In the inverted insulation, the damp proofing membrane is placed under the thermal insulation membrane in the roof layering series. A necessary condition for constructing a reverted insulation, also due to the fact that the thermal insulation material is in direct contact with the water, is closed cuvettes, therefore the use of extruded polystyrene is the only possible solution. In order to protect and increase the lifespan of the thermal insulation material, a layer for its protection should be applied.

ADVANTAGES:

• The damp proofing membrane is protected from intense thermal fatigue, extreme contractions and expansions, thermal shocks, frost and the negative impact of acid atmospheric precipitation.
• Increasing lifespan of the damp proofing layer functionality.
• Protection from injuries and undesirable perforation due to potential localized loads (eg dropping of sharp objects during maintenance works of air conditioning).
• Limited accessibility and reduced chance of failure due to anthropogenic actions.
• It is not necessary to use a water vapor barrier.
• Moisture creation is avoided due to potential entrapment of water vapor between the barrier and the damp proofing layer.
• The use of anti-root membranes makes it possible to build a planted roof, increasing the feeling of thermal comfort in the underlying space. “

DISADVANTAGES:

• The progressive immersion of the thermal insulation material in water requires the use of a thicker thermal insulation material, in order to balance the reduction of its thermal resistance over time.
• Discharged rainwater causes rapid cooling of the ceiling (rain cooler effect), resulting in the need to use thermal insulation material thicker by 10% than on a conventional roof, in order to achieve the required thermal transmittance coefficient of the roof
• Requires sufficient gradients in order for the drainage at the damp proofing membrane level to be smooth.
• Requires a good preparation of the substrate in order to achieve an even drainage surface (without any points that withhold water)
• The cost of repairing the damp proofing is higher than for a conventional roof.
• The overall construction cost of the inverted roof is higher than that of a conventional roof.
• It is not recommended for old buildings. The roof’s weight, due to the need for a layer of accessibility, is much higher than that of a conventional one and the static resistance measurment is therefore required.”

CONSTRUCTION STEPS:

1. Evenning the surface either by scraping gravel or by filling small cavities with cement mortar.
2. Detailed cleaning of the surface from dust, oil, loose materials etc.
3. Priming the surface that is to be damp proofed with elastomeric bituminous varnish with aqueous base, enriched with synthetic elastomeric additives and anti hydrophilic ingredients for achieving maximum adhesion of the damp proofing layer on the substrate.
4. Coating bituminous type membrane with granulate (if it is the finishing layer). The membrane will rise up to the circumfirencial guardrails by approximately 30cm, where it will be either welded or mechanically supported with a blade.
5. Creation of trench flows (cement mortar or lightweight concrete)
6. Placement of the thermal insulation layer
7. Next is the sealing of the blade with bituminous polyurethane mastic.
8. Placement of a thermal insulation layer with geotextile
9. If required by the technical study, the accessibility layering follows eg a ballast thick enough to rule out any potential drifting of the thermal insulation panels due to the wind. “

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The damp proofing membranes belong to the worked-to-shape solid damp proofing materials category, they are industry originated materials and are applied in the construction site.

They are categorized under the worked-to-shape bituminous materials and worked-to-shape materials. The first category is bituminous sheets and bituminous membranes, while the second category consists of Synthetic membranes. ”

Bituminous membranes (Bitumen)

In the market you can find many types of bitumen such as plastomeric (APP), elastomeric (SBS), elastoplastomeric or Self adhesive, which mainly consist of the main body and the carrier. The main body of the bituminous membrane consists of a bituminous mixture and an addition of polymeric resins.

For example the addition of polypropylene Plastomeric resins (APP), creates a more plastic and heat resilient bituminous mixture, while adding elastomeric materials (SBS) creates a mixture more flexible and therefore more resilient to low temperatures.
The carrier acts as reinforcement in the bituminous membrane and determines the degree of its elasticity, improves its mechanical resilience in tensile strength, perforation resistance, bending resistance and increases its capacity of receiving the contractions and extensions of the substrate.

Bitumen is superior to the brushable damp proofing materials in the fact that basically they achieve a higher and uniform thickness of damp proofing.

“Bitumen has the following advantages:

• Higher and uniform thickness of damp proofing.
• Long lifespan.
• Easy application without the use of specialized equipment
• Easy maintenance and failure detection
• They offer limited accessibility
• Satisfactory resilience to static perforation
• Increased resilience to fatigue due to solar radiation (membranes with granulate or aluminum sheet finishing)
• Increased albedo and high reflectivity when using membranes with white granulate
• In some cases, there is a possibility of fully damp proofing a surface anew by appending on an existing (well applied) single layer of pre-existing bituminous membrane.
• Increased dimensional stability
• Fast application, easy to restore details (eg guardrails twists, solar water heating bases, etc.) in relation to synthetic membranes.
• Cheaper solution compared to synthetic membranes.

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The above categories may be reinforced or not and may be of glass fiber, fibreglass, polyester sheets or other synthetic materials. Synthetic membranes are an excellent choice for water insulation on roofs, especially if the roofs have large dimensions, so there is an increased need for the overlapping parts to be strongly welded.
They are also selected in cases where there are standing waters and the inclination cannot be layered anew, as they are particularly resistant to the fatigue caused by standing water.

The great advantage of synthetic membranes is that they exhibit a very high adhesion level on their compounds, a very long lifespan due to their increased resistance to solar radiation and very high elasticity.
The main difference between PVC and TPO membranes is that the latter do not contain plasticizers. Given the absence of plasticizers, the temperature required for the welding of the sheets between the TPO membranes is significantly lower. The absence of plastisizers provides stability in time, increasing both the anticipated lifespan as well as the speed of application. TPO membranes present more powerful mechanic resilience in perforation and injuries than the respective PVC membranes. PVC are always layered above polyester protection textile (geotextile) in order to avoid transfer of plastisizers to their substrate.

EPDM (Ethylene Propylene DieneMonomer rubber) are similar to the inner tubes of bicycle wheels, having additional chemical components with increased resilience in UV radiation. They present great elasticity, very strong weldings between the sheets, exhibiting high dimensional stability.

An EPDM disadvantage is the initial high cost of application which, in the course of time, is balanced out by the long lifespan of the membrane and the fact that it is more sensitive to injuries than TPO or even PVC.

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The cost of interrupted function can be considered as an indirect cost (eg drinking water tank in a hotel, hospital, etc.). In addition, in case of failure and continuous leakage, there is a continuous cost due to both energy and water consumption for the continuous filling of the pool or the tank. In addition, the adjoining spaces are significantly affected. ” «DAMP PROOFING OF POOLS WITH CEMENT BASED MORTAR” “The damp proofing of concrete is made from the inner side of the swimming pool with damp proofing cement based materials, preferably of crystallic structure and with deep penetrating properties. This damp proofing is a barrier to any leakage of pool water, but also to the inflow of any groundwater towards the inside of the pool.

These materials, de facto, cannot be elastic. It is well known that cement based elastic sealing mortars are only resistant to positive (and not to negative) water pressure. Therefore, they could not possibly prevent the inflow of groundwater. ” “The below require extra attention: The pool walls should not exceed the specified length without the provision for an expansion joint, since the next layer, the flattening cement mortar, is not as elastic as the underlying layer (the elastic cement damp proofing material) and usually cracks. The point of inclination change of the pool is the point where the pool begins to deepen.

It is a vulnerable point and should be investigated by the static engineer during the phase of the technical study, because of the abrupt and uneven burden placed on the deeper part of the pool in relation to its elevated section. At this point, there is usually a differential subsidence observed and a subsequent failure of the damp proofing layer.

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