19 March 2021

Recently we faced an issue that hardly ever occurs in our work, but we consider it very important and it calls for a detailed discussion.

Look at the roofing picture below (Figure 1).

The roofing picture

Малюнок № 1 Figure 1
Запитували? Відповідаємо! You asked? We answer!

In the zone designated by squares and with the red arrows, you can see that the bitumen shingles have acquired a wavelike shape. This may suggest that the shingles were of low quality, leading in time to such a bad result. The personnel of the Aquaizol Plant Quality Department give a detailed explanation of what caused this defect. Analysing the roofing photo (a change in the photo perspective view, and comparing the distances between the waves with account of the scale), one can see that the character, shape and location of the waves on the roof slope follow the places of OSB butt joints (even horizontal waviness is present).

What is the reason?

The point is that, when installed on a roof, as shown in Figure 2, the OSB board has a minimal moisture content, conditioned by the board production process and storage conditions.


Малюнок № 2 Figure 2
Підкладковий килим Underlay
Гнучка черепиця Shingles
Вологість Moisture content

Of course, the moisture content of the material of a new board is much less than the air humidity.

However, since the air humidity is much higher than the board moisture content, the board moisture content increases during service until it equals the air humidity. This causes dampening and linear expansion of the OSB board.

The boards, due to the gaps between them, can move and, consequently, lead to the appearance of “waves” on the bitumen shingles along the short (more often) and long sides of the board as shown in Figure 3.


Малюнок № 3 Figure 3
Підкладковий килим Underlay
Гнучка черепиця Shingles
Вологість Moisture content
менше less than

Recommendations for avoiding the appearance of such a defect on the roof:

  • ⦁  Prior seasoning (conditioning) of OSB-3 boards before installing them. OSB boards must be kept outside premises for 1-2 weeks to be subjected to the effect of atmospheric air. In so doing, it is necessary to cover them to protect from precipitation, with bars placed between the boards for access of air to all parts of a board.
  • ⦁  Use for installing not whole OSB-3 sheets, but cut them to smaller pieces, for example, on the long side (2,500 mm) into 0.5-meter pieces. This will significantly reduce the linear expansion of each piece and prevent the waviness defect.
  • ⦁  Leave gaps no less than 5 mm between OSB-3 pieces.

Aquaizol – a long-standing reputation!