Glued Roof Trusses

In my previous article, The Evolution of Glued Trussed Joists, in the August 2021 issue, I explain that glued trusses have potential in floors. The new G-joist has high resistance. The chord shear failure is eliminated as the web finger punches the chord. The web-chord glue area is big. The mean glue shear stress can be fixed at an exceptionally reliable level, less than about 44 psi (0.3 N/mm2). Such a small shear value is reliable even in earthquake loads. The G-joist can be used for roofs of residential and commercial buildings up to 100’ (30 m).

Glue is the cheapest, stiffest, and often the strongest timber fastener. Trusses are the most effective structural model in horizontal structures with the least material and the best flexibility.

Prefabrication and automation are demands for effective construction. This means that the horizontal structures, floors and roofs, are made of shallow joists with uniform height. The glued timber truss has the lowest depth-span ratio, and it is the most cost-effective, flexible, and ecological horizontal joist with uniform height.

Currently, glued trusses are not applied to roofs. In this article, calculations are presented to show that glued timber trusses have potential in roofs. A recent master’s thesis shows that a roof made of parallel chord glued trusses is more cost-effective than a glulam roof or a metal plate connected (MPC) wood truss roof up to 100’ (30 m), which opens a promising new business possibility. [For all Figures and the Table, See PDF or View in Full Issue.]

Glued Residential Roof Trusses

The MPC wood truss dominates the residential roof market due to its low cost, sophisticated technology, and flexibility to accommodate multiple geometric and structural forms. However, the MPC wood truss has some deficiencies:

• The MPC wood truss needs height, which means that the prefabrication of the complete roof structure is not feasible. The roof must be constructed on-site from prefabricated trusses.
• The MPC wood truss is not effective for a house with an attic, because either the trusses will be very expensive or the attic space will be limited.
• Having a roof opening in an MPC wood truss roof is complicated and expensive.

Glued timber trusses are not currently applied in residential roofs. Joists with uniform height, I-joists, solid joists, or MPC wood trusses are sometimes applied for residential roofs. The glued truss would apparently be more cost-effective due to low material cost, flexibility regarding supports and openings, and automatic manufacturing.

Off-site construction and prefabrication are strong drivers and trends in construction. Therefore, I believe that joisted roofs will gain more market in residential roofs.

The mono-pitch residential roof is the most cost-effective for the glued truss as it suits the off-site manufacturing of panel components. A double-pitched roof can be prefabricated effectively too if the joists can be attached parallel to the ridge (see Figure 1). A ridge beam plus joists along pitches is feasible, too.

Glued Roof Trusses in Commercial Buildings

A commercial building with 60’ (18 m) breadth, 120’ (36 m) length, and 17’ (5.2 m) inner height is used as a reference. The walls and roof are rigid diaphragms to resist horizontal forces. The roof is constructed using G-glued trusses, center to center (c/c) 1.6’ (480 mm), glulam beams c/c 20’ (6 m) plus crosswise panel elements, and MPC trusses c/c 2’ (600 mm). Columns are needed in the glulam building; these columns resist vertical loads only. The roofs are designed for self-weight and snow load of 46 psf (2.2 kN/m2).

Glulam roof vs G-roof

In both cases, the roof consists of 8’ (2.4 m) wide and 60’ (18 m) long prefabricated elements (see Figures 3 and 4). The measurements of the glulam girder are 46”–57” (1164–1440 mm) × 7.5” (190 mm), GL30c, c/c 20’ (6 m). The 60’ (18 m) long G-truss has a height of 31” (780 mm), chords are 2.4” × 7.7” (60 ×195 mm2), and webs are 1.8” × 2.9” (45 × 73 mm2) doubled at the ends, with five trusses per 8’ (2.4 m) element (see Figure 2).

Figure 3 shows the cross-section of the G-roof element. Soft mineral wool insulation or loose insulation is applied to enable simple application despite the inclined webs. Semi-hard insulation panels are at the sides to make a good seam between the elements. The basic structure is flexible:

• Additional trusses can be fixed in cases when extra resistance is needed, e.g., in point loads or in roof openings.
• A crosswise girder for extra support, roof opening, crosswise overhang, or point load may be added inside the element.
• The G-truss can be supported at the top chord without a hanger, which is beneficial in the beam support.
• The G-roof element can be fixed to the wall or a beam by processing above the element, with no fixing nor sealing below the element needed.
• The sealing between the elements is simple, and only the upper membrane must be fixed on-site.
• The seam includes a substantial horizontal assembly tolerance, while it is air- and moisture-tight without any sealing onsite.

