May 12- How to select suppliersIn December we looked at essential design criteria and the huge impact that low carbon has on flat roof design. Part two looks at getting the work done and maintaining quality control.
WHO DOES THE WORK?
Single ply technology is not just technically different to other coverings such as bitumen and mastic asphalt, its supply is also different. Learning from the mistakes of other technologies, which allowed the reputation of high performing products to be undermined by poor quality installation, the first (mainly European) manufacturers based their route to market on the principle of registered installers who were required to undergo training before being allowed to receive materials. This rule applies today, accredited by the Single Ply Roofing Association, which approves the training course, assessment process and off-site facility of its members. SPRA also owns the sector apprenticeship programme. Further details are available at www.spra.co.uk
Once you have decided upon a product that suits your technical and appearance criteria, contact the membrane manufacturer and ask for details of registered installers nearby who work in small-scale residential projects. As well as the obvious checks for current public and employer's liability insurance, you can trim this list by calling in more details such as:
• Examples of similar work.
• Do you operate a quality assurance system?
• How long have you been trading?
• How long have you been registered with the membrane manufacturer?
• Do you offer an insolvency-protected workmanship warranty?
Make sure each is asked to price the same schedule of work (even if you intend to change it later, by mutual agreement). This will ensure you obtain genuine competitiveness. The schedule will be offered by the membrane manufacturer but you should ensure that it meets your needs (and those of Building Control)
A well designed and constructed structure is vital A neat chimney flashing will be durable
(courtesy of Sarnafil 'Roof Assured')
Next, look at each detailing situation and make sure you and the installer know how it will be tackled. Most details can be classified into the following:
• Abutments: abutting walls; parapet walls; door openings; steps; rooflights.
• Penetrations: soil and vent pipes; boiler flues; posts to support handrails.
• Terminations: eaves (where the water 'runs off at the edge'); verges (where there is a check-kerb to hold back run-off.
• Adjoining pitched roofs: flat above pitched (the typical 'mansard' arrangement); and pitched above flat (the typical rear extension scenario).
For all details, these basic principles should apply:
• Make sure the waterproof membrane extends at least 150mm upwards at all abutments and penetrations.
This will protect the adjoining materials (e.g. brickwork or joinery) from deterioration.
• Avoid air leakage at the interface to the non-roof element. For example, make sure the detail closes any gaps between the roof deck and a wall. This should be by using either the vapour control layer or the single ply membrane itself, not by relying on a sealant.
• Avoid thermal bridging: for example, make sure the insulation between roof and wall or pitched roof and flat roof is continuous, without a cold spot where condensation could occur.
• Isolate movement from the single ply membrane: the single ply is there to keep the rain out so avoid expecting it to stretch and recover where there is likely to be movement (for example at a pipe which passes through the roof).
• Get a feel for the sequence of operations: this will help to avoid unsightly appearance or wet trades such as rendering or painting from putting the roof membrane at risk.
As the self-builder you will want to maintain quality control, but don't pass up the opportunity to involve the manufacturer of the single ply membrane. SPRA requires a recorded inspection of all commercial work where a product guarantee is to be offered. In the private residential market this is reduced to a suitable random inspection of a reasonable proportion of the installer's work. The installer must also self-certify their work and maintain a photographic record. Do the same yourself, making sure to put the camera on 'record date' setting.
Generally, the key stages are as follows:
• Install the deck: plywood should be butted in broken bond pattern, with a 1mm/m gaps at edges (e.g. a 2.4m side should have a 2.4mm gap at the end) secured with ring shank nails at 150mm centres. Intermediate attachment should be at minimum 300mm centres. The falls should be provided in the deck, a minimum finished fall of 1:80.
• Install the kerbs, verges and slopes: good quality dry treated softwood, properly secured. Remember that perimeters are the line of defence against wind uplift forces.
• (Manufacturer-registered installer) Install the vapour control layer: continuous, undamaged, with taped and sealed laps and dressed up behind all insulation, so allow plenty of spare at the perimeter.
• (Manufacturer-registered installer) Install the insulation: in most situations, this is best mechanically fixed using thermal break (tube) fasteners, which is quicker, of lower environmental impact and less susceptible to changes in the weather.
• (Manufacturer-registered installer) Install the field single ply membrane: again, this may be best mechanically fixed but the alternative of adhesion to the insulation is often feasible. A specific type of facing to the insulation must be used if the single ply membrane is to be adhered.
• (Manufacturer-registered installer) Install the detailed single ply membrane: members of SPRA must have a comprehensive set of standard details available so you will know upfront how they will look and how they coordinate with elements adjacent.
Finally, never forget that final joint inspection with the installer, to pick up any snags and to agree any temporary protection so that there is no doubt as to how the roof was left before other trades started working.