Oh do me a favour 
joist thickness for loft conversion
- Thread starter jaysta
- Start date
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I asked a relative of mine who specialises in loft conversions and he also mentioned that calculation. Hes one of the old school. I have just under 4m span and my joist will have to be 9" x 2".
What are the different 'C' grades, is it something to do with the quantity of knots per meter which may weaken the timber when its under load???.
Of the top of my head the one I remeber reading on a price list was C16.
Here's an interesting link regarding timber grading:-
http://www.bsw.co.uk/processes/strength/animation.php
Worth a look.
http://www.bsw.co.uk/processes/strength/animation.php
Worth a look.
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Thats an interesting site, thanks for that. I'm looking around for prices today for 9 x 2" timber. I expect to pay around £16-20 per 4mt length.
Has anybody out there used a specific wood treatment on the new wood prior to fitting?? I was going to put a couple of coats of bog standard wood treatment from Wickes. Incidently is it a regulation that wood treatment must be used now on new construction or is it a matter of personal choice??
Where are you going to sit the joists is the next question?
Will they be going from gable to gable, running to the eaves is probably not an option surely?
If your going across the gables then you need a space between them & your ceiling joists which impacts on your head height. You could lessen the depth of joist by bringing in the effective span from the eaves. This could be achieved by using steel or using timber box girders & your joists hung from them via joist hangers. This method is a good way of support without steel, it narrows the loft down to the effective usable area, cuts down the joist size, saves on the effort of dragging large steels upstairs & you get a wall thrown in either end. --Brilliant!!
Will they be going from gable to gable, running to the eaves is probably not an option surely?
If your going across the gables then you need a space between them & your ceiling joists which impacts on your head height. You could lessen the depth of joist by bringing in the effective span from the eaves. This could be achieved by using steel or using timber box girders & your joists hung from them via joist hangers. This method is a good way of support without steel, it narrows the loft down to the effective usable area, cuts down the joist size, saves on the effort of dragging large steels upstairs & you get a wall thrown in either end. --Brilliant!!
H
hotrod
That's quite expensive. I've just checked online and my local merchant charges £2.14 per metre +vat for 47 x 225mm (220fs) Sawn Carcassing C16 Grade. Be wary of the list prices and discounting structures of the nationals (Travis, Jewsons etc) you will get stung if you are not careful. It is not unknown for them to discount upto 60% off list (I know for a fact Jewsons have in the past discounted timber at 60% off list!!) and this has been discussed in detail on here in the past. Take a look here for a rough idea on what you should be paying.
I get all structural timbers Vac treated as a matter of course, the extra cost is negligible. If you are sourcing from a decent timber merchants they will have the facilities to do this for you. Remember to treat any cut ends with a suitable preservative.
hth
ive just had calcs done for a loft conversion with joist spans of 3.9m and i can use either 9x2 C24 or 8x3 C24
Where are you going to sit the joists is the next question?
Will they be going from gable to gable, running to the eaves is probably not an option surely?
If your going across the gables then you need a space between them & your ceiling joists which impacts on your head height. You could lessen the depth of joist by bringing in the effective span from the eaves. This could be achieved by using steel or using timber box girders & your joists hung from them via joist hangers. This method is a good way of support without steel, it narrows the loft down to the effective usable area, cuts down the joist size, saves on the effort of dragging large steels upstairs & you get a wall thrown in either end. --Brilliant!!
I forgot about the timber box girder method. Have seen it done. The builder bolted 2 or 3 10" x 2" timbers together and spanned then across the area (over the room) and sat them on top of a wall. Cant remember how he sat them on the top of the wall tho, but it looked quite impressive. I bet that would need a calculation. Surely you would need to know if the internal wall you are transfering the load onto is load bearing. How would you know, apart from digging down underneath and looking for a footing???
I will search deeper for the timber costs, the company I work for have accounts at all the big builders merchants, I buy for my site frequently and know them reasonably well now so I will expect a good price. I may even be able to use the company discount.
Used a box girder on a job in Manchester. The internal walls were made of crap, so a neighbour, an engineer drew up the design. Used 9x3 spliced glued & bolted together as a base plate let into both gables, a 6x3 as a top plate, with 4x3s as verticals & diagonals. The structure resembles a "Bailey Bridge" which then has ply glued & screwed to it both sides. The joists are then attached using hangers.
This is an immensely strong structure which can be pre-fabricated below then assembled in the loft. If you doubt the efficacy of this, then try to bend a sheet of ply edge on, it's virtually impossible. Beats steel -has less deflection easier to get into a confined space & creates a wall either end.
(The span was over 20')
This is an immensely strong structure which can be pre-fabricated below then assembled in the loft. If you doubt the efficacy of this, then try to bend a sheet of ply edge on, it's virtually impossible. Beats steel -has less deflection easier to get into a confined space & creates a wall either end.
(The span was over 20')
The diags & verticals were cut in top & bottom & against each other, this acted to brace the whole thing against the roof & base plate. The ply etc bonds everything , there was no discernible sway or twist, in fact the structure was very solid without the ply.
Sorry misread your earlier post & editted above.
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