When choosing a beam, it is necessary to take into account weather conditions in the region. If you live in the Saratov region and you know that in winter the temperature will not drop below 30 degrees of frost, then you can choose a beam with a section of 15 centimeters. But residents of more northern regions are better to choose a beam with a cross section of at least 18 centimeters. Such a beam is perfect for external walls. For internal walls, a beam with a cross section of 10 by 15 centimeters is usually used.