Laying technique
The technique used to apply prefabricated membranes is an important factor on which depends the performance of the entire system. The choice is made according to the type of substrate, the degree of slope, the possible use of heavy protective layers and on the general make up of the roofing system. It must be reminded that there is a close interaction between the membrane and the substrate which, being made of different types of materials, react differently to the stresses to which are subject. In particular, elongation, shrinkage and elastic deformation and cracks of materials in contact with the membrane may be transferred partially or wholly. For this reason, whenever allowed by the degree of slope and other conditions, it is advisable to apply the membrane partially bonded to the substrate by uniformly spaced points. Depending on the situation, the membranes can be applied:
1) non bonded;
2) semi-bonded;
3) full
bonded.
In all cases, the type of application always refer to the first
sheet: in multilayered waterproofing systems, the subsequent layers are always
applied fully bonded.
NON BONDING
Non bonded laying, which
always requires the use of heavy protective layers, can be used on surfaces with
a maximum slope of 5% (3° approx.). The work is carried out by simply laying the
membrane over the surface and torch-bonding the sheets overlaps. The sheets must
in any case be attached to the substrate along the perimeter and to the vertical
areas, where it should be fully-bonded by torch.
SEMI-BONDING
Semi-bonding can be used on slopes not exceeding
40% (22° approx.) in case of rigid concrete substrates and not greater than 20%
(12° approx.) on substrate with thermal insulation covering. This type of
application consists the membrane to the substrate only at certain points or
along staggered strips. This technique is normally used to allow a continuous
pocket between the substrate and the membrane, thus permitting the circulation
and evacuation through air vents of the gases and air leaked through from the
substrate In this way the localised formation of blister is avoided. The number
of anchoring points or strips depends on the type of substrate and also on the
type of membrane. In all case the bonded area should never be less than 50% of
the total membrane area. To preserve the reliability of the head joints, it is
imperative the application of the membrane in complete adherence to the
substrate at least 1 meter from the joint on both sides. In the case of precast
concrete, a 20-30 cm wide strip of membrane spanning between the supporting
elements should be applied, torched only on one of the elements. On substrates
made of slabs of thermal insulation, two methods can be used:
-
50% spot semi-bonding of a first layer of light, fibreglass reinforced membrane, followed by the full bonding of a second membrane;
-
50% semi-bonding of a layer of bituminised fibreglass, followed by fully bonding of a membrane to the initial layer.
FULL BONDING
Full-bonding is carried out on rigid concrete
substrates with slopes greater than 40% (22° approx.). It foresees the full
bonding of the membrane to the substrate, following the instructions indicated
for semi-bonding laying. To ensure strong adhesion of the top edges of the
membrane to vertical or steeply inclined surfaces, it is advisable to use
mechanical fasteners such as flashing or similar elements. In these cases it is
better not to use membranes with fibreglass reinforcement as this does not have
enough elongation in the event of substrate breaking. In the case of substrates
covered with thermal insulation elements, the technique described above for
semi-bonding still applies, with the difference that the first membrane or
bituminised fibreglass must be completely bonded. For slopes greater than 20%
(approx. 12°), mechanical fasteners must be used as well.
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