Case 2

Good Practices

Construction

Surface Preparation

Preparation of the roof deck surfaces is important but it is often taken lightly. Without a sound, even and clean surfaces, the membrane will not be able to achieve a good bonding with the surfaces. Rainwater will not be able to drain off properly due to the uneven surfaces even if there is sufficient design of falls and drainage system.

Surface preparation is simply done using broom to remove dirt and debris. Any protrusion or substances that may damage the membrane are to be removed. If the roof surfaces are uneven, a layer of 25mm to 50mm cement sand screed can be installed to make it smooth.

Application of Preformed Membrane

During the application of preformed membrane using torch on method, it is important that full and continuous bonding is achieved. There are namely two methods of membrane application. The following steps serve as a guide:

1. The torch is placed near the underside of the membrane. The bitumen will be heated until molten coating runs down to form a continuous flow of hot coating infront of the roll.
2. The membrane is then rolled forward into the flow of bitumen whilst the torching operation continues.
3. The flow of bitumen is maintained infront of the roll. This allows the membrane to achieve continuous bonding, and the flow of bitumen ensures air is not trapped under the membrane.

When correctly applied, a small bead of bitumen should appear at the sides of the laps which indicates that the membrane is well bonded. The bead can be concealed by sweeping loose material granules onto the bead while it is still hot. The membrane is rolled with a minimum side lap and end lap of 50mm and 75mm respectively.

Lapping

Bitumen roofing is applied with minimum 75mm end laps and 50mm side laps. The laps are staggered between layers to ensure maximised membrane security. The second layer is formed by offsetting the the second layer against half the with of a full membrane roll.

Application of liquid-applied membrane

Coats of fluid bitumen are applied on the roof surfaces with the use of brushes or rollers.

• The coats are applied till a minimum thickness of 1.2mm. 2 – 3 coats are normally applied. Two type of bonds for the different layers are used.
  • Full Bond: This method involves fully bonding the felt to the roof deck and subsequent layers of felt to one an other by the pour and roll technique. This method ensures against any wind uplift and forms the bond of succeeding layers creates a watertight joint.It used when strong wind uplift is expected.
  • Partial Bond: An additional perforated glass fibre based felt is used beneath the first layer. The perforated first layer is laid loose over the roof deck. The first layer of bitumen felt is then placed on top of the perforated glass fibre felt. They are bonded together by the use of hot liquid bitumen. The hot bitumen between the bitumen felt and glass fibre based felt is forced through perforations in the glass fibre based felt and adheres the bitumen felt layer directly to the structural deck. A partial bond between the bitumen felt and roof deck is thus formed. Allowance of thermal movement within membrane prevents problems like delamination and cracks.

• After application of the first coat, the second coat has to be applied in perpendicular direction to the first coat.
• The second coat must be applied while the first coat is still green but sufficiently set so that it is not drawn off while applying the second coat. Thus, a strong bonding between the layers can be ensured.
• A membrane up-turn of 300mm must be done whenever there are areas that need to be up-turned (e.g. parapet wall and pipes protrusion).
• The finished membrane is left to cure for a minimum of 24 hours prior to water ponding test.
• A sample board can be used to show client the finished thickness of the applied-liquid membrane. This will allow the client to have a cross sectional view of the membrane thickness.