This unusual building feature requires a complicated series of design steps – and their related experts – to avoid problems. By Paul Davis
Every so often you are going to get a roof to design that includes parapets. Typically, there will be a box gutter behind at least one of the parapets. Other parapets may be required around the building perimeter to maintain a consistent look. The image in this article is taken from one of my drawings where we were commissioned to design the steelwork and the timber parapet was designed ‘by others’ – that being someone like you!
The hydraulic design of box gutters is often one of those aspects of construction that is left until late in the design process. There is a set of rules in the roof plumbing code that requires certain sizes, falls and space for sumps. So, before you tackle a parapet design you need to know from some other ‘others’ just how big the box gutter is and so how much space you have left to thread a bottom chord between the soffit and the gutter space.
If you are designing trusses supported on an extended bottom chord, as in the diagram, the chord extension acts like a beam and so any deflections from this area can’t be cambered out. So, depending on the spans, loads and geometry, the bottom chord may end up being deeper than you would expect for a truss that is of a more conventional shape. You need to make sure ASAP, preferably at quoting stage, that the roof, the deeper bottom chord, the parapet and the gutter are all going to fit in the available space.
As well as designing the truss proper, you need to design the parapet to resist lateral wind loads on its vertical faces.
As wind whips over a sharp edge of the parapet and spills onto the roof, considerable turbulence can be generated. The turbulence means that parapet wind pressures are typically very high. Not only that, but a parapet is subject to pressures on the windward side that then combine with a suction that develops on its leeward side.
It’s quite tricky to work out what the design wind load is, and I very much doubt that your truss design software can successfully do that automatically because of the complications I have described above. So, when presented with a parapet design you will probably need to get an engineer to work out the wind loads on the parapet – yet another ‘other’.
Additionally, there is a need to design the parapet so that it will not rotate over – you can’t just sit a wall on top of the trusses and hope for it to stay upright. Without a box gutter, the end vertical of a cut-off truss can just continue up to form the parapet. Or you can fix an additional vertical parapet timber to the side of the end of the truss
In a configuration as shown in the drawing, you can’t rely upon the steelwork – it is very poor at resisting twisting. In fact, the steel relies upon the truss to hold it square – not the other way around. So, the twisting effect of the parapet wind loads needs to be resisted by a large nailplate onto the bottom chord. The joint behaves much the same as a butt-joined piece of timber or a Z-sprocket, so once again you will need your nailplate supplier’s engineers to do the design. Yet another ‘other’!
And, as a finale to my word play, last night at my place we had a mother-another. My daughter asked her mother if she knew the correct way to use a feather duster. Her instant, and instantly classic, response was “Give it to another person!”
Paul Davis is an independent structural engineer managing his own consulting firm Project X Solutions Pty Ltd. The views in this column are Paul’s and do not reflect the opinions of TimberTrader News. Phone: 02 4576 1555, Email: paul@projectxsolutions.com.au
