|Carpentry for Vocational Schools - A Teacher's handbook (GTZ, 252 p.)|
A timber is nailed across the endgrain of the lowest batten. From the outside top edge of that batten, where you have nailed the timber across, you have to measure down 70 mm. It should not be more or less than 70 mm. If it is less, water could pour down between the gutter and fascia board. On the other hand, if it is more than 70 mm, the water could overshoot the gutter. Make a mark on the timber which you nailed across that batten and drive a nail in. At the other end of the roof you repeat this procedure and stretch a string between the two nails. Make sure the string is tight so the roofingiron will not touch it.
Now you have a straight line and can start laying the sheets, the lower edge of the sheet has to be in line with the string. One worker has to stand directly below the string and direct the worker on the roof to move the roofing exact position above the string.
NOTE: The battens have to be exactly straight.