Installation and details
What determines rafter size and spacing?
The cross-section and spacing of rafters are determined by calculation of the whole roof structure, not by one universal size. Check above all the span between supports, the anticipated loads, the timber class and the connections. Also consider deflection, not just strength, because excessive deformation affects the roof plane geometry and tightness. Insulation thickness should not be the sole reason for choosing the member height. A table of common sizes helps in understanding the design but does not replace calculations for a specific case.
Why does the rafter size alone tell you little?
A rafter of the same cross-section may perform in two roofs under completely different conditions. What matters is the distance between supports, not just the overall length of the member. A beam supported only at its ends behaves differently from one that is additionally supported on a purlin. The share of the eaves overhanging beyond the wall also differs.
The orientation of the cross-section and the actual condition of the timber also matter. Nominal dimensions do not replace measuring the member after processing. Notches, holes and local damage can reduce its load-bearing capacity. A table of common dimensions can therefore help in understanding the design, but should not be used to select the roof structure independently.
The way the ends are supported is also of great importance. A member does not act in isolation from its connections to the rest of the roof structure. A correct cross-section joined with random fasteners does not make a correct structure. The design must cover both aspects together.
What loads must be taken into account?
Permanent load includes the covering, the battens, the sheathing, the insulation and the attic finish. The plan must also account for equipment to be added later. Photovoltaics or an additional installation is another item to check. Lightweight metal sheets reduce part of the weight but do not remove the other actions.
Snow depends, among other things, on location and roof shape. Wind can press onto the slope or act in the direction of lifting it off. Particular load arrangements can occur at changes of slope and near higher parts of the building. The designer checks their combinations. It is not enough to assume a single average value for the entire surface.
Does closer spacing allow smaller rafters?
Reducing the spacing usually lowers the load on an individual member, but it also changes the entire system. The number of rafters and connections increases, as does the proportion of timber in the insulation layer. The wall plates, supports and battens must be checked. A saving on one cross-section does not necessarily mean a saving in the finished roof.
The spacing should also suit roof windows and the routing of installations. Matching it solely to the width of the insulation mat may be convenient on site, but the order of decisions is the reverse. First the structural requirements, then the details of the layers. For non-standard openings, designed trimmers are used instead of simply cutting through the obstructing rafter.
Why is deflection also checked?
An element may not crack under load and yet deflect too much. Large deformations affect the geometry of the roof slope, the cladding and the tightness of some connections. They can also hinder proper drainage. Strength assessment and serviceability assessment are two separate checks that cannot be replaced by looking at the cross-section alone.
In the case of timber, moisture conditions and the duration of load action also matter. One should not assume that a new element of random origin will behave the same as timber specified in the design. A documented material class is required. A biologically protective preservative does not repair insufficient timber strength.
How do you connect rafters with insulation?
Part of the insulation can be placed between the rafters, but the timber creates areas with a different thermal resistance than the infill. A continuous layer below or above the rafters limits the effect of these bridges. PIR boards are one option for above-rafter insulation. Their advantage is high thermal resistance at a relatively small thickness.
Such an arrangement requires the fixing passing through the insulation to the structure to be selected. The calculated fasteners must not be replaced with random long screws. The eaves, ridge, airtightness and connection to the wall insulation must also be resolved. The rafter height alone therefore does not determine the final thickness of the entire roof.
What should you prepare for a meeting with the designer?
Provide the roof drawings, spans, planned covering and the intended use of the attic. For a renovation, measurements, photos of the supports and information about damage will be useful. It is worth noting future solar panels and windows. The more complete the data, the lower the risk of changing the design after the timber and sheet metal have been ordered.
On site, compare the executed cross-sections and spacing with the documentation. Also check the connections, because they often determine how the structure behaves during strong wind. Do not change the dimensions just because the supplier has a different material available immediately. A substitute should be assessed for the same load and support arrangement.
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