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I want to build a timber ramp over a stormwater pipe that crosses a concrete pathway next to the house. The ramp needs to be 850mm wide (it is not possible to make it any wider than 850, and I don't want to make it any narrower). The top of the stormwater pipe is 225mm above the path. So the ramp needs to go from 0mm (path level), up to at least 225mm, then back down to 0mm.
I was thinking of using some 200 x 50 H4 treated pine sleepers (cut on an angle and stacked on top of each other) to support some merbau decking boards.
I understand that decking is usually laid over joists at 450mm centres.
If I space the sleepers apart at 450mm centres, the merbau decking will have a 175mm cantilever (or overhang) on either side. I've never read of a deck with cantilevered decking, but since this ramp is quite low and the overhang is only 175mm, I thought that might be OK.
Am I wrong here? Is there a risk of someone standing near the end of a merbau board and it collapsing?
I could modify the design so that it uses three sleepers (instead of 2) to support the merbau decking. This would result in an overhang or cantilever of only 100mm. But maybe that is still too much? Three big sleepers seems like overkill for such a low ramp.
What if I put a galvanised angle bracket under each piece of decking, on the outside of each sleeper, to reduce the overhang distance a bit? (eg a Carinya 75 x 50 x 20 x 4mm Galvanised Angle Bracket would reduce the overhang to 100mm with 2 sleepers, or to 25mm with three sleepers).
The Sketchup image shows a section of the pipe crossing the path, as well as some dashed lines (guides) indicating where the sleepers would go, at 450 centres, starting from the centre of the ramp. 850mm is the total width of the ramp.
There are some metal standoffs supporting the stormwater pipe and keeping it off the path. Their locations (which are already set) make positioning the sleepers a little more tricky. They could be repositioned if necessary, but I'd prefer not to.
The ramp would have a gradient of 1:12 (or maybe 1:10) on either side.
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