Need idea's and info on Joist spans

[r896neo]

I have just bought a bungalow. Its a very simple long rectangular shape with traditional cut roof and gable at either end.

We are considering changing the internal layout to move the living and kitchen space to the rear with bedrooms going to the front. In the long term they will move upstairs when we convert the loft.

So i have included a crap sketch to help explain.

[/img] View media item 49658
The rear of the house is split into 2 rooms which have a solid dividing wall. I would like to remove this wall but it carries the weight of a haphazardly converted room above and also the ceiling joists for both rooms bear on it.

The floor of the room above needs strengthened anyway so to save having to put a steel in I am considering dropping the ceiling height 8'' and putting new joists in spanning right across to both outside walls (standard cavity walls)

This would also give me an extra few inches in the room above. Obviously though i don;t want to have a very low ceiling so a big 300mm deep JJI joist may not work. The span is 5.9m will any standard timber joist make that?

Also other ideas (such as hanging them off the purlins which run perpendicular to them) welcome. We will be getting an architect to do some drawings at some stage.

Sorry its so long. cheers

 

[ABCwarrior]

I used this site when planning my shed base and decking frame.

http://www.diydoctor.org.uk/projects/load-bearing_walls.htm

The biggest cross section at 75x220mm at 400mm centres is only good for 5.11m span.
Know it's not much help with your span being longer than that, so unless a structural engineer can calculate the required dimensions in standard timber, you may be better with JJI or steel?

Don't know if it's possible to piggyback joists by bolting them together to gain more strength/span length....

 

[tony1851]

Your joist-span issue could probably be solved by using suitably-deep joists and possibly spacing them closer than usual, or doubling them up etc. You would also be allowed to reduce the total loading as the normal loading for domestic floors would not be going right into the eaves upstairs. An architect (or better still an SE) would advise.

This may not be the main problem,though. If you are taking out the existing ceiling joists, you will loose the triangulation of the roof. What would you do to prevent the rafters from spreading?

 

[r896neo]

The proposed new joists will be directly replacing the ceiling joists and can be tied in to the rafters at wall plate level.

 

[chickenlips]

If your looking to put a future dormer in, then putting new purlins in at the upper ceiling level (i.e at the new dormer ceiling height) this would stop the spreading of the rafters when you take out the old ceiling joists.

Although if there is already an existing room up there, then the purlins that are existing should be stop the rafters from spread. Although I would still recomend putting new purlins in at the upper ceiling height so that the rafters are then notched (birdsmouthed) over the purlin.

Obviously you would need to double up the rafters either side of the dormer and look at trimming between the dormer opening.