Architecture intouch

Adding an addition to your house to create an open plan living area.

The amazing Girder Truss

I’ve had many requests to write more on the topic of house construction after the first blog I did on load bearing walls and the two basic types of roof construction more commonly used. This blog is about existing roof support when you want to remove an existing external load bearing wall. If you haven’t already read my blog on load bearing walls it may be helpful as it explains the difference between roof trusses and framed roofs. It’s also worth noting that the girder truss also works for supporting older style pitched roofs.

These days it is very common to add onto the side of your house and create one large living area. This area could be a large family room or an entire kitchen / dining / living area. This design scenario requires a large part of the external wall to be removed and in this scenario I’m using here, the wall removed is 5.8m as the width of the addition is 6.0m overall the outside. The load from the 6.0m width of existing roof requiring support and the 5.8m span to support the wall removed is far too great for a solid beam to contend with.

It’s obvious to a lot of people that some kind of beam is required and this usually takes the form of a solid timber beam (for smaller spans), laminated timber beam (45mm deep strips of timber glued and pressed together in a factory) for longer spans, or sometimes a steel beam, or even a composite beam involving steel and timber such as a flitch beam, which is a length of thin steel usually between 6mm – 20mm wide sandwiched between two lengths of solid timber each around 45mm wide and up to 290mm deep.

All the above beams are heavy, ranging from about 13 kilogram per meter for a solid pine timber beam, to 45 kilograms per meter for a steel flitch beam and roughly 40 kilograms per meter for a steel channel. These weights all depend on the size required which is related to the length (span) and depth of the beam required.

I’ve prepared this 3D drawing above showing the typical way most people, including some Builders, Engineers, Architects and Architectural Designers and Council staff conceptualise supporting the existing roof (yellow trusses) when an existing external load bearing wall is removed. The blue walls and blue roof trusses represent a proposed new addition off the side of the original building (yellow), complete with polished wood flooring for effect.

From this, you can see with the section of external wall required to be removed, there is nothing to support the ends of the existing roof (yellow trusses). The most obvious solution is to install a beam in place of the missing wall and this is shown in orange.

The orange beam in almost all instances will be below the ceiling. In addition to the previously mentioned drawbacks to beams in this instance, is the beam interrupts the ceiling line and this creates problems with ventilation and acts as a visual division in rooms where you may wish to create a seamless feel to the space created by ate addition.

Beams are expensive especially when the span (length of the beam) goes over around 3.5 meters (the depth of the beam depends on the load carried by the beam and the span) and this is where the cost comes in. As an example, the building in the above [3D 1.] is the original building I used in the load bearing walls blog (yellow exterior walls, with green internal walls and yellow roof trusses). The width of this building is 6.0m with the length being truncated for convenience and space available in this blog. I’ve made the width of the proposed addition also 6.0m over all the outside walls and again the length is not important here. As I said previously, 3.5 meters is generally the maximum length a solid beam will span and this leaves only the laminated timber beam, steel flitch beam and or the steel channel as options, or are they?

Previous to writing this blog, I had a client with this exact situation and they insisted they needed a timber laminated beam. They had been told by a builder that this is what was needed and the timber beam made sense. I priced the timber laminated beam and the price was NZ$300.00 Dollars per meter, or $1800.00 for the entire 6.0m span. Steel flitch beams and steel channels while not priced at the time, would be more economical with the flitch beam probably being slightly cheaper of the 3 alternatives. The other problem with the laminated timber beam for this project was it would need to be around 450mm deep and this seriously divided the room as the underside of the beam was slightly lower than the top of a standard door.

I suggested to the client there was another way. I then proceeded to explain to them, my proposal which I had designed successfully many times before, and if I’m not mistaken, he fired me. This is what he said in an email to me, ‘If your not confident in designing our alteration I would be quite happy if you choose not to continue. That comment, I wasn’t expecting after40 years designing residential homes and additions, clearly he was convinced by the builder a beam was the only way to support this situation. I was debating weather or not to take him up on his offer, when I thought, no, I need to explain to him how girder trusses work as an alternative to a beam. After creating and sending my client a 3D image of his proposed alteration (similar to 3D.1 above) showing a girder truss fully supporting his existing roof, and backed up by my Engineer, I received a second email which read; “Thank you for the 3D image and full explanation of the girder truss system. This is why we hire experts”. Well, I was hired again which was a good thing… I think. I had few more surprises waiting for my client in the truss department which I will cover in another blog. My point is, people tend to believe builders as after all they are the people who build things. Unfortunately, their not always up with cutting edge technology. and in this instance the builder was about 40 years behind.

