DIY Doctor

how to prop to change a lintel + correct bowed wall plate

Postby Jonathand » Tue May 12, 2020 9:45 pm

this is a long one, sorry!
I am renovating a 1934 built semi.
There are timber lintels throughout. The timber lintel above the rear bedroom window is particularly undersized, and has bowed. The deviation is approx. 20mm at its most extreme and it’s pulled brickwork to the right the window, causing quite a lot of cracking. The wall plate is 2 brick courses above and it has also bowed, causing the ceiling to bow and also causing a corresponding dip in the roof above the window that is visible from the outside. It’s a full hip roof so the rafters sit on the wall plate above the window.

I’m going to use helical bars to reinforce the brickwork along the cracking and am replacing the timber lintel with a steel box beam lintel. I want to correct the bowing of the ceiling and dip in the roof so would if possible like to straighten out the wall plate. I’m going to reroof later this year, So I guess I can correct the dip in the roof by putting packers on the top side of the rafters before battening, but this wont correct the bowed ceiling.

So my questions are:
1. Will I be able to lift the bowing out of the wall plate using suitable means, without it breaking?
2. What’s the best means of supporting the weight of the roof while I install the new lintel, bearing in mind straightening out the wall plate?

The new lintel’s height is equivalent to the old lintel and 1 ¼ brick courses above it, so the only options I’ve got for propping it is either under the wall plate or directly to the rafters, as the 2 brick courses will have to come out. The rafters are spaced at 380mm centres and there are 5 of them along the entire span of the new lintel. The window opening is 1800mm and the lintel will sit 150mm on brickwork either side. So total 2100.

If I use say 4 strong boys on accrows under the wall plate at 400 centres, will this be sufficient to support the weight and to raise the dropped rafters without the wall plate being damaged? Or should i support each rafter individually via a plate of some kind bolted to each one?
Thanks for your help.
Attachments
window 3 sml.jpg
window 2 sml.jpg
winodw 1 sml.jpg
Jonathand
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Postby stoneyboy » Wed May 13, 2020 9:47 pm

Hi jonathand,
You will have to fit strongboys directly under the wall plate because the ceiling joists are not paired with the roof rafters.
Suggest you fit 3 above the window and one above the movement crack to the right of the window so you can rebuild this section.
I doubt you will take out all the bow - the whole roof width will lift.
Regards S
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Postby Jonathand » Thu May 14, 2020 12:01 pm

stoneyboy wrote:Hi jonathand,
You will have to fit strongboys directly under the wall plate because the ceiling joists are not paired with the roof rafters.
Suggest you fit 3 above the window and one above the movement crack to the right of the window so you can rebuild this section.
I doubt you will take out all the bow - the whole roof width will lift.
Regards S

Hi thanks for the advice. I had another idea: I prop the rafters individually close to the wall plate, leben the rug by raising these seperately, and cut out the bowed section of wall plate, put in a new piece, using lap joints to join it to the original. The lower the rafters onto this. Then I have a flat roof and can at the same time change the lintel. Do you think it likely I can level the roof this way, or do you think it's likely the rafters will have bent slightly?
Jonathand
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Postby stoneyboy » Thu May 14, 2020 10:02 pm

Hi Jonathand
Suggest you stick with your original proposal.
Your roof is a complete structure and it's better to jack it up complete. If you try to jack individual rafters you will need to separate them and the joists from the wall plate which will be difficult, also you will loose the structural tie across the roof which could be disastrous.
Regards S
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Postby Jonathand » Fri May 15, 2020 9:38 pm

lose structural tie as in the roof might start spreading once the joists/rafters are off the wall plate?
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Postby stoneyboy » Sat May 16, 2020 9:56 pm

Hi jonathand,
Your wall plate is tying the rafters and the ceiling joists together, it's a triangular structure dependent on the three element, rafters, joists and wallplate.
Regards S
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Postby Jonathand » Sun May 17, 2020 9:43 am

Yeah get that. Will jack as one. Thanks for the advice
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