For the newcomer to roofing, measuring & ordering the roof sheets can be a tricky business if the slope is to be covered by a single sheet and there is no old existing sheets in place to measure. Measure too short and you are in deep trouble, too long and you have to hire a nibbler or other tools to cut the sheets back.

If there are more than one sheet down the slope there is no problem as the lap can be increased or decreased. See the sketch below. The sheets must not be allowed to clash at "Top Dead Centre" (TDC) and they must lap over the gutter to between 1/3 and 1/2 way across.

If re-roofing an existing building with the side cladding in position, then measuring from TDC to the edge of the side cladding is enough. By sliding the sheet back to provide cut back at TDC can produce enough gutter overhang.

If a new steel structure without side cladding, remember to add extra length to cover side purlins if they are to be on the outside of the structure, the depth of the side cladding and any insulation, plus the width of a facia board for mounting the gutter on should you decide to fit one.

Never trust the building as being true and square, particularly if it is an old one. Measure the pitch length at both ends and if it is an old one in the middle as well. In certain circumstance you may have to settle for a compromise length to fit differing measurements.

Laying the first sheet.

Many DIYers make the mistake of presuming the building is square and laying the first sheet true with the gable end and then find they have a saw tooth edge at the eaves. Always fix a builders line at the eaves as per the red line in sketch below. Stretch it tight and fasten it at the position you want the roof water to drop into the gutter. Lay and fix the first and all the sheets true and square with this line.

See all your fixing are in a straight line. Nothing can look worse than fixings out of line.