5 Welding techniques

The overlap of several EVALASTIC® waterproofing membranes or with EVALASTIC® coated metal sheets is intended to provide a homogeneous connection of materials or welding, which is free from extraneous matter. The welding is almost entirely independent of the weather conditions.

The overlapping areas must be dry and free of dirt and debris. The contact areas are heated with hot air to a workable condition, or evenly wetted with solvent welding agent and fused with moderate pressure. If seam edges are welded over with the next membrane, a tape, or a preformed detail, they have to be chamfered over the whole width of the welding seam to avoid so called capillary effects (e. g. T-joints).

5.1 Hot air welding

The seams have to be seal-welded at least 2 cm from the top edge of the membrane.

hot air welding

5.1.1 Conditions

Hot air welding is possible without further measures at ambient temperatures of at least +5 °C. Depending on the weather conditions and the model of hot air welding machines the hot air temperature should be approx. 430 – 540 °C. By preheating the seam areas, welding is also possible at lower ambient temperatures.

Test welds are obligatory!

5.1.2 Hot air welding with welding machines

For the hot air welding of EVALASTIC® waterproofing membranes, all approved self-propelled hot air welding machines for synthetic membranes can be used 5.

With hot air welding machines 1.7 m to 3.5 m of seam per minute can be welded. The seams are welded in one operation.

Düse und Rollen einer Schweißmaschine

Bild der Temperaturanzeige eines Schweißgeräts

Depending on

  • the ambient temperature and possible wind
  • the temperature of the substrate and
  • the temperature of the waterproofing membranes

at the machine 6

  • the speed
  • if necessary, the volume of air and
  • the hot air temperature have to be adjusted

to ensure an even and homogeneous fusing in the weld.

Test welds are obligatory!

For welding machines without crawler drive, and depending on the weather conditions and the roof slope, a welding aid may be necessary to ensure a weld seam without wrinkles.

The alwitra welding aid, made of non-welding elastomeric material (150 mm wide and approx. 25 m long) is loosely placed onto the membrane immediately infront of the welding machine, level with the upper edge and is removed after welding for further use.

Welding aid

5.1.3 Manual hot air welding

All hot air guns 7 with an angled nozzle (approx. 40 mm wide), which produce the necessary hot air temperature of approx. 430 – 540 °C, can be used.

Bild eines Handschweißgeräts

The hot air gun is to be inserted at an angle of

  • approx. 45° to the membrane edge and
  • approx. 30° to the roof area

into the seam overlap.

Membranes are seal-welded in one operation by moving backwards, where as small tapes (up to 33 cm) are first tacked and then seal-welded.

When seal-welding, the nozzle must be inserted between the membranes in a way that the top membrane edge will also be heated and plastified.

The plastified lap areas will be fused by applying moderate pressure with a silicone pressure roller, following parallel to the opening of the nozzle. Due to the backwards movement, the nozzle always blows against already seal welded areas of the seam.

5 For further information please contact alwitra.

6 Please note the manufacturer‘s operating instructions, especially the connected loads! 230 V, 4,600 W, 20 A / 380 V, 5,000 W, 15 A.

7 Please note the manufacturer‘s operating instructions, especially the connected loads! e. g. Leister, Type Triac.

5.2 Welding of T-joints

T-joints must be formed very carefully, in order to prevent capillary action.

T-joints must be thoroughly welded with hot air (hot air gun) only.

5.2.1 Butt joints of unbacked membranes

Unbacked membranes are laid, correspondant to the longitudinal edge, with an overlap of at least 4 cm at the end of the membrane. The top corner of the membrane must be rounded off!


Before welding the T-joint, the respective inner (middle) membrane end must be chamfered over the whole lap width (min. 4 cm).

This can be done with the hot nozzle of a hot air gun, if necessary, with a metal plate serving as an underlay.

Bahn wird abgeschrägt

5.2.2 Butt joints of backed membranes

Backed membranes (EVALASTIC® V / VGSK), at the end of the membrane, are tacked or bonded with hot air with an overlap of approx. 2 – 3 cm 8 and covered with an unbacked tape 9 of at least 16 cm width. The length of the tape: membrane width + approx. 5 cm. At one end of the tape, both corners have to be rounded off.

