Full width 6 metre knock through

[sambotc]

hi,

I have a mid terrace house which I'm looking to add a rear extension to.

The biggest issue for me is the knock through. The current cavity wall is 6 metres long, with 2 upvc Windows and a door already in place (the door and one window are joined and share the same lintel)

Both lintels are concrete.

My concern is how to take the weight of the floor to remove existing and replace above along such a wide opening?

I'm looking to do most of the work myself under building control inspection and getting a structural eng. calculation for the steel.

How would the pros here support the above in this situation? Also, does anyone have any idea roughly what size steels they would expect to see used on something like this? I'm trying to find out how heavy it will be!!

Much appreciated in advance

Sam

 

[tony1851]

It will be heavy. To address the deflection issue alone your beam(s) would probably need to be 254 deep. rather than the more usual 203 deep.
Probably something like two 254 x 146 as long as they were well-fixed together.

 

[chappers]

As Tony said they will be very heavy and deep in section.
As for support, there are varying factors;
Do your 1st floor, floor joists run into the cavity ? If so then you could support the joists and hence the inner skin with acrows and boards across the joists and strong boys on the outer skin (get your steels into position inside them first though.
If your floor joists run parallel to the wall in question you may be better considering putting some needles through both skins above the first floor joists

 

[sambotc]

Hi,

Thanks both for the replies. The steel you mentioned is about 32kg a metre from what I can find so, yeah, fairly weighty but manageable just about I think.

The joists do run into the inner skin at 90 degrees to the wall so that sounds like a possible idea with the acrow props. How would you support the outer skin though?

Would you knock each one out separately I'm guessing, then use some of those supports that slot into the brickwork above the external lintel to support the weight above?

Either way, it's not something I'm looking forward to doing!

The removed render shown is to find the existing lintel and to check the construction type. They seem to have used some form of concrete in between the standard block and the end of the lintel.

Thanks for the advice, any further help is appreciated.

Sam

 

[tony1851]

Assume you would be doing this with a Building Regs application?

If so, be prepared for your building control people asking for a steel frame rather than just a beam.
This will involve a steel post at each end, bolted to the beam, to form a 'goal post',

 

[sambotc]

Hi,

Yes, all through building control.

What's the purpose of the goal post then as I have a good 150mm bearing on each end for it to sit on?

What sort of span wouldn't require this out of interest. It's possible I could reduce the span by 500mm either end by building up the windows first, although I'd prefer the full width ideally.

As you can see, the house isn't worth a great deal of added expense where it can be avoided, however we are running out of space and I'm self employed so moving isn't currently an option.

Thank again

Sam

 

[jeds]

What's the purpose of the goal post then as I have a good 150mm bearing on each end for it to sit on?

The goalpost provides lateral restraint to resist overturning. Normally this is provided by piers either end but if you go full width there are no piers and a strong wind could take it over. Of course, in practice, your house would be restrained by the adjoining houses but BC often don't accept that and will say the house must be capable of standing on it's own.

 

[wessex101]

They seem to have used some form of concrete in between the standard block and the end of the lintel.

Whenever I see that roughcast render on what looks like an ex-local authority house I think "is it non-traditional construction" and the comment above didn't help. Probably worth some more thorough investigation just to be on the safe side. Otherwise what everyone else said but it is quite a big job which could go horribly wrong if you aren't totally sure what you are doing.

 

[chappers]

Yeah I would definitely be wanting a bit more investigation, this job has the added complication, that that looks like a lintel that was cast in situ during the construction of the building and removing it and replacing with a steel makes the job, more complicated.
However it may be that with some lesser additional support it may be possible to leave it in place

 

[sambotc]

Hi,

This is exactly what I originally thought and I posted a thread about it a while ago.

However, it appears not. We were originally led to believe that the house was a wimpey no fines when we bought it 9 years ago.

It's not, it's standard block work.

Then thought as above the white strip was one giant lintel cast in situ, so as you can see I removed the render which was applied on a metal mesh. Behind there is a concrete lintel to the right, a small in fill and then block work the the right. I'll add a picture later today.

I'm quite happy to have piers on each end of it simplifies the whole process.

Thanks for the replies, all very helpful

Sam

 

[sambotc]

Picture attached. From right: block, approx 50mm of some form of concrete fill in, mortar line, concrete lintel.

 

[wessex101]

Don't want to sound argumentative, it is just difficult forming an opinion on the basis of a couple of photos.

Are you sure that is a block to the left of the lintel and not in-situ concrete. I vaguely remember working on some houses a very very long time ago, possibly Laing Easiform (not dissimilar to the Wimpey No Fines), I really can't remember and they were built using an in-situ concrete mix like that poured into shuttering that makes up cavity walls (but I don't remember the pre-cast concrete lintels so I'm probably wrong).

 

[chappers]

I would have thought that to the right is also a lintel as that would then bridge over the door and window below.
I would expose the whole lot a and get a good look at what you have there, your going to have to do it at some point anyway.

 

[sambotc]

I've quickly knocked out a bit more, it's quite tough going as it has a mesh covering the render and is well bonded to the block work.

It's definitely block. I have found the mortar lines. The infill I referred to, seems to be the same material used as the padstone below the lintel? Not sure if that is common practice or not?

There is also slate under the lintel on top of the padstone as a packer? and some block infill around the lintel.

Thanks for the help all, all appreciated.

Sam

 

[sambotc]

I would have thought that to the right is also a lintel as that would then bridge over the door and window below.
I would expose the whole lot a and get a good look at what you have there, your going to have to do it at some point anyway.

Yes, that would be my next point to investigate. I'm fairly happy that it is standard construction. My concern is about installing the steel. Would my local planning inspector insist on something a structural calculation didn't include?

There seems little point in getting a structural engineer to do a calc if the planning officer saying you need X Y Z, i'd be better off just agreeing with it and doing it surely?