Joining joists


Postby chrisbpt » Sat Jul 11, 2009 5:25 pm

Hi apologies if this has been covered in the forums before. I recently bought and moved in to a terrace that was built in 1895. I am about to try and strengthen the joists in my loft as they are pretty bendy when walked on (think crawling around on your hands and knees praying you don't go through).

The existing joists are only 2" x 3"s and and span 6.5 metres (front to back) with a loading bearing wall right in the middle which supports the centre. The joists don't show any sign of woodworm or rot but do have some small cracks in them. Basically I think they are fine for holding up the plasterboards but I do want to feel safe when I am walking up there.

The loft is only going to be used for storage and I am not planning a loft conversion. I am planning to double up the joists (or sister them as they say in the US) with 200m x 47mm treated c16 timber. I managed to get hold of some span tables and this size of timber seems more than adequate.

As I can't find 6.5 metre timbers and I don't want to rip the whole ceiling down to get them up there anyway. I am planning on using 3600mm long timbers which will be cut to size and nailed to the existing joists. The joists will have to meet over the load bearing wall and there lies the problem. The rafters on both ends of the house join on the same side of each existing joist therefore I have to run the new joists all 6.5 metres down one side with them butted up over the central load baring wall.

Can I use splice plates to join the timbers (the join will be over a load bearing wall not as shown)?

[img]http://www.batmetalwork.com/graphics/fixing_6.jpg[/img]
chrisbpt
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Postby stoneyboy » Mon Jul 13, 2009 10:08 pm

chrisbpt,
Leave the existing joists as they are and do not try to fix new to old. Put your new timbers in the gaps between the existing joists and fit cross bracings to stabilise them.
end
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Postby chrisbpt » Tue Jul 14, 2009 7:18 am

Thanks stoneyboy,

One other question... I have had to take down part of the ceiling; as belive it or not it was only made of hardboard nailed to the joists. Having done this I don't think the existing joists will safely take the weight of two layers of plasterboard.

If I stand underneath and pull the joists downwards there is quite a bit of play. The wooden wall plates on the supporting walls are in good condition and as I mentioned the existing joists do have some cracks in them.

Therefore my question is ...can I fix the plasterboards to the new joists I'm putting in?
chrisbpt
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Postby stoneyboy » Fri Jul 17, 2009 6:39 pm

chrisbpt,
"can I fix the plasterboards to the new joists I'm putting in?" yes, but you may have problems getting the new boards flat - the old rafters will have sunk so you'll be using the plasterboards to push them back up again.
The easiest solution will be to notch the new rafters where they fit fit on the wall plates 15mm should be enough.
end
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Postby Perry525 » Fri Jul 17, 2009 7:07 pm

There are many homes with 3x2 inch joists.
Joists are held in tension by the weight of the roof and as such are capable of holding your body weight and plasterboard and a skim coat and various storage items, there is no reason why they should not hold floor boards as well.
But, there is also the question of joist spacing, 16 inch or less centres are OK.
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Postby chrisbpt » Sat Jul 18, 2009 4:11 pm

Thanks Stoneyboy I hadn't thought of that. Will go and buy the new joists next week.

Thanks for the reply Perry525, unfortunetly contrary to my inital post the old joists in one area are in pretty bad condition. This may be the reason why the previous owner had used hardboard instead of platerboards on that part of the ceiling. If I stand underneath the open part of the ceiling and pull on a single joist there is around 1cm of movement (bending) and thats without putting any real weight in to it. Put it this way I wouldn't dare stand in the middle of one of those joists and I'm only around 12 1/2 stone.

The exisiting joists are spaced about 40cm apart and I am planning on using 47mm x 200mm c16 treated timbers inbetween the existing joists as Stoneyboy suggests. I won't bother using splice plates or butt joining the new timbers I will probably just run them past each other so that they overlap by about 70cm and screw them together.

I got the span tables from here if anyone wants them:-

Google "nelincs.gov.uk/NR/rdonlyres/F77068E2-150C-4733-B005-3CDDC2AD0734/0/BC_ceilings.doc"
chrisbpt
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Postby chrisbpt » Fri Aug 07, 2009 8:38 am

Ok managed to get all the new joists up there and screwed in place. As mentioned above I have used 200mm x 47mm C16 treated timbers spaced 400mm apart.

To avoid the joists twisting I am planning on putting in a three rows of noggins between the joists. What size should they be?

Should I place them over the central load bearing wall and at both ends close to the enternal walls?
chrisbpt
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Postby stoneyboy » Tue Aug 11, 2009 9:38 pm

chrisbpt,
You can either make up cross noggins from 50x25 batten or buy the special metal ones.
Should I place them over the central load bearing wall and at both ends close to the enternal walls? - YES
end
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Postby chrisbpt » Mon Aug 17, 2009 5:06 pm

Thanks mate
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