On 21/10/2012 21:42, firstname.lastname@example.org wrote:
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.
On 22/10/2012 20:28, email@example.com wrote:
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
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
3) Joists will survive loadings far beyond those specified in the
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?
On Sat, 20 Oct 2012 22:33:14 +0100 John Rumm wrote :
Our old rule of thumb when I was a BCO which matched the tables pretty
well was that for floor joists double the depth in inches and subtract two
to get the permissible span in feet; flat roof joists, subtract one (2"
As you say, in most cases practicality requires all joists to be the same
depth (you need tops of joists to be level and want them all to bear on
wall at one level) so except for the largest span they are generally
oversized. There's also more in reserve in that for virtually all joists,
deflection governs the size, not bending stress and few floors are loaded
to BR design loads (1.5kN/m2 30lb/ft2).
Tony Bryer, Greentram: 'Software to build on',
Melbourne, Australia www.greentram.com
Slightly sideways... is it possible that the quality of timber available
to the Victorians was better than that of today?
I use a lot of recycled 4"x2" and generally find that the grain is more
dense and the knots smaller than that supplied new.
A dense grain means a timber gown in more northerly climes, and I don't
think the climate has changed significantly since Victorian times. There
has always been, and still is, a great variation in quality and price
for what is loosely termed "4x2"
Most timber Victorians used in buildings would have been imported and
Baltic or Scandinavian, slow grown 14 rings to the inch and the snow
load knocks off dead suppressed branches.
We were major importers of wood on the global market as our economy
grew earlier than others.
Since the 1950s more home grown timber has come on line and it has
benefited from machine stress grading where in the past it failed
visual grading. Also I suspect much Canadian lumber is second growth
On Sunday, October 21, 2012 2:16:39 AM UTC+1, Tony Bryer wrote:
So for a 14ft span (as here), the 8" joists I originally proposed would be about
I wonder if the tables are constructed from that rule of thumb or from a complex
calculation that gives the same result?
I guess most of the discussion (now) is about what you might be able to get away
with, rather than what should be done, but I'd prefer to over-engineer than
under, for the sake of a couple inches.
Of course, I'm equally concerned that they are mounted securely, as Mr R
If you can rule out that the space will ever be converted to habitable,
then you can undersize a tad from the tabulated values. As Tony
mentioned above, its normally the deflection limits that dictate the
size rather than the shear or bending limits. (i.e the floor would be
likely to damage decorative finishes, feel to bouncy, and upset
inhabitants of rooms below, long before the timber is in danger of
For your application (i.e. with the new beams some distance above the
existing ceiling, and not ceiling to be mounted on the underside of the
new joists), deflection beyond normal limits is a non issue. So it
reduces to a problem of what is adequate in terms of bending and shear
loading on the timber (assuming you don't mind it feeling a little more
bouncy than "normal" given that you know it is still structurally sound).
Perhaps a play Tony's excellent bit of software might be in order
(assuming there is still a demo version available for download?)
On Sunday, October 21, 2012 4:25:44 PM UTC+1, John Rumm wrote:
Yes John - it may potentially be that 2 x 6 joists (which are significantly
cheaper per metre) could do an
adequate job in this application, if they are available (my local timber yard
couldn't supply that length
for the living room when I was costing it: There, the lower spec was due to
the wall running along the
middle, effectively halving the span).
TBH, I didn't spot Tony's software - I thought he just mentioned the rule of
thumb(!). If I did go below
spec (say 4m of 2 x 6), it would be great to have some idea of how wobbly such a
floor would be. The
joist tables just give maximum length for size, as far as I can see.
I'd still feel a little uncomfortable that it would deny the option of making
the space habitable in the
future though, even though the rest of the house is big enough that it shouldn't
be an issue.
Although I get the point that BR specs change over time, they surely can't ask
for joist that are much
deeper than they require now, so I would have thought the current specs won't
(Apologies if my posts are hard to read. Someone told me a while ago they
weren't wrapping, whilst in
a recent thread someone else told me they had a lot of empty lines!)
A timber merchant ought to be able to get 6x2 in 5.3m lengths at least.
I just checked, there is still a demo available. It has printing
knobbled - but that won't be a handicap for your needs.
(the usual caveats about it letting you design unsafe structures faster
apply, if you don't stick in sensible values!)
It will show you the calculated deflection for whatever load you apply,
and also tell you when you are exceeding the safe working limits on the
If you model your longest timber that should let you get a feel for the
changes. The loadings to apply for a normal floor appear further up the
thread (if you know what you are storing etc you may be able to use
Might be worth working out what you could "get away with" and then
comparing the cost difference to doing it to full spec.
I would not expect them to change in substance at all really. They may
grow to include more on composite joists (i.e. man made beams with
struts top and bottom and some sheet material webbing)
Its a combination of not wrapping and all the lines being double spaced.
The former is easy to fix in a reply with a quick CTRL + R in
thunderbird. That latter takes slightly more editing!
On Sunday, October 21, 2012 7:33:24 PM UTC+1, John Rumm wrote:
Thanks for the link - Now I have to find a Windows PC I can spark up and run the
software - should be interesting.
I suspect a narrower joist could be significantly cheaper for these timbers,
although it might not make a substantial impact on the job cost overall.
I've rather taken a 'do it properly and do it once' approach to this house,
rather than being cost-driven.
I did toy at one stage with the prospects for making up composite joists in
situ, given the access issues, but decided that would just add another variable
(and potential disaster) to the equation (!)
Must have another go at Thunderbird for this group. I had it working, then it
wouldn't post for some reason.
Probably not as "industry standard" or trustworthy, but there are a
number of smart phone apps about that will do calcs at least on a single
beam. Might be worth trying one of those if you have platform for that.
It depends a bit on the layout and how many joist hangers you need etc.
Those will be the same price regardless.
It ought to be fairly painless for newsgroups if you are using your ISPs
newsserver. If using a third party one, you may need to turn on the
"request authentication" option in the account settings.
I should take a browse through the phone apps (without wishing to turn
the thread into another debate about which phone is best!)
Your comments about ISP newservers made me realise that I set
Thunderbird up for home, then might have failed to post over a different
WiFi connection, so if this posts, problem solved (ish)
Hence why I did not mention a platform. I have seen them for mine, so I
presume they exist for the others... I wonder if Tony is planning on an
Android or iOS port of superbeam proper?
Normally if the newsserver accepts authentication then it will allow
posts from any network. If it does not, then it will usually only accept
them from its "own" network address block.
BTW the quoting looks much better - no double spaced lines anymore ;-)
On Tue, 23 Oct 2012 01:43:39 +0100 John Rumm wrote :
No current plans. Would it be an interesting project? Probably. Would it
make any money, given what people are used to paying for mobile apps?
Probably not. If I was looking for an alternative platform, then the most
likely route would be a web-based app, accessible via any browser.
Tony Bryer, Greentram: 'Software to build on',
Melbourne, Australia www.greentram.com
One option would be to only sell/give away the mobile app as an extra
for the full PC version - gives you the sales hook for some added value
on the main product, stops folks who already use it going to alternative
platforms since they can still use the mobile version of the "industry
standard", but without cannibalising your desktop sales.
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