Flat roofs waterproofing with polymer modified ,asphalt membranes
1. SBS modified asphaltic membranes were firstly used for flat roofs waterproofing in France during the early ’70’s. Styrene –Butadiene – Styrene is a thermoplastic polymer with exceptional flexibility which was found to modify asphaltic membranes in an optimum way.
SBS modified membranes are called elastomeric.
ADVANTAGES
a) Enhanced elasticity: resistance to creep and fatigue.
b) Very good behaviour in lower temperatures.
2. APP modified asphaltic membranes were firstly used for flat roof waterproofing by Italians round late ’60’s. Atactic polypropylene is an elastic type of polypropylene with disordered polymer chains. APP bitumen mixes are heat resistant with softening points up to 150oC.
APP membranes are more preferable in equatorial climates than SBS membranes due to their elevated heat resistance.
ADVANTAGES
a) Very competitive prices. Perfect value for money.
b) High resistance to heat and UVs.
Both APP and SBS give – with the use of modern technology – asphaltic membranes of very high quality and are widely used all over the world.
3. Non woven polyesteric reinforcements are the most advantageous ones to fully exploit the excellent characteristics of APP and mainly SBS membranes. Some spunbond polyester, continuous filament, needlepunched reinforcements really exhibit exceptional mechanical characteristics.
Not all reinforcements are the same! Actually there is an enormous span regarding qualities and don’t forget that reinforcement is the membranes’ armour.
4. When exposed to sun, both APP and SBS membranes need an autoprotection. Natural slates or artificial granules are the mainstream protection for both types.
Quality of asphaltic mass and type of granule facing will dictate the granule-loss profile for the years to come.Excessive granule loss is considered to be unacceptable.
SBS membranes can also be protected by metallic foils e.g. aluminium (anodized or not), copper, inox etc. Usual thickness: 80 microns.
French people have found the technology to harmoniously combine two materials with very different thermal expansion coefficients! Their patented technology, in essence permits the two materials to “slide” against each other independently.
Aluminium foils need a minimum slope of at least 3% and are prone to corrosion near sea front!
5. Flat roof slopes: Ponding water will sooner or later find its way through SBS/APP waterproofing membranes. Especially when it stagnates over the joints (overlaps).
To ensure a positive drainage, a minimum slope of 2% is mostly desirable. Well, in Greece at least, it is heard like a joke!
Another issue is with roof drains. Put them in the right way and in the right numbers to facilitate quick and efficient drainage.
6. Vapor barriers: Vapor barriers or vapor retarders are layers of minimal water vapor permeability placed most usually on the warm side of the flat roof to prevent condensation in the waterproofing – insulation system where dewpoint is met by rising water vapors.
Usually they come under a form of 3mm bituminous membrane, or an aluminium foil sandwiched between two bituminous layers.
To select the right vapor barrier one has to take into account:
- the local climate
- the moisture levels generated in the protected rooms.
7. One or two layers?: Opt for two layers. The reliability level is far superior!
Parallely displace joints of the two layers for at least 30cm.
Never use one layer when:
- slopes are insufficient
- protected areas contain valuable objects
- money is not the limiting factor.
8. Diffusion layers: Diffusion layers are obligatory in buildings where high moisture loadings are produced. They are placed directly below the vapor barrier.
Most of the times they are combined with appropriate flat roof ventilators so as excessive water vapor pressure is conveniently relieved. Water vapors are led due to pressure differentials to the ventilators through diffusion layer’s voids.
As with all roof penetrations, ventilators need utmost waterproofing attention.
9. Bonding to the substrate: Depending on each different situation SBS / APP membranes could be applied on the substrate:
- totally adhered
- totally independent
- semi-adhered (a compromise).
TOTAL INDEPENDENCE means less creep and fatigue and avoidance of local vapor pressures. On the other hand leaks location is difficult and heavy protection is needed.
TOTAL ADHERENCE incurs exactly the opposite facts and finally…
SEMI – INDEPENDENCE is a compromise between the two extremes.
10. Basic rules for the application of SBS/ APP waterproofing membranes.
Please check: “SBS – APP MEMBRANES: 9 FATAL MISTAKES”.
Chris Strogilis
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