REPLACING FLOOR JOISTS

[gsmith]

I had a chimney removed and the shorter joists to the chimeny on the left hand side of the room were not supporting anything so I have removed them, the cavity is left in the wall. I thought I should leave the joists overhanging on the right as they are supported on the steel beam and run into another room.

Could I have some advice on completing the joists. Should I make a cavity to the side of the existing ones, joist hangers and run into the steel beam. I could then bolt the two existing ones to this for addittional strength? I'm not too sure, none of the old beams have DPM or joist hangers due to its age. Are tehre regulations that you have to adhere to when repairing/adding new beams.

Many thanks.

Untitled

• gsmith
• 15 Aug 2011
• 1

Untitled

• gsmith
• 15 Aug 2011
• 1

Untitled

• gsmith
• 15 Aug 2011
• 1

 

[RedHerring2]

I'd just replace the new joists so that they sit next to the others and go into the wall at the other end.

You could bolt them together, if you wish but not necessary, as long as they're sitting on the steel beam.
The strutting will prevent twisting/movement, which should be replaced, including between the wall and the joist. You don't need to use herring bone strutting. You can use solid timber. I think it must be ²/³ the depth of the joist, minimum.

Any regs would apply to complete floor replacement, eg. size of joists.

Any dpc requirements would apply to ground floor installations rathre than first floor replacements.

To provide sufficient floor board support, if you were intending to place the new joists in front of the existing, looking at your first picture, nail a piece of 50mm X 50mm or similar along the top side of the joist that goes closest to the wall, before installation. You won't have any reasonable access to fix anything once it's installed.

 

[gsmith]

Thanks for your advice. Can the joists just sit in the wall the same distance they have done in the past or do they need a form of joist hanger?

 

[RedHerring2]

100mm bearing in the wall will be fine. You can sit them on a piece of timber, slate, anything like that to give them a decent bearing.

 

[gsmith]

Thanks.

There is a timber wall plate so that should be fine. Should I cement the bearing in place?

One other thing is that the timber for the other joists is 165mm x 65mm. An awkward size!

When I went to order some new treated timber I could only order 150 or 170mm x 47mm. Is 150mm x 47mm ok and then I will need to pack out the bearing end and add a strip of timber for the plasterboard ceiling to be attached to?

 

[RedHerring2]

By cementing in place I assume you mean fill in the sides with brick/cement. Then yes, that's normal. Take care if there's a cavity, not to drop stuff down the cavity.

As for timber sizes, have you asked if they'll cut the 170mm down to size for you? Any half decent timber yard will do that. You only need two. You don't need to worry about the 65/47mm dimension being adjusted. Although there is an outside chance of 165mm X 47mm not being sufficient, we need to know the span.

As to whether 150mm X 47mm is sufficient, again we'd need to know the span.

 

[gsmith]

Yep I meant cementing to the sides.

The span is 4.1m. The centres of the joists are 380mm

 

[RedHerring2]

Anyone got their timber tables handy?

I'll look for my mine in about an hours time.

 

[RedHerring2]

According to the timber tables you need 170mm X 63mm, (at 400mm centres), so, as there's only two joists to be replaced and they're at 380mm centres, I'd settle for 165mm X 65mm. Or 165 X 63.

BTW, Strutting/blocking should be not less than ¾ the depth of the joists.
One at mid-span is sufficient.

 

[gsmith]

Thanks. I’ve ordered 75mm x 200mm C16 treated timber and the timber merchants are cutting it to 165mm (so the thickness of the timber is slightly oversized – the timber could only be purchased in 50mm or 75mm and it would be unnecessary to cut down the thickness as it is offering more strength).

To create the noggins I will use the offcuts which will be the full depth of the joists. Are these just screwed in from either end, offsetting them to enable you to screw them? How do you normally attach the noggin bwteen the wall and the first joist?

Thanks for your advice. It would have cost me a substantial amount of money to get someone in when some sound advice solves the problem.

 

[RedHerring2]

Yes, noggins are just nailed through the joists into the ends of the noggins. as you said, off-set to allow access to all ends.
The end blocking is simply cut to size, wedged in and nailed through one joist.