Replacing joists

[highleigh]

As part of my renovation I have decided to replace a couple of joists. The joists in question form a T shape over the hallway/stairs with one side of the T open for the staircase and the other side has short joists (1m) hung on it supporting the landing floor above. The two main joists are 175x75s and about 2m long each. They both used to have a brick wall build directly over them which has been removed and will be replaced with a bannister rail. One is quite badly warped and the short joists hanging from it notched and are precariously hung on a piece of 2x2 nailed to the foot of the joist. It seems surprisingly stable but I really want to improve it.

Whilst replacing the joist I would quite like to move the top of the T joist and extend the joist that is hung from it. It is currently 2m but I am thinking of increasing this to 3m as this would provide extra ceiling height above the stairs.

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• highleigh
• 25 Jun 2012
• 1

Not a great picture but I have marked where the joists are even though you can't actually see them.

Couple of questions I have are:
How can I work out the loadings on the joists to confirm they are fit for purpose. Each end of the joists making up the T are resting on brick walls and I will use a hanger to hang the joists on each other.

Would it be better to use 2 175x50s bolted together rather than a single 175x75?

Is there anything else I need to consider when doing this job. Building control are involved so I want to make sure it meets any regs that may be relevant. I am sure the replaced joists will be far better than the current arrangement no matter what I do.

Any advice would be much appreciated.

 

[tony1851]

If the stalk of the"T" is being extended from 2m to 3m in length, and it is only supporting 1m long joists from one side, nothing on the other, and a bannister rail above, this does not add up to much.
Presumably the area of floor supported will be 3 x 0.5 = 1.5 sq m, giving a load of approx 3kN (about 700lbs). Two 7" x 2" bolted sie-by-side would be more than adequate.

 

[highleigh]

Thanks for the reply. It will actually support 3 x 1m floor + bannister but in my mind this still isn't very much? It used to support 2m x 1m + a very heavy concrete block wall.

Are you suggesting 2 bolted 50x175s are better than a single 75x175 joist?

 

[tony1851]

Even with double the floor load, the beams you propose will still be well within their stress-limits.

(Structurally, two 175 x 50s are better than one 175 x 100 by about 10%. This is because any weaknesses in the timber (such as loose knots or splits) are statistically evened out when 2 or more similar-sized beams are fixed together. This allows designers to use higher stress values. Therefore, two 175 x 50s will be significantly better than one 175 x 75.)

 

[highleigh]

At the weekend I took the plunge and cut out the existing joists.

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• highleigh
• 4 Jul 2012
• 1

I ended up cutting them all out to the last one adjacent to the end wall and replaced the main joist with 2 175x50 joists. I have fitted just one for now, hung the short ones on with speedy hangers and just placed the other along side it although I fully intend to bolt it to the other.

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• highleigh
• 4 Jul 2012
• 1

My quandry now is what to do at the far end. I had originally thought I would put in a new double joist, and still could, however I could also just hang to the last joist, with a heavy duty joist/truss hanger. The joists are 60 years old but I can vouch they are very strong after sawing through some of them. Another option is to put another along side the top one and bolt it to it and then use a truss hanger.

2 questions

What do I actually need to do. The last joist is only 50x175 but looks strong enough to support the landing so can I just hang onto this as it is?

I need to get a truss hanger (think that is what it is called) and bolt/nail this to the joist to hang the doubled up joist. These new joists are 45 wide so I have a total width of 90mm. Does anyone sell these as singles as anything I have found so far seem to come in packs of 10.

As always, any advice greatfully received.

 

[tony1851]

If the existing joist is 7x2, and spanning approx, 6ft, in good condition, and not supporting anything else. it will be OK to support the point load from the double trimmer (remember, it will only be supporting the weight of about 1/4 of the landing).
For the hanger, you do need the h/d type (not the bendy tin things). If you cant get one 90mm wide, you could use one 75 wide if available; you would just need to chisel a bit off the side of each pair of the trimmer. The bearing area on the shoe of the hanger will still be adequate for the load involved.

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• tony1851
• 4 Jul 2012
• 1