Choosing lowest profile floor joists - On Trada limit with this - Help please :)

[lordgaz]

Hi all

We are currently in the process of an extension build and are trying to choose the best sized domestic floor joists - By best, I am meaning that we need the joists to be as low profile as possible so as not to steal head height in the upstairs room which has a sloping ceiling.

Our exact "block to block" measurement is 3980mm, and we have purchased 45mmx 195mm C16 timber - however, I am looking at the C24 trada tables, and, based on the minimum over coverage of 90mm onto the blocks, we would have a span of 3960mm which is right on the limit of 75mm x 145mm C24 timber -

Some questions

Is the span taken from the outside or inside of the block (or is it just the length of unsupported joist) -

Is being on the limit a problem?

This is based on 400mm centres, and 0.25 dead load - Is a 0.25 deadload quite standard, or unlikely to be achieved (based on 1 x double bed above and 2 adults)

Any help welcomed

Many thanks
Gaz

 

[freddiemercurystwin]

The spans in the tables are from inside of block to inside of block or the 'clear' span in this diagram.

 

[Mr Chibs]

Until someone more knowledgeable comes along, I seem to remember deeper joists are better than wider.

Just a thought/question to others, could you not make the centres of the less than 400mm?

Also I thought dead loads are things like the weight of the materials etc, live loads would be adults and moving loads.

Might be out of date...

47 x 195, c16, load >25 = 4.25m span @ 400c (on a old 2008 trada table I have.)
4.09m @ 450c
3.7m 75x145mm @ 400c

 

[lordgaz]

The spans in the tables are from inside of block to inside of block or the 'clear' span in this diagram.

Thank you.. that is very helpful

 

[lordgaz]

The spans in the tables are from inside of block to inside of block or the 'clear' span in this diagram.

Until someone more knowledgeable comes along, I seem to remember deeper joists are better than wider.

Just a thought/question to others, could you not make the centres of the less than 400mm?

Also I thought dead loads are things like the weight of the materials etc, live loads would be adults and moving loads.

Might be out of date...

47 x 195, c16, load >25 = 4.25m span @ 400c (on a old 2008 trada table I have.)
4.09m @ 450c
3.7m 75x145mm @ 400c

I have no problem using more joists at centres less than 400mm, but like yourself I am unsure if it is acceptable.. for the 75x145mm, I am referring to C24, and not C16.. is your trada information above for 75x145 based on the C16 table?

 

[lordgaz]

The spans in the tables are from inside of block to inside of block or the 'clear' span in this diagram.

Until someone more knowledgeable comes along, I seem to remember deeper joists are better than wider.

Just a thought/question to others, could you not make the centres of the less than 400mm?

Also I thought dead loads are things like the weight of the materials etc, live loads would be adults and moving loads.

Might be out of date...

47 x 195, c16, load >25 = 4.25m span @ 400c (on a old 2008 trada table I have.)
4.09m @ 450c
3.7m 75x145mm @ 400c

Based on Freddie's span advice picture, the clear span is 3780mm, which I think helps a lot with the trada grading

 

[Mr Chibs]

Previous all for C16,

With the C24 figure it's shaded and there is a lengthy sub note... mainly about loading if there is a bath full of water sat on the joists. [double up with alternate joist]
If you want to get out of the shaded figures, then go to 75 x 170, this gives you a span of 4.06m @ 600c, yes, 600c... and 4.62m @ 400c (but in the shaded zone)

 

[lordgaz]

Previous all for C16,

With the C24 figure it's shaded and there is a lengthy sub note... mainly about loading if there is a bath full of water sat on the joists. [double up with alternate joist]
If you want to get out of the shaded figures, then go to 75 x 170, this gives you a span of 4.06m @ 600c, yes, 600c... and 4.62m @ 400c (but in the shaded zone)

Thanks again! This room above will be max 3.8m x 3.8m and have enough room for a double bed and 2 humans due to the sloping ceilings.. I am keen for someone to say 75mmx145mm at 400mm centres will be ok obviously so as to save 50mm in joist height and increase the headroom upstairs.. I saw the shaded area in the trada (not got it with me right now as I am on my phone typing)... as there will be no bath and only a spread bed load, will C24 145x75mm over a clear span of 3780mm at 400c be acceptable to building regs?

 

[^woody^]

Span tables are conservative. Joists in a load sharing situation such as in a floor, can typically be proved to be suitable for greater loads by calculation.

Spacing joists closer together could work, but you will need to provide calculations for that - so may as well calculate the joists at normal centres.

Or use engineered joists

 

[tony1851]

As per the guy above, in critical situations, it's usual to ignore the tables and calculate the joist sizes and centres manually.
How low does the room come at the sides? The reason this might be pertinent is to do with loading. The standard tables allow for
domestic floor loading of 1.5 kN/m². But when headroom comes low, such as in loft conversions etc, it's often not possible to load the whole area of the floor to the
1.5kN limit, and a lesser load is often appropriate at the perimeter of the room. This reduces the stress on the joist and often allows a smaller section.
But as above, it really needs to be calculated.

 

[Nakajo]

You may find yourself repainting one bit of ceiling more often than the rest!

 

[lordgaz]

Thanks Tony& Woody - So reading between your lines - The Trada tables are over cautious , so being close to the Trada limit should be ok seeing as a fair proportion of the room will not be "standable in" due to the sloping roof (it will go down to as low as 600mm at the sides of the room (depending on which joists we can use and final drawings) -
So, going back to the Trada table - 75mm x 145mm C24 joists are permitted for 3.96m clear span @ 0.25kN/m2, and 3.79m @ 0.25 - 0.5kN/m2, which would be in limits for my clear span of 3.78m (all at 400mm spacing) -

The dead loads do not include the weight of the joist - so is 0.25kN/m2 plenty take account of board + insulation on the joists+ underlay+ carpet + plasterboard and light fittings/cables as a dead load? - Does anyone know what a typical deadload for a domestic floor would be with carpet?

 

[tony1851]

I don't think underlay and carpets are included as dead load.
With 6x3 joists @ 400c/s, 22mm boarding, plasterboard & skim, plus insulation and strutting I think you'd be looking at something like 0.4kN/m².

Ps, yeah, without the weight of the joists themselves, you'd be around 0.25kN/m².

 

[lordgaz]

I don't think underlay and carpets are included as dead load.
With 6x3 joists @ 400c/s, 22mm boarding, plasterboard & skim, plus insulation and strutting I think you'd be looking at something like 0.4kN/m².

Thank you Tony - That is really helpful - So, based on your info, and referring back to the C24 Trada tables for 6x3 joists - Fd(kN/m2) of more than 0.25 but not more than 0.5 section at 400c/s spacing allows a clear span of 3.79m and my clear span is 3.78m - right on the borderline, but would this be acceptable to the average building reg inspector? -

 

[tony1851]

would this be acceptable to the average building reg inspector? -

It would have to be!!!
Don't hold those people in such awe!!!