- Design guides
- Flat roof design considerations
Assessing requirements of a refurbishment roof
The following factors should be taken into consideration when undertaking the refurbishment of an existing flat roof system;
Condition and nature of the existing deck
The first thing to examine when considering a refurbishment of a flat roof is the condition of the deck and supporting structure. If this is at all questionable, a structural survey should be commissioned.
Generally speaking, decks made from chipboard or strawboard should always be replaced as their lifespan is limited and any water ingress that may have occurred will have reduced their load bearing strength. These products would normally be replaced with a suitable grade plywood or OSB. Metal decks should be checked for signs of corrosion and deflection; and concrete for cracks and movement. Woodwool is now classed as a fragile material and should be treated in accordance with the latest Health and Safety guidelines.
Condition and nature of the existing waterproofing
It is important to determine the condition of the roof so that the correct remedial action can be taken. There are three main methods of diagnostics available:
Core samples - Investigative core samples assess your current roof construction and the condition of the substrate. This diagnostic method can be completed by your area technical manager and is included as part of our no obligation survey.
Moisture mapping - Non-destructive method of testing for trapped water within a flat roof build up. A moisture gauge is used to pinpoint the presence of dampness within the roof structure to depths of 300mm with the results delivered as a topographical map.
Thermographic imaging - Non-destructive testing method that identifies defective roof areas currently allowing water ingress, which could remain undetected using standard inspection techniques. Thermographic surveys are conducted using infrared technology to identify thermal irregularities, with recordings converted into a clear image of the roofs condition.
Overlay or strip the existing waterproofing?
If the intention is to overlay the existing roof system, the following basic criteria should apply:
1. The existing roof system should be generally sound, dry, free from water damage and suitable to take an overlay system.
2. Overlay materials must be compatible with existing materials. In the case of Bauder bituminous systems generally means asphalt and in some cases other bituminous membranes. Bauder Liquid systems can be used to overlay almost all types of roof membrane. Synthetic single ply can also be used for refurbishment applications in some circumstances.
Other points to consider when proposing to overlay:
1. Waterproofing upstands and details should generally always be stripped and replaced.
2. There will be additional weight imposed on the structure by the installation of a new system.
3. An increased surface level will be imposed by the new system (especially when upgrading insulation) and its effect on adjacent detailing must be considered. Upstand heights should not be compromised and it may be necessary to raise DPC’s, cavity trays, edge kerbs and fascia boards etc.
4. Products being overlaid may well have a shorter life expectancy than the new system and ultimately cause premature failure of the overlay system. For this reason, Bauder generally recommend that existing systems incorporating insulation are stripped rather than overlaid.
5. Building Regulations now require insulation to be upgraded in most overlay situations.
The existing roof coverings should always be removed if there is any doubt over their long term integrity or if moisture is present; this may even include any underlying screed. Consideration must be given as to whether stripping the roof will also mean removing the drainage falls, if so, it will be necessary to incorporate new screed, or more conveniently tapered insulation to re-introduce new falls and maintain adequate drainage of the roof.