Insitu Padstone on cavity wall!

[STORYTELLER]

Hi,

A confusing one. For our knock-thru and main steel at rear of house, our SE has specified insitu padstone as follows through onto the new extension wall (steel actually only 100mm dimension now, not 250mm):

The builder has produced this with a day to go before steel installed!

we checked with SE who said basically drawings need to be followed and builder can't just do what he wants.

Question here is builder now unhappy and asking about insulation problems due to shuttering in cavity...

As anyone come across a situation like this before? I also read something about these padstones needing 7 days cure before being string enough to take load...?

thanks for any input!

 

[Koolpc]

Builder needs to follow the plan. Simple really...

 

[martygturner]

Casting insitu is pretty easy and in any installation you need to prop the steel to wait for any mortar to go off if you are using precast pad stones, insitu or even none at all and it would defiantly need to be propped if its in situ cast.

are you absolutely sure that your builder is not going to cast as those blocks just look like they are making up the wall to rough height. Your builder also is correct on the insulation front... in-situ can be a huge cold bridge, its a big lump of concrete that acts as a massive iceberg in winter.

I am really surprised that the SE specified just in-situ... they are normally reserved for when getting a beam in and up is a bit tricky. They offer nothing over an precast.

 

[STORYTELLER]

Casting insitu is pretty easy and in any installation you need to prop the steel to wait for any mortar to go off if you are using precast pad stones, insitu or even none at all and it would defiantly need to be propped if its in situ cast.

are you absolutely sure that your builder is not going to cast as those blocks just look like they are making up the wall to rough height. Your builder also is correct on the insulation front... in-situ can be a huge cold bridge, its a big lump of concrete that acts as a massive iceberg in winter.

I am really surprised that the SE specified just in-situ... they are normally reserved for when getting a beam in and up is a bit tricky. They offer nothing over an precast.

exactly... should be 215mm insitu cast. It looks like about 200mm combo of 100mm pre cast lintels, concrete blocks and maybe even a cut-down aircrete block... also, the blocks in that stack are at different heights.... Assume he specified insitu as it steel onto nib and into side wall and then load distributed onto new side wall... so its one piece spreading the load evenly. Builder now saying he wants SE to specify a design for the shuttering! Steel due to be installed today or tomorrow and he has ad the drawings to review for about 6 weeks.... maybe more...
Surely it's not rocket science to shutter the cavity off there.... then seal small gaps with foam or something....

 

[freddiemercurystwin]

How do the architects details/drawings overcome the cold bridge?

 

[STORYTELLER]

How do the architects details/drawings overcome the cold bridge?

I dont know for this situation... we aren't using an Architect. We made the current mistake of trusting the builder.. The SE said this is usually allowed given the need for it... and the relatively small area in the whole extension (30M sq)_

 

[martygturner]

I would also point out that the beam will need building control...as that's at your builders risk what are you worrying about ?

 

[noseall]

we aren't using an Architect.

The builder need to follow the SE's instructions to the letter. If there are any obstacles, then a conversation is needed. In-situ pours are not that difficult...

situ1.jpg

• noseall
• 5 Dec 2024

situ2.jpg

• noseall
• 5 Dec 2024

situ3.jpg

• noseall
• 5 Dec 2024

 

[STORYTELLER]

The builder need to follow the SE's instructions to the letter. If there are any obstacles, then a conversation is needed. In-situ pours are not that difficult...

situ1.jpg

• noseall
• 5 Dec 2024

situ2.jpg

• noseall
• 5 Dec 2024

situ3.jpg

• noseall
• 5 Dec 2024

indeed! Thanks for the pics - really tricky to find examples of this and drawings online. This could be quite useful in a conversation.

 

[STORYTELLER]

I would also point out that the beam will need building control...as that's at your builders risk what are you worrying about ?

is a fair point... it depends if the BC officer looks at the SE spec and compares it to the built padstone spec... I've seen so many examples of BC inconsistency - from vague OKs, to accepting Pics, to absolutely not accepting a micron of deviation from any rule, reg, drawing etc

 

[Ivor Windybottom]

Is the bridging of the cavity acceptable practice? Looks weird to me, I always thought the whole point of a cavity was to be a cavity.

 

[noseall]

Is the bridging of the cavity acceptable practice? Looks weird to me, I always thought the whole point of a cavity was to be a cavity.

Structural integrity takes precedent or supersedes thermal etc.

We often get asked by the SE to build in 7n dense blocks, directly beneath steel beams, in place of more thermally efficient Fibo's or other aerated blocks.

Steel beams with a plate welded beneath, are a bu££er for wicking the cold. We get around it by fixing insulated boards to the steel or, if you don't have the reveal tolerance - use ex foam-fix as an insulating cushion.

 

[Ivor Windybottom]

I'm guessing that the designer of it shoved its end out into the new extension to avoid the padstone jutting into the interior on a nib wall.

Seems weird to me. That will be cold and potentially damp.

 

[noseall]

I'm guessing that the designer of it shoved its end out into the new extension to avoid the padstone jutting into the interior on a nib wall.

I'm guessing that he's not banking on the existing foundations being man enough on their own. So he's gaining some additional support from the new masonry (250mm of additional steel and 400mm of additional padstone), in order to satisfy the calc's criteria.

 

[Ivor Windybottom]

I would have made the extension half a brick wider, then made that section of the inner leaf 9" wide under the steel by widening that part of the wall into the room. Then the steel wouldn't have crossed the new cavity so wouldn't be bridging cold and damp and would also be fully supported by brickwork to the ground, rather than sitting on a pad that's 50% over fresh air.

Perhaps you could have even extended it further so eliminated the need for any nib wall at all within the original house.

But... the original house doesn't have a cavity anyway, so it's all a bit academic.