Place the tape at the centre of the joint with the rounded-off end extending approx. 5 cm onto the previously laid membrane, which is through at the joint. The other side of the tape should end level with the membranes to be joined. Tack the strip in this position, in the middle, with hot air and seal it all around the perimeter.

Butt joints of backed membranes

The longitudinal seam edges of the membranes to be joined, which are under the rounded-off end of the tape, must be chamfered at a width of approx. 5 cm or 10 cm each. As must the longitudinal seam edges at the other end of the tape itself, at the width of the lap.

After chamfering, the strip will be seal-welded at its longitudinal edges and at the rounded-off end. Be especially careful when welding the chamfered areas with hot air.

The welding edge of the following membrane overlaps another longitudinal edge of the joined membranes and the tape in the chamfered area by at least 4 cm. Again, be especially careful when welding the chamfered areas with hot air.

chamfered areas


8 Advantageous at low temperatures.

9 16 cm tape width for mechanically fastened waterproofing membranes, e. g. if fasteners are placed at the butt joint.

5.3 Seam checking

After the seam has cooled down to the ambient temperature, the sealing layer from the seam edge (min. 2 cm) must be cohesive without any foreign matter over the complete welding width, and the seam has to be permanently waterproof and highly resistant. The performance of the roof sealing depends to a great extent on the quality of the seam.

In order to detect possible voids, the welding seams must be tested over the complete length, paying special attention to the T-joints.

Bild einer Nahtkontrolle mit einer Prüfnadel

A non-destructive test along the seam edges can be done by the roofer with an alwitra seam checker. 10

At voids the point of the seam checker can penetrate into the overlap. Seal-weld voids with hot air (hot air gun). Welded seams on coated metal angles can be tested only by means of a peeling test; therefore the sealing has to be cut.


10 Other test methods, such as vacuum testing, are normally not available to the roofer.

5.4 Leak test

5.4.1 Leak test by flooding

Leak testing of the roof waterproofing by flooding is possible. After completion of the roof waterproofing incl. seam testing and, if necessary, sealing, the roof area is flooded with coloured water (e.g. fluorescein sodium) for several days to an average height of at least 10 cm (=1 kN/m²). All outlets are to be temporarily closed. The water level on the roof must be checked every day and there also have to be visual controls carried out inside the building (e. g. also with UV lamps) and the results recorded.

After completion of the flooding test, the roof must be drained (be careful when opening the outlets!), inspection samples of the complete build-up structure are taken at the lowest points.

In case of leakages, considerable moisture may intrude into the roof build-up. 

A leak test by flooding is only possible if the additional load will not exceed the bearing capacity and the safe deflexion of the supporting substructure.

5.4.2 Leak test by measuring

Specialized organisations like building diagnostics companies provide leak testing of roof waterproofing using appropriate measuring methods. The results of the leak test should be included into the Acceptance Report.

5.5 Welding of weathered membranes

Generally, weathered EVALASTIC® waterproofing membranes may be fused with new material (EVALASTIC®) on site after years. Necessary technological changes (closing of roof penetrations, flashing of new penetrations to the existing roof waterproofing) and repairs of damage resulting from inexpert handling, can be carried out with the same material.

  • Where it is to overlap with the new material, the weathered surface must be thoroughly cleaned with low-surface-tension water (water with dish-washing liquid) and dried.
  • Precleaned areas must be wiped with a clean cloth soaked with alwitra membrane cleaner to remove dissolved particulate matter.
  • The overlapping areas must be absolutely dry and free of dirt and debris.
  • Weld the EVALASTIC® respectively EVALASTIC® R tape with hot air.

Test welds are obligatory!
Welding is carried out with hot air (hot air gun)!

Weathered unbacked membranes – if the underneath is clean – can also be welded to underlying new material. Humidity and moisture trapped under the membranes may impair weldability. The described pretreatment, particularly the drying, is essential for the welding area.