Now I know this roof is not exactly heavy and has stood for over 30 years but the design of roofs and the various regulations/code have changed too.
I decided, not only to raise the roof truss lowest level by 300mm, but increase the number of joists and change the pitch angle to about 18 degrees. The existing one is just less than 15 degrees and is not acceptable. Raising the roof truss lower level necessitates extra masonry and lintels.
My builder has suggested a low density block work skin on the inside. I wanted to do this anyway but the trusses will now bear on the internal skin rather than the external skin.
My shop dimensions are 5.7m x 7m and is a combination of pre-cast concrete panels bolted together with brick columns. This method of construction was very popular in the UK in the post war years as it was quick and easy to construct. The gable is in the 5.7m span.
I had a look at the latest regulations and the recommendation is to have spans a maximum of 650mm between centres.
I have opted for 615mm and this gives me 12 trusses. Here are the overall views I wish to attain (the cladding will be western red cedar that will go silver/grey over time and I don't want to paint it):
Then I needed to draw the trusses out as I originally intended to make them myself. I based my design on the existing design but beefed them up a bit. However I discussed the design with my builder and asked him to source the trusses as it may be better to have them professionally made and installed. He thought my design was using too thick members and said that thinner members would suffice. I then found this superb suite of applications from finesoft.eu that has an application to design trusses for you. I downloaded the free version and put in my parameters. Sure enough it worked out what I needed and had thinner timber sections.
You can even put in snow loadings based upon where in the world you live. This is great for our woodworking friends who live in more northerly (or southerly) climates such as Canada etc. Although as Penelope says in the comments "it would be best to seek professional advice for safety reasons."
The truss manufacturer is making them for me using just the overall dimensions, angles, heights etc as a guide. I have left it to them so they are free to do whatever they consider to be suitable for the integrity of the building.
Here are the trusses in position: