Kn question



I am just working out the weight which will be sat on my loft conversion floor and think it is easily accomodated by what joist i plan on using.

i need the floor to take no more than 0.5 Kn/m2 in weight.

1Kn = 100kg, so 0.5Kn = 50kg. This means i can load the floor up to 50kg/m2. Anyone confirm this rough calculation?

We have a double bed, 2 side tables, a chest of drawers and a chair in the bedroom. i reckon this will weight 200kgs max. So my floor area in the part of the conversion is 20m2.

So i get our weight on the joists to be 10kg per sq m, easily inside the 50.

Does this make sense to anyone out there?

It basically means i need a 9x3 c24 joist for this area and it will easily withstand the loads on it.

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Hi marsaday,

what spans do you need the joists and what centres are you going to install them at?


span = 5.18 centres = 400.

a 9x3 c24 seems to easily be ok if loading between 0.25-0.5 Kn/m2 (it will span 5.25m)

if it is more than this then i can only span 4.88m with this joist, so would have to increase the size.


For imposed loads 1.5kN/m² and dead loads as you've specified (>0.25-0.50<) you're right on the limit using C24 9"x3" on a 5.18m span @ 400mm centres according to the Trada tables. I don't know where you have got your figures from?

I would consider using 220x75 C24 instead. Using this size gives you the option of using C16 instead if you have issues sourcing C24.


EDIT: Ignore the above - see below - I had a blonde moment :rolleyes:
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24 Aug 2009
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You write " are right on the limit using a C 24 9" x 3".

" I would consider using a 225 x 75 C24 instead ."

What's the difference, except that your sugestion is 1 mm thinner nominally ?


Sorry mointainwalker you're quite right. :oops:

For some daft reason I had transposed the the OPs 9" joists as 195mm in metric on the Trada tables, and then just looked to the next size up when that looked a tadge close. I blame The Cube :LOL:

Apologises to the OP for any confusion. 9"x3" will be ok.


yes thanks (confused initially).

I think i am well in the limits with 9x3 c24 and i can get hold of these no problems. 10 x 6.6m = £333 plus vat.

The thing i am surprised by in this conversion is the cost of basic materials (joist, plasterboard, wood, insulation). It all comes in pretty cheap. Its the bathrooms and wood flooring and velux which bump up the prices. i have a costing of £13k to do my loft conversion and that includes sticking in 2 new gable windows and blocking up another. This price covers materials only and some small labour charges. i will be doing the job on my own.

i think this job would be £30k at least if a contractor did it. the loft is big as well - 8 x 7m

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