Roof Trusses

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(Information in this document was gathered from an interview with a roof truss plant operator in Ontario)

Engineering and Lead Time

Trusses should be booked 4 months in advance to ensure that they are on site in a timely manner. Usually at this time it is enough to supply the basic building dimensions if finalized drawings are not available at this time. trusses are designed independently of building.

A preliminary design is made by the truss company with pricing, and this takes about3 days). Then, the drawings are sent to an engineer at the truss plate manufacturer, who does the final draft of the drawings and stamps them. This takes about one day. , yYou should plan for a week of design work before production can even begin.

In total, it will take 12 days for the truss design to be finalized.The truss company does not need to see the engineered drawing, (some farm buildings don’t require engineered drawings  anyways).  Truss systems from a truss manufacturer will be engineered, regardless of the size of the building.

Once the foundation has been poured and measurements are confirmed trusses can be built within 1-2 weeks!

Extra Loading

Trusses are designed with a standard ceiling load to account for the weight of insulation, cladding and light equipment that has its weight shared by multiple trusses (water, feed lines, small fans etc.).

However, in cases where substantial equipment will be concentrated on only a few trusses, those trusses will be upgraded. Examples of heavy equipment include:

  • suspended manure equipment
  • suspended cages
  • hoisting equipment in a machine shop.

 It is important to let the truss manufacturer know what sort of equipment you intend to install before trusses are designed.

As well, if you’re interested in harnessing solar power, your truss company needs to account for that. Solar installations substantially add to the roof load of a barn, so trusses need to be upgraded to support their load.

When it comes to load bearing, it is much more cost effective to do it right the first time, rather than having to go into the attic and add bracing when you want to add solar five years from now.

Usually it’s a matter of keeping lumber the same dimensions, but upgrading to a higher grade of lumber will increase the size of truss plates.

Similarly, if future expansion plans  mean that an addition to the building could have a different roof line and influence snow load, we recommend you  inform the truss manufacturer up front so that reinforcing won’tt need to be added after the fact.

Truss Plates

Truss plates, also called truss connected plates, are used to connect the premade wooden trusses. There are a variety of materials truss plates can be made from.

Most truss plates are galvanized to a minimum standard, typically G90 coating of zinc. A higher standard of galvanization G185 is available and has twice the amount of zinc as G90.

Stainless steel plates are available as well, both the G185 and the stainless plates are significantly more expensive, come in limited sizes, are limited to 20 gauge and must be made to order.

Another corrosion protection option is a duplex system where an embedded truss G90 plate is painted, or powder coated to improve protection. This is a good option in high stress joints where corrosion resistance and a thick truss plate are both needed. Hangers and fasteners can be G185, hot dip galvanized or Gold Coated.

It is important to use the correct fasteners with each product to prevent galvanic corrosion.

Truss plate corrosion is almost completely avoidable with a proper ventilation system, soit’s good practice to invest in proactive measures such as a proper ventilation, insulation and vapour barrier rather than buying expensive corrosion proof plates.

We strongly recommend you consult with your truss manufacturer and other experts to weigh the options if you are concerned that your truss system will still be susceptible to corrosion.

Truss Damage

Trusses loose structural integrity when material is removed from the bottom chord of the truss.

This means that notching is never acceptable, nor is drilling holes in the wide face of the bottom chord.

However, it is okay to drill pilot holes for fasteners vertically into the narrow face of the truss, and this is necessary for mounting equipment and won’t damage the structural integrity.

Span and Spacing

90-100 ft is a reasonable width for a free-span truss. Around this threshold it, you should alsohave an intermediate support.

In cattle barns, this might be a row of posts at the feed bunk, or in a pig barn it could be a central alley way. For very wide broiler barns it may be advantageous to have a solid centre divide so that the barn environment can be controlled more accurately.

If building under SB-11 in Ontario, truss spacing is required to be 4’ and even for engineer designed buildings 4’ is the standard for agriculture.

Pitch

4/12 is considered the most economical pitch for small and mid-sized buildings. On excessively large buildings a shallower pitch is necessary so that oversized load restrictions are not exceeded. If a steeper pitch is specified on a wide truss it is necessary to manufacture the trusses in two section. The top section will ‘piggyback’ on the lower section. This should be avoided if possible, as is significantly increases labour, crane and material costs.

Vaulted Ceiling

Vaulted ceilings are great for natural ventilation, however they can be double the cost with with free span trusses.

It is important to note that the interior slope of the truss can only be a fraction of the exterior slope. For example, with a 4/12 exterior pitch a 2/12 interior pitch is usually recommended.

Raised Heel

Raising the heel of the truss for an air inlet increases truss cost but can reduce the cost of framing the weather hood on site.

Truss Delivery

Your trusses will be delivered with an engineered drawing that specifies the requirements for installing bracing and the truss ties. Typically, the truss ties (‘hurricane ties’) are supplied by the truss company.

When trusses are delivered to the building site, trusses should be delivered in multiple piles so that the crane can reach the entire building moving locations a minimum amount. For work to flow well when installing (‘setting’) trusses, it is important to have the crane and truss piles properly situated.

Truss Erection

This image is taken from OMAFRA’s 1995 book “Construction Guide for Farm Buildings” and shows lifting methods for trusses of various lengths.

Roof Truss Diagram