UNIT 2 - Conventional & Inverted Flat Roofs

Conventional & Inverted Flat Roofs

COURSE 3 / MODULE 2 - FLAT ROOF STRUCTURES / UNIT 2 - CONVENTIONAL & INVERTED FLAT ROOFS

Constructing a non-accessible, conventional system flat roof with nailed plastic waterproofing membrane

1. The reinforced concrete slab surface must be dedusted with gaps filled and high spots levelled.
2. Apply a primer coat to the entire surface.
3. Position the interior drain base elements with factory-made insulation sleeves in slab penetrations and secure using sealing collars.
4. Install vapour barrier / vapour-retardant membrane to the sealing collar of the interior drain and to the attic walls to protect the insulation from moisture loads.
5. Lay the first insulation layer of expanded polystyrene in a staggered pattern to offset longitudinal joints.
6. Form the second, slope-forming layer of insulation above using factory-cut tapered boards with a minimum slope of 2.5%. Laying is carried out in a brick pattern i.e. in both directions offset from the bottom layer. The interior drain must be lowered with a minimum radius of 40cm in the surface of the thermal insulation to create a slope of 5%.
7. Since PVC board and expanded polystyrene foam are incompatible materials, a protective and separator layer must be laid on the surface of the insulation. This is done together with laying the 2 mm thick PVC plastic waterproofing membrane insulation in a single step, moving sideways in rolls. Fixing is carried out mechanically, by processing in precise lines, using galvanised clout nails to pierce the edge of the plastic membrane considering the density specified in the wind load analysis. The drilled clout nails are approximately 1 cm from the edge of the sheet (calculating from the edge of the clout nails’ heads). For a building height up to 20 m, 3 nails per square meter are required in the inner field, 6 nails per square meter in the edge field, and 9 nails per square meter in the corner field. The rolls are laid side by side with an overlap of 10 cm, starting with a longitudinally-realised offset. On the interior side of the overlaps, hot air welding creates a 3-4 centimetre weld strip over adjacent membranes, concealing the clout nail heads.
8. Weld the waterproofing membrane edges to the foil flashing at the top of the parapet walls. The parapet coping consists of impregnated wood panels with insulation between. Both faces of the parapet wall are insulated to eliminate thermal bridging.

Constructing an accessible, inverted terrace roof with torch-applied bituminous waterproofing membrane

1. Form the sloping substrate on the slab surface using lightweight concrete ensuring a minimum slope of 2% on the surface and 1% in the roof valleys. The sloped concrete begins at the edges of the square-form sealing collar of the interior drain, with a minimum initial thickness of 5 cm. A 5 cm thick extruded polystyrene insulation is placed under the sealing collar of the drain as blackout. Thermal insulation expansion joint fillers are installed around the slab perimeter at all wall connections. Finally, cast the lightweight concrete substrate.
2. The substrate is dedusted when the sloped concrete has hardened, and any gaps are filled and high points are repaired.
3. Prime the sloped concrete substrate and vertical parapet and superstructure surfaces using a cold-applied bituminous compound on dry surfaces or a bitumen emulsion primer on wet surfaces.
4. Construct approximately 10 cm coved joints (fillets) using trapezoidal profile insulation elements along all wall and superstructure interfaces.
5. Install the bottom layer with modified bitumen membrane perpendicular or parallel to the roof edge, commencing from the interior drain. Drainage outlets must be integrated with factory-fabricated insulation components. Secure the membrane by full surface torch-bonding, melting the bitumen substrate layer on the underside of the roll so the adhesive melts and bonds. Slowly roll the roll into the heated bitumen melt and press down smoothly immediately behind the unrolling section. Sheet edges should be sufficiently torch-heated to melt a 2-3 cm bead of adhesive extrude from the edges to join the lateral seams. Rolls are laid side-by-side with an 8 cm overlap, ensuring a longitudinal offset to prevent seam alignment between courses. For seams, it is again the torch-on technique that ensures proper fusion bonding.
6. Form the corner joints at the coves and fold the bottom bituminous sheet over the insulation wedges (fillets) at the interfaces. A splice (upstand) is then created along the parapet walls by folding the waterproofing membrane over the coves and fixing it to the wall structure. Extend the membrane upstand to a minimum height of 30 cm above the future walkway on the parapet walls and fasten mechanically. For lower parapet heights, terminate the membrane at a pre-formed metal flashing profile installed at the top of the parapet wall. Secure with battens and fix atop the thermal insulation.
7. When installing or bonding the second layer of modified bitumen sheeting, follow the same procedure as at the bottom layer, but offset the overlap seams by half a roll width (staggered seams).
8. Complete the corner joints at the coves, extending the top layer to cover the horizontal section fully.
9. Install the protective separation layer above the thermal insulation.
10. Install the extruded polystyrene foam insulation boards with staggered joints, laid in offset rows, ensuring the specified board thickness is achieved.
11. At the parapets, install protective extruded polystyrene foam perimeter insulation along the wall with a minimum thickness of 5 cm.
12. Layer the following: filter/separator geocomposite layer, 6 cm gravel ballast bedding, and 5 cm concrete paving slabs.
13. Install the top outlet bowl of the interior drains and the drain dome.
14. Finish the parapet wall cappings with sheet metal flashing, and mechanically fasten the bituminous membrane.

Inverted Roof Assembly for Intensive Green Roof with Torch-Applied Modified Bitumen Waterproofing

Assembly procedures are carried out using the standard methodology for accessible roofs, with the following differences:

1. Two layers of modified bituminous waterproofing membrane, each a minimum of 4 mm thick, must be certified as root- and rhizome-resistant as per FLL Guidelines.
2. Install a drainage mat above the protection and separation layer over the thermal insulation. This will function as a combined drainage and water retention layer.
3. For extensive green roofs, install another filter and separation layer before placing a growing medium or substrate layer above it. For intensive green roofs, add a granular filter layer above the filter and separation layer before placing followed by a geotextile layer, and complete the assembly with a growing medium layer.