The Girder truss.

The girder truss is really not a lot different to a standard truss as shown in the previous 3D [3D 1.] (yellow trusses and blue roof trusses), but, they are a lot stronger due to a modified web layout (webs are the internal struts that look like the letter “W” or an “M’).

Firstly, let me explain where I get my information. Trusses first became popular in 1960’s New Zealand as an alternative to pitched roofs (refer my blog – Removing walls from your house – what to look for). The system or at least one of the systems were provided by a company called Gang-Nail New Zealand, another was Pryda New Zealand. These Companies did not invent the system, the Greeks did during the Bronze age around 2500 b.c.

Trusses are not only found in residential roofs but also bridges. The kapellbrucke bridge above is the worlds oldest surviving trussed bridge.

William Howe patented the Howe truss in 1840 (you can see the trusses I’ve used in my 3D drawing [3D 1.] are Howe trusses. A. Carol Sandford of Pompano Beach, Florida invented the metal truss plate called Gri P late in 1952. Gang-Nail New Zealand call their plates “Gang-Nail plates. There are many more instances of truss inventions and componentry on the web.

I developed my knowledge of trusses back in 1976 where I was working for a timber Company specialising in Pre-cut and Pre-Nail timber frames as well as trusses. I then went back into Architectural Design, (previously I had been involved in high-rise buildings for one of the largest Architectural firms in the Southern hemisphere), starting my own Architectural Design Company and fully utilising the truss system in residential applications. My roof designs were always designed using trigonometry and backed by the Gang-Nail New Zealand’s design manual. Girder trusses were mainly for supporting hip end trusses (I will create a blog on hip end trusses at a later date) and I saw an opportunity to use them to hold up existing roofs instead of using beams.

We have finally reached the point where you can see how this all comes together. Below is a 3D drawing [3D 2.] which shows the same 3D [3D 1.] but without the beam. In it’s place is a girder truss, coloured orange.

The first thing to take note of is the orange girder is completely above the ceiling line unlike the beam which hangs below the ceiling line. The girder has a different web layout to the other trusses and the bottom member (The lower horizontal part of the truss at ceiling level otherwise known as the bottom chord), is vertically deeper than the other trusses.

The existing trusses (yellow) have the overhangs removed to allow the orange girder truss to be placed hard against the outer edge of the existing exterior wall. The orange girder truss is then fixed to the ends of the existing yellow trusses with pre-made folded metal plates screwed to the girder truss. The plate incorporates a shoe and this supports the existing yellow truss. The girder truss is then fixed down to the top of the new wall (blue).

The amount of material in a girder truss is minimal, the timber is around 0.2 of a cubic meter, the equivalent of around 40 meters of 100 x 50 Pine or Douglas fir framing timber and around 22 nail plates holding the parts of the girder truss together. Nail plates are metal plates with teeth or spikes every 10mm or so and these are pressed over the timber joints to make up the truss. A girder truss spanning 6.0 meters as the one in [3D 2.] weighs around 60 kilograms or 12 kilograms per meter (slightly less than the solid beam) and can be easily handled by two people. The cost is a fraction of the cost of a laminated beam at about 20% of the laminated beam cost. For larger spans and or larger loads from the existing roof there are stronger girder truss designs as well as they can be doubled up by nailing two together.

The girder truss really is a win / win situation; they are light, easily handled and cost efficient. The also double as a roof truss performing both functions (support and roof framing) and in my Architectural design career I have used these hundreds of times without failure. The girder trusses are made at the same time in the factory as the main trusses and brought to the site together with the roof trusses.

Girder trusses are the unsung hero of roofing. They can even support other girder trusses sometimes girder trusses that support other girder trusses creating very complicated roof layouts. They are the main component in hip roofs sometime supporting 40 or more square meters of roof without fail. Another type of truss is a cantilever truss and these trusses can create covered outdoor spaces off the main roof without support posts. There are trusses that can create attic spaces as well as coved ceilings (coved ceilings are where the outer edges of the ceiling slopes up and then goes flat). Then there are scissor trusses which don’t cut anything but they create a sloping ceiling without the need for ridge beams (the beam often seen in the upper middle of the ceiling where the ceiling slopes away on both sides).

One last little know truss is a saddle truss, and I’m mentioning this as there are two in both [3D 1.] and [3D 2.]. Saddle trusses are used to form the space between the existing roof (yellow) and the new girder truss (orange), I’ve shown these trusses as light blue. The saddle truss is named from the fact that it sit on or saddles the existing roof structure be it a trusses roof or pitched roof. These trusses are simple and cheap and save a of of construction time otherwise spent filling in this space with pieces of timber and small ridge beams.

