Raising rafters

Start with the approved drawing and the structural calculation. From these you take the truss cross-section, centre spacing and span direction — these are designed, load-bearing conditions that you never guess at. Check that the delivery (size, strength class, CE mark) matches the calculation.

Cross-section, spacing and anchoring are structural matters — they come from the calculation, not from memory.

Check the wall-plate height, that the plates are level and plumb, and that the building is square before you raise. Measure the diagonals of the wall-plate frame — if they are equal, the base is square. A crooked wall plate carries through to the whole roof, so corrections here are cheap compared with corrections up in the rafters.

The rafter is the hypotenuse of a right-angled triangle: rafter length = √((span/2)² + rise²), and pitch = atan(rise / (span/2)). Set the length and pitch before you cut the first rafter — then all the following ones will fit. Remember that the sloping overhang is slightly longer than the horizontal measurement (overhang / cos pitch).

Even though you are not doing the design, it helps to know the loads behind the calculation: the characteristic snow load is s_k = 1.0 kN/m² on horizontal ground throughout Denmark, and the basic wind velocity is 24 m/s inland and 27 m/s in the coastal zone (closer than 25 km to the North Sea/Ringkøbing Fjord). Snow and wind are most often the governing loads — which is why anchorage against wind uplift is critical.

Set out the rafters' centre lines along the wall plate from one fixed starting point, cumulatively, so that the last spacing is not off. The centre spacing is given by the structural calculation — setting out is pure geometry (the fence-post principle), but you do not choose the cc value yourself.

Erect the gable trusses (or the two outermost trusses) first, plumb them accurately and brace them temporarily and firmly to stable points. The two outermost trusses are the reference for the whole roof, so take the time to get them exactly plumb and in line before the rest are set.

Stretch one line at the ridge and one at the top edge/overhang between the braced gable trusses, and set the intermediate trusses to the line. Hold each truss temporarily until it is anchored. Check continuously that the trusses are plumb and that the line is not pulled out of true by the trusses it is meant to guide.

Fix each rafter to the wall plate with an approved rafter hanger/rafter anchor that resists wind uplift. Number of nails/screws, bracket type and position follow the supplier's data sheet/ETA and the structural calculation. The choice of material follows the service class — e.g. hot-dip galvanised (Z275) in AK1, stainless (1.4404) for pressure-treated timber and outdoors (AK3).

Fill ALL bracket holes with the specified fixings — a half-filled bracket does not have its data-sheet load capacity.

Fit collar ties, struts/braces and any wind bracing according to the design, so the roof is braced both within and across the rafter plane. The temporary bracing may only be removed once the permanent bracing is in place and the roof is stable in itself.

Check that the ridge runs in a straight line, and that the overhang is the same all the way round. Finish with underlay, counter battens and battens according to the instructions. The batten gauge for roof tiles is pure geometry: the tile's covering length minus the manufacturer's overlap — but always look up the final gauge in the manufacturer's laying instructions.