On 22/10/2012 20:28, wrote:
On Monday, October 22, 2012 2:38:42 PM UTC+1, John Rumm wrote:
On 21/10/2012 21:42, wrote:
On Sunday, October 21, 2012 4:07:53 PM UTC+1, John Rumm wrote:
On 21/10/2012 00:37, wrote:
On Saturday, October 20, 2012 10:33:20 PM UTC+1, John Rumm
wrote:
On 20/10/2012 20:33, wrote:
On Saturday, October 20, 2012 4:42:00 PM UTC+1, John Rumm
wrote:
On 20/10/2012 01:49, wrote:
On Friday, October 19, 2012 3:53:36 PM UTC+1, John Rumm
wrote:
On 19/10/2012 13:38, wrote:


Also building regs are not retrospective. So if a
floor was designed as a floor, and was compliant with
the standards of the time, you would be able to use as
the basis of your room in the roof, it even if the
standards applying had changed since it was built.

There's no way a BCO will accept a loft conversion in a
1924 house on its original 3" loft floor joists.

I doubt a loft with 3" joists would not have been deemed
acceptable as a proper floor for a habitable space - even
in 1924. However, my point was, that if you upgrade
something now to the current standards of a floor in a
habitable room, then there would be no need to upgrade it
further if one later made the space habitable - even if the
standards for a floor have changed by then.

3x3 was the smallest standard habitable flooring joist size
in Victorian houses. It was much used for short spans, such
as across corridors & landings.

And it still might be acceptable now (for short lengths)

IIRC the 1924 BR didn't specify joist sizes, so 3x3 would
still be compliant for habitation then. It could be used in
loft floors above corridors, where the span was short.

A loft floor is not a floor in the accepted sense though - its
not expected to carry significant load.

I challenge you to find any BCO that would accept that in a
loft conversion today.

A BCO would be happy with a loft using 3x2 - its a good deal
better than many a lofts built with modern trusses. However
that is a very different thing from a loft floor which going to
be used for a habitable room. If you are converting the loft,
then the same spec as would apply to any other floor in the
building will kick in. Out of interest I had an experiment with
superbeam to see what you can get away with on a 3x2 (well
72x47mm) and a typical floor load (uniformly distributed
0.8kN/m on each joist). 1.3m seems to be about the limit - so
you could probably still do a landing with it and comply with
modern building regs. (having said that, its generally simpler
to use one depth all over to save having to buy lots of timber
sizes)

For clarity, lets take it a step further. Say the loft got 2x2s
in 4' spans in 1924, hopelessly unsuitable for habitable rooms,
but still compliant for them in 1924.

I don't think 2x2 would have been used for the floor of a habitable
room in 1924 or at any other time. Its seems to are engaging in a
little reductio ad absurdum.

That's exactly the point. All Victorian loft floor joists were
compliant for habitable use at time of building, but no BCO is going
to accept them in a conversion today.

I don't quite follow the line of thought. A loft floor in 1924 was *not*
designed for habitable use then, so its no surprise it would not be
considered adequate now.
Your point seems to be that the definition of "habitable" did not exist
in 1924. However to cut through the confusion, look at what would have
been installed in 1924 for a normal 1st floor set of joists, since that
will have been designed for what we would today call habitable. If the
loft had similar spans but thinner joists, (which which is a safe bet)
then its not habitable - then or now.

I don't think you've followed what I'm saying at all.

So what is your point then? I thought we had covered:

1) Standards have edged up?
Agreed. But not hugely.

2) Skimpy joists in a loft were acceptable as a habitable room floor in
the past?
Nope, don't buy it for a moment. They were acceptable as a loft floor
and that is all - but there was no anticipation that was going to be
used as a bedroom of office etc. They were also to a much lower standard
than was deemed necessary for all the other "proper" floors in the same
building.

3) Joists will survive loadings far beyond those specified in the
building regs?
Yup agreed. The specs are primarily intended to limit deflection, and
excessive deflection occurs long before structural failure.

4) You can still use skimpy timbers if they are short enough?
Yup agreed, nothing much has changed there.

5) Joist sizes are specced on sound transmission?
I don't believe that really comes into it. Specs on sound transmission
have got much stronger in recent times, but joist sizes themselves have
not increased as a result. Joist support techniques have changed -
reducing wall penetrations, ensuring adequate insulating material is
included in floor construction to reduce noise. Eliminating gaps and air
paths are all partly related to reducing noise transmission as well as
improving thermal performance. Fire protection rules have also
tightened, and that has had a knock on on ceiling coverings (i.e. 1/2"
PB and skim, not 9mm etc, intumescent covers over ceiling penetrations).

Have I missed any?


--
Cheers,

John.

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