I`m going to build a suspended floor in a small downstairs bathroom so that
all the drainage & pipes can go underneath.
The room is approx 1.89m by 2.46m and I plan to batten out the wall and then
hang joists inbetween using joist hangers at 400mm intervals using the
shortest span (1.89m).
What size joist should I go for?, one book I read says I can use 100x50mm
(4x2) but I`m not sure. On top of this will be plywood screwed down and then
tiles. There will be no bath in the room, just a bog, shower and basin.
If necessary I can supplement the span with a sleeper wall (I think they
call it) to provide rigidity to the floor (don`t want it flexing with tiles
What's the floor now? Are you digging out? If so, I'd be inclined
to put a sleeper on each side, allowing for existing ventilation,
on in the middle of you're using 4x2, and be very careful about
plane, and be careful about the timber you use to keep it plane!
Anyway, I get the feeling I want to know a bit more about the
current and intended set-up!
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A 47mm x 97mm joist on 400mm spacings with SC3 timber and 0.25-0.5 kN/m2
allows a span of 1.92m, making it just compliant with building regulations.
If you had wanted a bath installed, you would double up the joists
If your intended flooring weighs more than 0.5kN/m2, you'll need 122mm (5
A sleeper wall or thicker joists would improve rigidity, if required. If you
install one, ensure ventilation is adequete. In fact, you must ensure
ventilation is adequete anyway. Ensure you install good insulation and
airbricks. Take the time to install underfloor heating for a new floor.
Spookliy enough, I'm probably about to do something very similar;
building a raised floor in bedroom 3 (formerly bathroom) to enable
soil pipes etc from what will be an internal bathroom to reach the
external wall. (I can't see any other way of doing it given the
layout of the house and joists etc).
What I was going to do was essentially nail new joists parallel to and
directly above the existing structural joists (not sure whether its
worthwhile/necessary to remove the floorboards first - may do so just
to ensure I can see any hidden wiring/pipes!). The soil pipe will be
traversing the structural joists diagonally (which is why I can't
'bury' it below floor level), so the new timbers will just have a gap
where the pipe goes through. (This is upstairs in a terraced house
Given that my new timbers (I hesitate to call them joists, cos they
aren't!) are not a structural part of the house as such, I was going
to use much less substantial wood for them than the real joists; just
using my common sense. Or is that wrong - do I need to fulfil any
building regs by using stinking great 5x2's? Would it make a
difference if I left the original floorboards there or not?
You should calculate the dead load of your proposed timbers and floorboards
and ensure that this loading does not make the existing joists too small.
What is the span and dimensions of the existing joists?
Hmm, that aspect hadn't occurred to me!
The overall span is about 12 feet; the raised area where the timbers
would sit is about half that. The existing joists are varied in size,
but are all at least 5.5 x 2.5 inches.
As regards loading; there is a stud partition down the middle of the
span, ie at right angles, and the non-raised area will need to bear a
bath and shower.
What d'you reckon?!
email@example.com (Lobster) wrote in message
While I haven't ruled out the above, an architect visiting the
property has suggested a Plan B which would not involve raising the
floor, but having the soil pipe traverse the kitchen ceiling concealed
within cladding to make it look like a beam. Visually it would look
OK but I have my doubts on two fronts:
(a) Would building regs object to a soilpipe basically attached to the
(b) I can get adequate fall on the soil pipe, but it would probably
need to have 4 or even 5 90-deg bends between the bathroom floor and
the entry to the stack. Although I'll be able to cover the whole pipe
from suitably positioned rodding access points, it doesn't sound like
great plumbing practice to me to have that number of corners. Is this
normal and is it likely to work OK? What does the uk.d-i-y panel
think? (Note I will be selling the property once work is complete, so
personally wouldn't be too bothered if we're only talking about a very
small increased risk of blockages...)
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