an extension and the need for underpinning?

[DIYedboy]

I am planning to put a single storey extension across the back and side of my 1930's semi.
the back wall will need the existing lintel above the patio door replaced with a longer one to make a larger opening into the extension. Does this work ever require the existing back wall to need underpinning?

 

[limerockconstruction]

Depends on the extent of the new opening versus the old one. Sometimes yes and sometime no....more info please.

 

[DIYedboy]

the rear wall is roughly 18feet across, the only opening in it is the patio doors that are 7 feet across (kitchen door n window are on the side of the house). I would like to increase the size of the opening to roughly 14feet, thereby reducing the right side of the rear wall to 3 feet and reusing the existing bearing on the left side

 

[jeds]

I'd say you're right on the limit there. And that means it needs to be checked carefully. Wouldn't be so much a problem in a more modern build but your foundation could be almost anything. You need to check the foundation and ground bearing, carefully check the loads involved and also check lateral stability. My guess is you will end up needing a goalpost design.

 

[DIYedboy]

Jeez Jeds, that don't sound good.

I live in a predominantly clay area, I presume I will need to dig a hole next to the wall to see how deep the footings are, what depth would be considered acceptable for the footings?

I have no idea what you mean by a 'goalpost design', are you suggesting that the lintel bearing walls will have to be rebuilt in concrete blocks for the lintel to sit on, or be replaced with vertical RSJ's that support the lintel?

 

[limerockconstruction]

agree with John - I believe that masonry returns are to be a minimum of 665mm in larger knock-throughs such as this, but i'm no SE so not 100%. My guess is that your close to the limit and to get a trusted SE involved.

 

[^woody^]

I've never known foundations to be underpinned or even checked for work such as this

 

[limerockconstruction]

Dont panic DIYedboy - its pretty common practice - i'm not sure its a DIYer job, but if you do a lot of the preparation and making good work it could be a small job for an experienced builder.

You are right to assume the vertical Steel posts.

 

[limerockconstruction]

I've never known foundations to be underpinned or even checked for work such as this

Seems to be more common place in recent times it has to be said...had to underpin masonry returns a couple of times last year.

 

[^woody^]

It may be common, along with goalposts, when crap clueless engineers are involved who want to over design, and cover their own backsides

 

[jeds]

A square house is a fairly stable shape - the 90° walls buttress each other. If you remove a large propertion of one wall there's a risk that the two adjoining walls become unstable. You aren't completely removing all the wall so there's a chance you may be ok but it needs to be checked. You do also have the advantage of the adjoining semi - so thinking about it it's more likely you will be ok - but it still needs to be checked.

The other problem is you are concentrating the loads along the beam, down through the supporting piers and into a much smaller area of foundation than previously. The key thing is to check the loads and make sure the foundation can take that concentrated load. It's not so much the depth - it's more a question of the capacity of the soil, and the width and condition of the foundation. Once you know the loads, divide it by the area, add safety factors and hopefully it will be well within the capacity of the soil.

You also need to make sure the piers can take the extra load. If not, one option is to rebuild them. Another option is to use a column instead. If the foundation is dodgy (or it's difficult to be certain) a further option is to use a column and a new pad foundation. What you now have is a beam connected to two columns - i.e. a goalpost. The beam can be connected to the columns in such a way as to resist rotation - which gives you some extra lateral if needed. The main advantage of this design is that the pad foundation is new - and therefore predictable. The column is designed for the job - therefore predictable. And you don't have to worry about lateral stability.

You might end up with some or all of these or, if you're lucky, you might just get away with a beam on some padstones. But it all needs to be checked to be sure.

 

[^woody^]

But the loads are tolerable by typical domestic walls/piers and foundations in common soil conditions. And the typical rectangular structure will not twist either

Not everything needs to recalculated

People forget basic construction principles which have gone on for centuries, and feel the need to check, calculate and put in steels and struts and supports for no reason

 

[RonnyRaygun]

It may be common, along with goalposts, when crap clueless engineers are involved who want to over design, and cover their own backsides

"Crap" is designing a beam and not bothering to consider where the loads are going...or the lateral stability of the building...

 

[RonnyRaygun]

But the loads are tolerable by typical domestic walls/piers and foundations in common soil conditions. And the typical rectangular structure will not twist either

Not everything needs to recalculated

People forget basic construction principles which have gone on for centuries, and feel the need to check, calculate and put in steels and struts and supports for no reason

But it's a recent phenomenon to have large openings in domestic dwellings. Footings in pre war properties might only be 12 inches deep by 16 inches wide. A 665 pier will then utilise no more than 1 metre of footing giving a capacity of about 45 kN (assuming clay with a capacity of 100kN/m2).
There's a good chance there will be a more than 45 kN on that foundation.

If so the beam shouldn't bear directly onto the pier without strengthening the foundation.

Or using a goalpost frame on a new pad footing.

Or using a box frame that can spread the load back along the existing foundation.

 

[DIYedboy]

Flippin 'eck, this job is panning out to be a lot more involved than I thought, I've replaced lintels and removed walls, but I think underpinning and bunging in a 'goalpost' frame is waaaaay too rich for me!.
Perhaps I'd better go and see my BCO and see what they say.

I personally like woody's approach, but I suspect the BCO will prefer goalposts..................what a shame!