Whether that's true depends on how you define acceptable. Limiting deflection to
3mm over 10' has absolutely nothing to do with safety or what I would call acceptable deflection limits.
Well, not in any sense of acceptable that I would buy into. Lots of old houses have ceiling deflection levels numerous times as big as BR allows, and I don't see any problems or complaints resulting.
Timber that light isn't being discussed, and the OP's new joists won't be supporting a ceiling.
If 3.1mm deflection would upset the op, go with the tables. I personally wouldnt have any problem with half an inch on a heavily loaded section
However, its not just about safety, but also producing an adequate quality of building, where the floors don't bounce up and down, and all the joints crack in the plaster because there is too much movement.
There are other requirements that are also taken into account with the sizes commonly used, such as minimizing twisting of joists (although strapping will still be required over 2.5m), and providing lateral restraint of walls where required.
I have yet to be in a house where the floor deflects 6" while walking across a room. Personally I would take that as an indication one should get out fast!
However it is true that what was acceptable in the past would no longer meet current standards. That is partly a reflection on more stringent requirements for air tightness of properties, and also changes in materials. Lime mortar, and soft bricks will accept more movement than modern materials for example.
I was responding to your comments about the 2x4 which can deflect 6" safely rather than the OPs storage floor. A 6" deflection may not cause it to fail, but it would be excessive by any measure, and would not be suitable even for loft storage.
The OP can use lighter timbers than those that would be required to meet the standard of a floor if he wants, although if there was a suggestion that at some point in the future it was upgraded to a habitable room it would be sensible to build it to the required standards now obviously.
3.1mm would be fine even for a floor (the limit on that length would be around 8.4mm IIRC).
4x2 joisted lofts usually feel reasonably solid underfoot. However it is surprising just how much they creep over time, even without too much weight on them.
If you look at:
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is looking at a new floor joist over a ceiling supported by 4.2m
4x2" joists.
one of the "D" joists as per:
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spacing under the ends of the new beam was approx 3/4 to 1" at the wall plates. Mid span the gap under the joist is getting on for 4" - so the ceiling had sagged around 3" mid span since when it was originally built in 1956 (photo taken some time 2004). That was with just light storage use on chipboard loft panels.
No, its not. Millions of old houses have timber a fraction the size of the current requirements and don't suffer any of those problems.
obviously that's not relevant
I've never known a Victorian ceiling/loft floor be draughty
Slight movement of loft floor does not move the brickwork. Modern PB is much more tolerant of movement than lath & plaster.
Obviously a floor structure that deflects 6" in use has never been proposed. My point was that deflection levels encountered in real life floors are a fraction of failure limits
Standards change, so its not obvious, its just an option
True, so long as the fraction you have in mind is say 7/8ths...
Many places in the past would have used say 7x2 where these days 8x2 would be deemed adequate.
There are places which use significantly shallower timbers, but then tend to have more frequent cross members and hence shorter effective spans.
Many of our downstairs floors here are only on 4x2" joists - but these span dwarf walls often at 4' spacings - so they are quite rigid.
Modern buildings are in general very much less draughty than Victorian places. There have been many changes in building practices to achieve this. One example being joist support. It is no longer common to set joist ends into openings in walls since (among other things) it also creates a potential air gap. The switch to joist hangers also frees you from the limitation of needing to use joist sizes that match a brick height multiple in height.
I did not say it did - it was an example of building material that is more tolerant of movement. Perhaps horsehair reinforced plaster would have been a better example.
Its different - not necessarily always better.
No one (including the building regs) is attempting to suggest otherwise.
Indeed, but they rarely reduce, so its fairly obvious (to me at least).
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.
Its possibly a reflection of the fact its actually quite difficult to observe a sag of a few inches over a 4m+ span, unless you stick a straight edge against it like I had in effect done there, or have some way of sighting along the timber.
The sag was not visible in the room below, although I expect that had you have gone round with a measuring stick you would have seen it.
I just noticed that you superficially said *loft* floors there, and my comments that followed were more generally about "floors" in general - my fault for not reading carefully.
There is a general drive to cut down air currents through void spaces in direct contact with heated surfaces. If you have a penetration through a wall, then air will flow through it. Even if that never discharges directly into the living space, it lowers the thermal performance of the building.
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.
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.
I challenge you to find any BCO that would accept that in a loft conversion today.
And it still might be acceptable now (for short lengths)
A loft floor is not a floor in the accepted sense though - its not expected to carry significant load.
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 fo= r them in 1924. So it was built in compliance with BR standards for habitat= ion at the time, and you can indeed walk on them, just about. But no BCO in= their right mind would accept a conversion to habitable now on 2x2s.
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" joists).
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).
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 nowadays.
So for a 14ft span (as here), the 8" joists I originally proposed would be about right? 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 outlined......
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