Our 10mm hollow soffit boards can be used either lengthways or cut into short pieces used side by side with the grooves running back to the wall - this is the most common method.To calculate how many lengths of hollow soffit you will need when using it in short sections, first measure the soffit gap in millimetres (the gap between the fascia and the wall). Divide 5000mm by this number. This will give you the number of pieces that you will get out of each full 5m length. Now multiply this by 300 and the resulting number is the length in mm along the wall that these pieces will cover.Example - soffit width is 200mm.5000 / 200 = 25. 25 x 300 = 7500, so one length of 300mm hollow soffit will cover 7.5m.Hollow soffit is secret fixed through the tongue of each board using 30mm cladding nails or countersunk screws.