation layer. Waterproofing to be installed by an Approved Derbigum Contractor under a ten year guarantee.


Specifications crushed stone diagram


Outlets

The waterproofing is to be fully dressed into the drainage outlets so as to ensure a watertight seal. Outlets are to be positioned so that they are easily accessible both for the Waterproofing Contractor at the time of installation and maintenance purposes at some future date. Ensure that the level of the flange is kept below the screed level to prevent ponding around the outlet.

Typical Fullbore outlet


Derbigum provides a flanged water outlet that is sandwiched between two layers of Derbigum thus forming an integral part of the system ensures complete drainage and is simple in its installation.

Specifications DrainageSpecifications dariange Diragram

Coves and Rounded Edges

All angles, internal and external, should be rounded. Provide a min. of 35 mm radius rounded cove at all internal angles. Horizontal and vertical surfaces are usually of different materials and are subject to different heat transference rates and thus differential movement.  The waterproofing membrane is subject to unnecessary stresses and practical installation problems if it is sealed into or over a 90º angle. Sharp edges and angles are to be avoided. The cove will greatly reduce these stresses and provide a neat finish.

Cove Diagram

 The basic principles to be followed:

Sketch one

Derbigum is sealed by 'torch-on' fusion over the cove and to the vertical surface to a minimum height of 100mm. (Sketch 1)

Sketch two


Turn-ups greater than 200mm high should be treated as turn-up and capping. (Sketch 2)

It is good waterproofing practice that all exposed terminations on vertical surfaces are either flashed or counterflashed. A variety of flashing methods and systems are available. The nature of vertical surface and aesthetic requirements will determine the flashing type.

Parapets & Kerbs

Attention to detail is important. The general principles involved are noted in the example sketches.
It is recommended that brick parapets be rendered and protected with one layer of Derbigum SP3 or SP4.

Specifications parapets and kerbs diagram

Specification
One layer Derbigum SP3 or SP4 waterproofing membrane, with 75mm side laps, sealed by 'torch-fusion' to side and top of parapet wall to receive paint (elsewhere)

Alternative

Acrylic waterproofing system to parapet dressed down over counter flashed Derbigum turn-up

Specifications acrylic waterproofing diagram

The top of the parapet is plastered with an inward slope and rounded edges.
A counterflashing of fibre-reinforced bitumen, metal flashing or similar over the Derbigum turn-up is recommended.        
   
Inverted Beams

Inverted beams should be capped in Derbigum similar to any kerb. Water should not be drained through an inverted beam. Where this is unavoidable, the opening is to be sufficiently large as to enable proper dressing of the membrane. Alternatively, a flanged outlet pipe is to be inserted through the beam from the 'higher' side ensuring that the screed level on the 'lower' side is sufficiently below the base of the pipe as to be able to achieve a proper seal at that point.

Plinths

Ensure that double or multiple concrete supports are constructed to such a height to allow adequate access both for the Waterproofing Contractor at the time of installation and for cleaning and maintenance at some future date.

Plinths or bases built on the waterproofing membrane are to be placed on an additional layer of SP4 and not directly to the waterproofing membrane.

Specification
One layer Derbigum SP4 fully sealed (as described for roof areas), up and over plinths with a sheet metal capping detail over the plinths to serve as protection to the membrane. Allow sufficient height for maintenance purposes (600mm).

Specifications Plinths Diagram

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