Hopefully this helps everyone to see the versatility of the timber truss system including build professionals. I can say building professionals as I once went to a Gang-Nail truss seminar, paid for by a structural Engineer who did structural work for me. Even with a masters degree in structural Engineering he struggled with the truss concept and he would come to me for advice on this topic. At the Gang-Nail seminar which went from 9am until around 3pm there were Architects, Architectural Designers, Auckland Council building inspectors, Builders and other Structural Engineers. The people at Gang-Nail initiated the seminar by explaining every aspect of the Gang-Nail manual (the blue manual shown above). In the afternoon there was a segment that involved attendees using the manual to identify the correct truss for a given situation. This is going to sound ridiculous, but it is true. For around 30 minutes we were given various roofing configurations, with five or so minutes to find the correct truss in the blue manual for each scenario. We were then collectively asked what truss to use. I was the only person out of around 30 to 40 attendees who got it right mainly because I was the only person prepared to answer! It would appear I was also helping to avoid an otherwise painful few minutes of silence for the other attendees.

I always felt that part of the reason for this dismal performance by a wide cross section of building professionals was the lack of early training or at least out dated training. Part of my employment with the Architects I mentioned earlier was I had to attend Architectural school, now known in New Zealand as UNITEC. As I said earlier, I was involved at a young age in the design office that designed and administered high rise buildings or some would call these sky scrapers. Below is the first building I worked on which is situated at No. 1 Queen Street, on the Auckland waterfront.

This building was cutting edge in 1970. You can imagine how I felt when one day I would be on site watching columns being poured into steel casings bolted together and the next day I would be at school being taught that pouring concrete columns required timber boxing and timber yokes (clamps) fastened around the timber at intervals. My point is, schooling can be behind the times and still is from what I’ve heard. Clearly no one teaching building or Architecture has been updated on trussed roofs which is apparent from what I experienced at the Gan-Nail Truss seminar.

I worked at the Architects previously mentioned with someone who became the head of the building department of one of these training centers. Years later I had a conversation with a building Forman who complained about the antiquated methods being taught by these centers. He mentioned the person teaching (what he called garbage) by name and it was this person I had worked with at the Architects. I’m sure it was more the institution rather than the individual but my point is they are out of date or aren’t aware of new technology.

After learning about trusses working outside of Architecture, naturally I took this information with me into my Architectural practice. I found that designing the roof without assistance (the timber company I worked for had access to the brilliant people at the likes of Gang-Nail and Pryda), I had to work very hard to understand the system. I studied the blue book and was lost at times but I personally needed to turn out a product that I knew would work and was correct. I had developed the ability to design trussed roofs over several years. I was honestly shocked at what I had experienced at the Gang-Nail seminar. Sometimes learning is exhausting and the frustration felt when you don’t understand can be painful. In my opinion a lot of these people are lazy. It’s ironic that I hold the highest Design License available yet I am no longer allowed to design my own trussed roof system according to Councils. I now have to pay a truss designer to do this for me, and he/she provides Council with a design statement. This has slowed down my process and cost my client’s more money, why, because no one in the industry wanted to update, or learn about modern building systems and go through the pain of learning a truly amazing roofing system especially the Council.

Moving on, I have to thank everyone who have taken the time to write the most amazing comments on my web site. I have to say, your enthusiasm for my blogs and kind words were pivotal in producing this blog. I’m absolutely over the moon that I have been able to enlighten non building public on some aspects that I know even a lot of building professionals don’t understand. Strangely enough I don’t seem to get any comments from Building Professionals. This is the first building blog since I first uploaded my building blogs three and a half years ago, and I mentioned in the blog previously why this was so. Even a simple blog like this takes a lot of work and I had intended to include a small video but I just ran out of time, maybe next time. I hope you enjoy this blog as much as the previous ones and I actually have another one waiting in the wings for some text.

I thought some of you might like to see my virtual office I created as it shows off some of the work I’ve done. Plus I like shiny reflections and my real office is a jumble of computers, survey equipment, books and a 1.2 meter by 0.8 meter Model house I built for a client.

Cheers, Ross.

Ross Newby Architectural Designer, Licensed LBP (Design 3). email ross.newby@ait.kiwi.nz All rights reserved. 3D images created by Ross Newby. Photos curtesy of Google – Ross Newby