In the following example, we present the construction of a double purlin roof, featuring a hip roof on one side and a gable roof on the other. Construction shows a combination of traditional carpentry joints and engineered joints. Traditional techniques include the mortise-and-tenon joints of the posts, inclined struts, and knee braces, as well as the purlins, posts, and post plates. The rafters’ birdsmouth joint on the purlins also belongs to the category of traditional carpentry joints.
In contrast, engineered solutions include the anchoring of the wall plates and post plate with anchor studs, as well as the stud bolt fastening of rafter ties and collar ties. A monolithic reinforced concrete slab supports the roof truss.
1. Preparatory stages
The first step in constructing roof trusses is to select appropriate building materials for the project. This is important because achieving and maintaining uniform quality in timber is more difficult than with other construction materials. With improper storage, wood may deteriorate, deform, crack, become mouldy, be attacked by fungi or insects, or decay. It is therefore crucial to have proper storage, ensure a dry environment, and provide fire protection. Selected timber must be evenly planed, have a standardised cross-section along its entire length, and not be twisted, infected, or cracked.
It is essential to treat the purchased timber before construction with fire-retardant, fungicidal, and insecticidal solutions. Treatment can be carried out by soaking, spraying, or brushing the timber with the recommended dilution ratios, coverage, and number of layers as specified. The solutions are often coloured and are available in blue, green, yellow, or colourless forms. Timber elements, such as beams, planks, and battens, must also be appropriately stored on the construction site.
For adequate ventilation, they should be stacked with transverse battens with gaps on the sides (spacers). The stack must be elevated from the ground to protect it from water accumulating below. Above, it should be covered with a tarpaulin to protect it from precipitation, unless it is stored under a roofed area.
To accurately cut and assemble the elements preliminarily, a full-scale layout platform is prepared. This platform can be a suitably-sized, flat, horizontal paved or tiled surface. If space permits, it can be the uppermost floor slab of the building whose roof structure is being constructed. If these are not available, the layout platform can be created on site using beams and planks, sized so that the roof cross-section can be drawn on the surface at a 1:1 scale.
2. Marking out and preparing the roof structure
On the full-scale layout platform (or other suitable horizontal surface), a full-scale (1:1) drawing of the planned roof cross-section is created based on the actual built condition. The drawing must indicate the cuts required for making the joints. Elements whose accurate dimensions are not visible in the standard sections should have their elevation (side view) rotated into the projection plane according to the rules of descriptive geometry. Each element is then placed in its appropriate position on the drawing, and the cutting lines showing the parts to be removed must be marked on the rafters, beams, etc. The positions of the holes and the wood joints must also be marked. Tools used: carpenter’s pencil, quick clamps, ruler, carpenter’s square.
On a separate workbench, the lumber is cut to size precisely based on the markings, including the notches, cutouts, and holes needed for the joints. Tools used: hand drill, small screw-driving machine, circular saw, chainsaw, hand planer, chain mortiser, cutting guide, trestle.
After cutting and machining, the connected elements are returned to the layout platform together, with joint accuracy checked using trial fittings. If the joints are not accurate and significant gaps appear, the cuts must be corrected. If correction is not possible, the elements must be recut using new lumber.
3. Erection of the Roof Structure
Assembly of the roof structure members may only begin once the top floor slab and all masonry or concrete structures connected to the roof cast-in-situ such as party walls, chimneys, flues, etc. have been completed. A completed attic floor slab also requires the installation of the anchor bolts for the wall plates or post plates before concreting.
To begin construction, the beams and other timber members must be transported onto the slab. This is typically carried out by hoisting and bundling. On larger construction sites, the members may be lifted to the working level by crane, bearing in mind the quantities needed that can be safely accommodated.
The assembly steps are as follows:
Throughout the process of completing the entire structure, we use measurements and spirit levels for the precise placement of the elements and to check their horizontal and vertical alignments.
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The assembly begins with the positioning of the wall plate and post plates.
Here, we place 20–25 cm wide strips of waterproofing sheet, threading them onto the anchor bolts affixed to the ring beam.
Drill holes for the anchor bolts through the wall plates at the appropriate points, if necessary.
Next, put the wall plates onto the anchor bolts in the order defined by the connections.
Finally, secure the wall plates to the ring beam with washers and nuts.
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Next, posts and inclined struts are erected.
Posts may need temporary bracing in two directions.
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Place the purlins on the posts, reinforcing them with knee braces.
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Set up the principal trusses by installing the rafter ties.
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Fix the ridge beam with the collar ties.
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Erect hip rafters and attach them to the purlins.
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Finally, erect the general trusses, secure them with collar ties, and fasten the rafters of the hipped section.