Table 1 includes wood volume, approximate wood cost, and approximate CO2 emission for the G-roof and for the comparable glulam roof in 60’ (18 m), 78’ (24 m), and 105’ (32 m) spans. The calculation is based on the sawn timber cost of $700/MFPM (€300/m3), and the glulam and LVL cost is $1400/MFPM (€600/m3). The assembly cost of the G-truss is $1/ft (€3/m) in automatic production. The table includes the wood cost only; the lower and upper cladding and the insulation are the same in both structures, so these costs are excluded.

We see that, in all cases, the overall wood volume is about the same in all alternatives, but the G-roof is about $1.8/floor-ft2 (€20/floor-m2) cheaper as the G-roof consists of low-cost lumber only. The G-roof has lower CO2 emissions.

In the G-buildings, there are further advantages resulting in lower costs not considered in the data of Table 1:

• The wall area is about 1.070 ft2 (100 m2) less, meaning a cost advantage of about $1.4/floor-ft2 (€15/floor-m2).
• The volume of the building is less, resulting in lower heating/cooling costs.
• Columns and column foundations are not needed when the roof is supported on the walls.
• In both cases, the assembly of the 8’ (2.4 m) × 60’ (18 m) roof element is manual. The assembly of the G-element (Figure 3) costs less as it has only 8 wood components per element whereas the glulam roof element (Figure 4) has 75 wood components.
• The construction work is simple including the walls and the roof only; the building skeleton of columns, girders, and column foundations are not needed.

The factors listed above contribute further benefits for the G-building, making the overall advantage about 10% of the overall cost of a commercial building.

MPC wood truss roof vs G-roof

Next, the comparison is made between a 60’ (18 m) × 120’ (36 m) MPC wood truss building and a corresponding G-truss building.

The plated truss has a height of 30.7” (0.78 m) at the walls and the pitch is 1/5, i.e., the ridge is 25.6’ (7.8 m) above the floor. The cross-section of the plated truss building is given in Figure 6, with the ridge in the longer direction.

In the corresponding G-truss building, the ridge is at the same height but it is in the shorter direction. The cross-section is shown in Figure 7.

The G-building has a 340 ft2 (32 m2) higher wall area meaning it costs $3200 and further the G-trusses cost $6600 more. On the other hand, the G-building is constructed using prefabrication and automation which gives it a 10% advantage in the roof cost, $10.000. In this calculation, both buildings cost the same. However, the G-building has a 21,000 ft3 (600 m3) higher net volume. The overall cost is about 7500/floor ft2 (€700/floor-m2) meaning 4700/net-ft3 (€135/net-m3). This gives an advantage of $81,000 for the G-building. It is about 18% of the overall cost. Thus, in this calculation the G-building is superior.

On the other hand, the excess volume of the G-building may be useless. In that case, we can fix the volume of the G-building as being the same as the volume of the MPC building. Then, the height of the G-building is 3’ (0.9 m) lower. The wall area is 700 ft2 (65 m2) less, meaning $6500 lower cost, which compensates for the higher G-truss cost. The outcome is that the G-building costs €$10,000 less due to prefabrication and automation. The G-building has three further advantages:

• The G-building height is 3’ (0.9 m) lower, so it has lower wind and earthquake loads and it fits better in the environment.
• The G-building has a higher inner height in the middle of the building, which is advantageous in most cases, and often scissors MPC trusses are applied. However, such trusses are expensive and can’t be applied over long spans.
• The G-roof has lower CO2 emissions and a higher carbon sink.

Conclusion

This study shows that a building with a glued truss roof, applicable to commercial buildings up to about 30 m spans, is about 5% to 20% more cost-effective with lower CO2 emission than a comparable building with a glulam roof or MPC wood truss roof.

A professor of Civil Engineering for the Faculty of the Built Environment at Tampere University in Finland, questions about this article may be sent to Tuomo via email: tuomo.poutanen@tuni.fi.