Replacing a First Floor Lintel which supports the roof?

[djaychela]

I've got planning permission and SE calcs done for an extension. House is a chalet bungalow with an existing rear dormer (which was done back in 83), and this will be replaced with a larger dormer. As a result, there's a 203x133 beam needed in the loft space, and to support that on one outer wall, a new lintel has been specced.

However, above this lintel in the roof space, there's a whopping (220x70, angled over) wooden beam which runs the entire width of the house - it's about 150mm over the window opening. There's also another beam which is going to come out which was put in when the dormer was created.

Can anyone give me some ideas on how this is replaced safely, while supporting the wooden beam? This is on the first floor, and there's nothing firm under the floor - no structural support under the floor, etc.

I've attached two drawings (admittedly from the outside perspective, but I think you'll get the idea), showing the current and proposed situation.

I'm building as much as possible of this myself (keen DIY-er, I've done concreting, brickwork and building large wooden structures before), but this seems to be next level.

Current layout with two wooden beams:


Proposed Layout from SE:


Thanks for any advice given.

 

[tony1851]

Because this is a safety issue, the SE should not just give you beam sizes, but also has a responsibility to outline a method statement of how the work should be safely done.

 

[^woody^]

You need a prop under the beam and a prop at ground floor level.

Has the engineer confirmed that a lintel is ok with that point loading from the purlin?

 

[djaychela]

Thanks both - much appreciated.

Tony - No, the SE hasn't provided anything other than sizes, so I'll see what he has to say on that front.

Woody - Sorry to be dumb, do you mean that there would be two props - one from ground floor level to first floor, and one from there to support the purlin? (I've attached a diagram). No, he hasn't confirmed if the lintel will be OK with the point loading from the purlin. I'll see what he has to say about that as well. Looking at the calcs, it shows 19.45kN load on the new beam, and that (I think) is the load of the roof above it, so I don't know if the purlin will be heavily loaded once the beam is in place.

Support idea:

 

[^woody^]

Yes that's right. Directly on and below a floor joist.

Standard lintels don't like point loads directly above them. See the manufacturer's guidelines and then confirm with the engineer. It may require additional measures or a different lintel.

 

[djaychela]

Thanks again Woody - much appreciated. Just amazes me that I get clear, direct answers from people here, whereas it takes days (sometimes over a week) to get any response from my SE, and when I do, it's of the "well, you didn't ask the right question" ilk, rather than clear, actionable advice.

 

[djaychela]

Does anyone know who I should be searching for to find a company who would take this work on - in terms of replacing the existing lintel? This is the one part of the build that is keeping me awake at night - pretty much everything else is something I'm OK with, but replacing the lintel without the house falling down and making sure it's all good is something that is probably beyond me, and I know you only get one shot at it!

 

[^woody^]

General builders do this work. It's not specialist in nature, but it does require experienced people.