The links below will take you to a very helpful website called the Timber Beam Calculator. I've highlighted the two main areas that most people building a garden room are likely to require - the floor joists and the roof joists. Obviously if you've got a solid concrete slab as a base then you won't need the former, which is only applicable if you're building a timber framed base on concrete piers, ground screws, steel rods, etc.
You'll see that there are a few flat roof options based on the "imposed load" applicable to your build. Imposed loads are temporary, dynamic loads put on a structure, such as your weight when you're working on it, the weight of snowfall, etc. The Timber Beam Calculator defines these imposed loads as follows:
I live at just below 200m altitude so went for the 1kN/m2 imposed load to calculate what I needed on my build, but I'm pretty sure the lower one would have been sufficient. If your garden room's overall height is constrained then you will clearly want to use the smallest option permissable. You'll see reference in the tables linked below to timber with widths of 63mm and 72mm. Rather than buying this thicker stock, many people will glue and bolt two lengths of common 2" timber together to form stronger joists in order to keep the height down, though I haven't seen any technical data specifically on that method.
Floor joists
C16 timber
C24 timber
Flat roof joists
C16 timber with an imposed load of 0.75kN/m2
C16 timber with an imposed load of 1kN/m2
C16 timber with an imposed load of 1.5 kN/m2
C24 timber with an imposed load of 0.75kN/m2
C24 timber with an imposed load of 1kN/m2
C24 timber with an imposed load of 1.5 kN/m2
You'll see that there are a few flat roof options based on the "imposed load" applicable to your build. Imposed loads are temporary, dynamic loads put on a structure, such as your weight when you're working on it, the weight of snowfall, etc. The Timber Beam Calculator defines these imposed loads as follows:
Imposed loads of 0.75 kN/m² for maintenance and snow load are applicable where there is no permanent access (no fixed ladder or staircase) and most areas where the altitude does not exceed 100 metres (refer to BS 6399-3)
Imposed loads of 1 kN/m² for maintenance and snow load are applicable where there is no permanent access (no fixed ladder or staircase) and most areas where the altitude exceeds 100 metres but does not exceed 200 metres, but excluding parts of Scotland, Pennines, North East England (refer to BS 6399-3)
Imposed loads of 1.5 kN/m² are applicable where there is permanent access to the flat roof. For example, a fixed ladder or staircase (refer to BS 6399-3)
I live at just below 200m altitude so went for the 1kN/m2 imposed load to calculate what I needed on my build, but I'm pretty sure the lower one would have been sufficient. If your garden room's overall height is constrained then you will clearly want to use the smallest option permissable. You'll see reference in the tables linked below to timber with widths of 63mm and 72mm. Rather than buying this thicker stock, many people will glue and bolt two lengths of common 2" timber together to form stronger joists in order to keep the height down, though I haven't seen any technical data specifically on that method.
Floor joists
C16 timber
C24 timber
Flat roof joists
C16 timber with an imposed load of 0.75kN/m2
C16 timber with an imposed load of 1kN/m2
C16 timber with an imposed load of 1.5 kN/m2
C24 timber with an imposed load of 0.75kN/m2
C24 timber with an imposed load of 1kN/m2
C24 timber with an imposed load of 1.5 kN/m2