Case Study: Lightweight screed insulated roof

Project Background

The project posed an interesting technical problem. The client has an existing three storey steel frame house in Pretoria. It has an accessible roof space of 12m x 12m. The client uses the roof space for social events. The roof slab consisted of waterproofing on screed to fall on a 110mm concrete slab in permanent steel shuttering. The waterproofing was exposed to regular foot traffic which is not ideal. There was no insulation for the concrete roof slab and thus the internal temperature of the living spaces directly below where hard to control.
Two design problems had to be addressed. Firstly, the concrete roof had to be insulated to SANS 10400 XA requirements (i.e. R value of 3,2). Secondly, a tiled finish was required on the roof for ease of maintenance and the protection of waterproofing.
Because of limited floor to ceiling space and the unique aesthetics of the permanent steel shuttering, adding insulation board to the slab soffit was not an option.

For a new building the order of finishes (from top to bottom) would typically have been:

  1. Tiles
  2. Waterproofing
  3. High strength concrete topping (say minimum 100 thick) to fall (this thickness is required for the topping to have its own structural integrity as it sits on the boards)
  4. High density extruded polystyrene (XPS) insulation boards (e.g Isoboard)
  5. Concrete roof slab

The problem with the above solution is the weight of the concrete topping. A new topping here would have replaced an existing screed of about minimum 30 thick, thus a great addition of weight. We could not guarantee that the existing structure could handle the additional load.

We required a material with the following characteristics:

  • Strength and stability to receive waterproofing and tiling
  • Own structural integrity as it lays on the the XPS boards
  • Minimum weight
  • Laid to fall

The material chosen was the Politerm Blu lightweight mortar system by Isover Saint Gobain. The product consists of EPS (expanded polystyrene) balls mixed with water and cement. The balls are chemically treated to improve their mixing ability and to ensure an even distribution. The ratio of cement in the mix determines the final strength of the mortar. The Politerm Blu screed was installed by Advanced Waterproofing. See www.isover.co.za/products/politerm-blu and http://advancedwaterproofing.co.za

The final order of finishes (from top to bottom) was:

  1. Tiles
  2. Waterproofing
  3. Politerm-Blu lightweight mortar (mix ratio 250kg cement per cube), min. 100 thick, laid to falls.
  4. High density extruded polystyrene (XPS) insulation boards (e.g Isoboard)
  5. Concrete roof slab

Case Study: Lightweight screed insulated roof

The result was a insulated layer system with a total weight very similar to the original screed-only system.  The final tiled finish is very neat. The internal climate of the building is much more pleasant due to the increased insulation.

CONSTRUCTION IMAGES:

1. Laying of Isoboard on the existing, stripped, concrete roof slab. Note the conduits recessed into the boards to maintain a level surface to the top of the boards.

Case Study: Lightweight screed insulated roof

  1. The Politerm Blu lightweight screed being mixed by machine on ground level and pumped up 3 storeys to the roof.

Case Study: Lightweight screed insulated roof

  1. The lightweight screed mix being pumped out and levelled on the insulation boards (taped).

Case Study: Lightweight screed insulated roof

Case Study: Lightweight screed insulated roof

  1. The screed is floated to the required falls and left with a smooth, even finish. The small polystyrene balls are visible in the end-product (bottom left).

Case Study: Lightweight screed insulated roof

  1. Mineral finish peel and stick bitumen waterproofing being applied to the lightweight screed (prepared with bitumen primer). The mineral finish allows for tiling directly onto the waterproofing.

Case Study: Lightweight screed insulated roof

6. Cleaning of the finished tiled product.

Case Study: Lightweight screed insulated roof