Case 3

Good Practices

Design

Adopt design considerations to prevent potential construction and maintenance issues. Select the proper waterproofing membrane for the roof, with consideration for the type of roof loading, environmental
exposures, thermal insulation and aesthetics. Thickness of membrane in accordance with manufacturer recommendation (normally >1.5 mm). Provide proper detailing for bridging of membrane over expansion joints in accordance with BS EN 1107-2, BS EN 12039, BS EN 12730, BS EN 1108, SS 637 (formerly CP 82) or equivalent. Carry out evaluation of preformed waterproofing membranes for roofs. Refer to proper specifications in waterproofing in accordance with BS 8217, BS 8747, SS 374, SS 133 or equivalent.

It is essential for the designer to have an understanding of the effect of joint movement in order to select specifications which eliminate the risk of movement being transmitted to the waterproofing covering.

To prevent potential construction and maintenance issues, consider good design practices such as allowing proper flow paths to suitable discharge points, specifying a suitable waterproofing membrane, and using the necessary movement joints. Properly detail all openings, penetrations, upturns and corners to ensure watertightness in accordance with BS EN 1107-2, BS EN 12039, BS EN 12730, SS 637 (formerly CP 82) or equivalent.

Provide waterproofing (materials and details) to prevent water leakage through external joints in accordance with BS EN 15037-5, BS EN 13693, BS EN 12730, BS EN 13369, SS 677 or equivalent. To avoid the risk of movement being transferred to the waterproofing membrane, detail expansion joints in accordance with BS 093. Refer to the code of practice for structural use of concrete in accordance with SS EN 1992-1-1, SS EN 1992-1-2, CP 65-1 or equivalent for RC roof design. For precast roof slab design, refer to the guidelines in accordance with BS EN 15037-5, BS EN 13693, BS EN 13369, CP 81 or equivalent.

Ensure positive drainage in all low-slope roofs. Design roof gutter in accordance with the performance requirements set by BS EN 12056-3, SS 525 or equivalent. Roofing components should follow the allowable thermal stresses under temperature variations.

When preformed sheet waterproofing is specified, ensure proper lapping with min. 75 mm end-to-end and min. 50 mm side-toside. Waterproofing membrane when reaching parapet wall should turn up at
least 300 mm upwards. At congested or difficult areas for the application of preformed sheet waterproofing, e.g., near penetrations and vertical services and shafts/walls, a combination use of liquidapplied sheet waterproofing as well as integral waterproofing should be considered.

Design flat roofs made of reinforced concrete with an adequate fall to prevent ponding and maintain minimum pitch variation over the entire roof to avoid water ponding at the perimeter and water leakage through the roof slab/parapet wall junction in accordance with BS 6229, BS 8218, SS 637 (formerly CP 82) or equivalent. Ensure positive drainage in all low-slope roofs. Do not obstruct joints of areas prone to water, ice and freeze/thaw damage.

Construction

Before laying the liquid applied membrane, ensure that the surface is cleaned, dried, and free of surface defects/sharp protrusions (concrete surface to have a plain and even finish prior to receiving the membrane). Ensure that all openings, penetrations, upturns, corners, etc., are properly installed to ensure
watertightness. Additional consideration should be given to flashing and kerbs for movement. Allow concrete/screeding to cure for at least 7 days prior to laying of the membrane. Protect the waterproofing
membrane from traffic and weathering. It should be applied soon after the primer is cured in accordance
with BS 6229, BS 8218, BS 8000-0, SS 637 (formerly CP 82) or equivalent.

Use low-permeability concrete for roof structure construction. Mix concrete appropriately (maintain specified water-cement ratio) and ensure proper curing to prevent excessive evaporation, which can lead to defects.

Consider storing aggregates under shade in accordance with SS EN 1992-1-1, SS EN 1992-1-2, CP 65-1 or equivalent. Waterproofing and drainage should be coordinated to form an integrated waterproofing system. Maintain construction quality control to ensure proper curing of concrete prior to membrane application.

Ensure consistency of construction quality in terms of installation of fixtures and fittings, proper dressing of pipe penetrations, and good lapping and adhesion of the waterproofing membrane to the base with no mortar stains. Check for any signs of leakage during construction. The sealant material used should be tolerant of climatic variations on-site and be able to accommodate high movement if necessary. Joint sealing slots need to be cleaned (i.e., remove all dust). Proper installation of a backer rod is necessary to
avoid sealant adhering to the base of the slot. Tightly pack joint filler to a slot.

Consider providing an upturn at expansion joints (with metal capping). Expansion joints should be properly constructed in accordance with BS 6093.

Laying waterproofing membranes on a roof requires skill, experience and patience. The moisture content of the substrate must be monitored to ensure proper adhesion. Conduct proper surface preparation. Special attentions must be paid to locations with openings, penetrations, upturns, corners and vertical services and shafts/walls. When mixing adhesives, ensure the manufacturer’s instruction is strictly followed. Where preformed sheet waterproofing is specified, ensure proper lapping for both end-to-end
and side-to-side as well as an upturn tucked into a parapet wall according to the specification.

Ensure quality waterproofing by maintaining adequate and correct fall directions and angles to gutters in accordance with BS EN 12056-3, BS 8490, BS 8000-0 or equivalent. The waterproofing and drainage should be coordinated to form an integrated waterproofing system.

Carry out visual inspections and ensure that all gutters are tested for ponding upon construction in accordance with BS EN 12056-3, BS 8000-0, SS 525 or equivalent.

Drainage of flat roof

A roof will not be able to serve its function, resulting in ponding of water unless means for rainwater flow are provided. The main components of rainwater goods are rainwater gutter and rainwater downpipes.

The removal of rainwater can be effected in 3 ways:

1) by providing at the eaves or low points of flat roofs, gutters of either metal, asbestoes cement or plastic. From outlets in the bottom of the gutter, the water will pass through rainwater pipes into the stormwater chains at the foot of the wall.

2) By forming falls to flat roofs and inserting at the lowest point a rainwater outlet which is securely sealed to the weathering membrane. The outlet will discharge into vertical rainwater pipes that in turn discharge into the stormwater drain at ground floor level.

3) By collection through parapet walls into hopper heads

Rainwater Downpipes

Downpipes convey rainwater from the roof gutters to underground drains, often through a back entry rainwater gully at the ground level. The size of the rainwater pipe should be at least the size of the outlet from gutter. A downpipe which serves more than one gutter have an area at least as large as the combined areas of the outlets.

General installation recommendations for corners, upstands, drainage points and pipes are as follows:

1) Lay horizontal roof surfaces first, but do not adhere with 30cms of any obstacles; this will ensure that the sheet can be folded back to enable work around the obstacles to be carried out.

2) All membrane sheeting joins should face drainage points as far as possible so that water does not collect between the sheet and lap tape in joint. This applies to vertical or near vertical flashing as well as other areas.

3) Work slowly and methodically on upstands, as these are the points on a roof where leakage may occur.

4) Whenever possible work from obstacles towards the edge of the roof. This will facilitate laying of sheeting near the obstacle.