You forgot about the pipe thickness!!! The TOP of the pipe will drop about 5" as you say, but the bottom is 110mm below that, so it would need a void 8.8" deep to achieve this. In addition any bends at either end take up additional space to allow for their radiused curve.
Phil The uk.d-i-y FAQ is at
I forget the exact fall. I read that 1:60 to 1:80 would be fine, but not too sure on that. You will be surprised at how slight a fall a soil pipe needs. They don't want a fall too great as they tend to suck out the water in the traps.
If the fall can't be got, it may be work installing a false ceiling below to get the fall. Depends on how desperate you want the toilet.
If I read it right, the soil pipe fall is a minimum of 18mm fall per one meter length of soil pipe to a maximum fall of 90mm per one meter length. That is 90mm fall over a 5 metre length (3.5 inches). The pipe diameter is 4.3 inches, which gives a floor gap of just under 8 inches over a 5 mter length. The pipe in question is about 4.5 meters so I will need a fall of 81mm (3.2 inches) plus 4.3 inches which gives a floor space of 7.5 inches (190mm)
You are looking at pages 15-16, right? There are lots of "ifs and buts" there, but it does indicate a minimum gradient of 1 in 80 for a single WC (from Table 6, and Diagram 9), which is 12.5mm fall per 1000mm run.
The bends at either end of the pipe do take up significant height as well, and also consume some of your available horizontal run. I have tried to show in the diagram below (use fixed font to view it or it will look a mess).
The + signs indicate the bulge at the pipe collar. The \ signs indicate pipe radius that takes up extra space.
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Phil The uk.d-i-y FAQ is at
A bend to outside can be 100% outside and if the soil stack pipe is proud of the wall, spacer clips can be used. Also solvent welded elbows don't take up much space to pushfit elbows. At the other end, a slot can be in the floor so the collar of the elbow/bend is clear of the floor boards taking no extra space. 12.5mm fall per 1000mm is 62.5mm fall over 4.5 meters. which mean it will fit under most floors. I haven't read the doc, but if 12.5mm fall is for one wc then a 18mm fall per 1000mm sounds about right if two WCs are taken off the one underfloor soil pipe. Or is it?
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