Joists

[Anonymous]

Greetings to the Forum and my new found friends.

Thank you all for your answers that were not inspiring, it must be down to my vagueness with a joist dimension enquiry.

We speak of a 140-year-old farmhouse with a roof height of. 9 metres

There are 7 rafters (16 x 12cm) on each side of the roof at 100cms or 1m apart and they have 7 collar beams (14 x 13cm) at ceiling height with a further 7 higher up the roof.

The lower collar beams held a 3.5cm-board floor, heavy bales of hay and farm machinery for over 100 years, but I removed infested flooring. These lower beams are supported in mid span by a strong wooden pillar/beam wall on a load bearing area, so the length from rafter to wall is 3m.55cms.

We wish to increase the height of the lower collar beams and fix joists between them at an equal height for a new floor. If you imagine a sectional plan of collar beams with added height and joists between them at that height you will see that this is a floor platform and gives more strength to a roof with a slight inward bow provided the right surrounding is fixed.

So based on joist heights being twice their width please can you give me the joist dimension for a span of 3m.55cms?

Warmest regards
Art King

 

[Anonymous]

Art - a couple of points:-
Firstly, I don't recommend criticism of people who respond to your questions. You may have found the responses "not inspiring", but they were doing their best to help.
Secondly, I don't know whether I'm alone in this, but I'm afraid I didn't understand your further description of your plans.

 

[Anonymous]

Art
Afraid I too am not very clear about your description.
From purely a `span` point of view you will need at
least 150 x 50mm joists; I would go one over and put
in 175 x 50mm - either way at 400mm centres.
From the structural point of view ie. load bearing
on collars and rafters I would recommend you seek advice from a structural engineer.
rgds
Phil

 

[Anonymous]

While I agree with Phil's thoughts about your joist sizes, I am still concerned about the word "collar". This suggests the possibility that the new joists will be hanging entirely off the rafters. Is this the case? If so I would definitely hesitate to place any new loads on these roof members without a structural engineer's OK.
However, if this is wrong, and the joists will actually be trusses resting on top of wall plates, then you may have less to worry about. But it's difficult to be certain without a clearer idea of the proposed final arrangement.

 

[Anonymous]

I have to say that I dont really understand your exact meaning - and I think playing with the collars isnt as simple as you seem to imply.

however I stumbled on this link which has an abreviated building regs table of joist sizes.


joists etc