Three of these 4 metre lengths bowed by as much as one inch in the centre. I came up with the following procedure to straighten them.
1) Starting at one end and about one inch down, draw a string line from one end of the joist/beam to the other end, again finish about one inch down. This should give you a shape that is one inch thick at each end and two inches thick in the middle. Assuming that you started with a one inch bow in the joist.
2. Using a table saw cut right down this straight line from one end to the other.
3. This thin section of wood can now be lifted out and placed on the under or opposite side of the joist/beam.
4. Now apply some good PVA glue to both faces and screw the two pieces together with suitable length screws, about 3 inches.
If you don't like the sound of this then just draw it on a piece of A4 paper first. It is so simple.
Regards Chris McBrien.