The 1x4 s are usually applied right over the shingles once the sheeting and shingles overhangs are cut off.
1x4 spacing for metal roof.
To determine how far apart to set rafters measure the width of the metal panel and then the width of the first hill and valley of the panel.
However the screw placement diagrams commonly are based on the on center spacing center to center of the purlin rather than the gap between purlins to determine the length and spacing of the screws needed to prevent wind blowoff for various design wind speeds so pay close attention to that difference.
We do recommend it however.
Wider panels allow for greater spacing between rafters.
Twenty four inches is a common spacing of furring strips for a metal roof.
Trim the most common flashing for metal roofing is the ridge cap which is used at the peak of a roof.
A metal roof must also allow for some overlap on each side of the panels which will reduce the effective width of the panel.
Place corner pieces where panels will meet bends at the edge of the roofing.
Do all roofing projects require purlins.
Use 1 1 4 inch nails to cover the outline of the roof with strips of metal eave flashing also called edging.
Metal roof installed on 1x4s that is how the tin often gets put on homes up here on acerages and farms.
Purlins are non pressure treated 1 x 4 x 12 lumber that is installed on to the roof in a specific layout pattern and screw pattern.
See page 6 for more information on screw spacing and ordering.
Some contractors feel that 7 16 osb roof decking is a poor choice for the supplied screws preferring the solid 1x4 s.
If an open grid framed roof or widely spaced old solid roofing planks as the sheathing it is important to tightly stretch this so it does not sag down and hold water between the framing roof sheathing boards and the metal roofing obviously far better to put down a solid sheet sheathing first when doing a reroof in that sort of situation.
If there are gutters installed on the eaves the eave flashing should be installed to overhang the lip of the gutter by about 1 2 inch.