Hollowcore Conrete Slab 1st Floor and 100mm Blockwork

[Gaf]

Some other posts I've made about insulation and airtightness have made me wonder a bit about the structural side of the construction drawings we received for a 1.5 story build.

 

Going concrete slab 1st floor and locally they all do hollow core. Conversations with airtightness testers got me reading a little about the structural side of the concrete slabs and I have seen posts about some manufacturers are fine with the slabs sitting on 100mm block work, but others requiring 215mm block work, with some engineers also unwilling to sign-off on slabs on 100mm concrete regardless of what the manufacturer says.

 

For our construction drawings, the slabs look to be sitting only 50% onto the 100mm concrete inner leaf, never mind the full 100mm (below).

 

Curious about whether this is structurally sound? The architect we hired for this reviewed our planning drawings and very confidently told us our house was straightforward and that we would not need a structural engineer, and that he was comfortable/happy to sign-off on the structural elements. For another reason I was recently chatting with a SE who just asked if we had a SE for sign-off on structure during the build and he said it was "very unusual" for an architect to say what ours said.

 

Edited January 31 by Gaf

[Conor]

All our slabs sit on 100mm block walls.

 

However, your detail above will not work as the slabs need a min of 100mm bearing. That means the slabs will sit full on the inner leaf. Not an issue, you just need to do some airtightness work at the end of the slabs where they are open to the wall cavity.

 

When the slab company comes out to do their measurements, they will point these out and do their own drawings and calculations regardless of what the SE / architect has done.

[Mr Blobby]

There is no way the hollwcore slabs will sit on 50mm of 100mm blockwork, they aren't made to that level of prcision.  100mm blockwork is ok to support them, although we have 140mm blockwork because, I think, we have some very big slabs.  Our SE specified this.  You definetley need an SE.  They cost far less than an architect and give very good value. 

 

For airtightness around the slabs on a cavity wall there are three things that we did, but you can choose from the list:

 

1.  Get the ends of the slabs filled with concrete at the factory.  This is the minimum I think should be done and is common practice in Ireland where airtightness building regs are stricter than GB.  The ends are not always completely airtight however.

 

2. Put a bed of mortar under the slabs.  This closes most of the gap ready for airtight paint later.

 

3. Wrap the slab ends in solitex, with mortar bed in place to provide a slip plane.  We did this.  Probably overkill and it can't be left to the trades on site to get this right.

 

Any service voids at the boundary will leave a gap into cavity to be closed.

 

 

The filled ends are not complete (on ours they forgot to fill one of the slabs) but better than nothing for very little cost.

 

 

 

Mortar on wrapped ends:

 

 

 

Service void:

 

 

 

 

 

 

Edited January 31 by Mr Blobby

[George]

Quote

For our construction drawings, the slabs look to be sitting only 50% onto the 100mm concrete inner leaf, never mind the full 100mm (below).

 

Should be OK if fully sat on the 100mm blockwork, I would defer to the slab supplier. The 50% is just a bad drawing - although appear it was done in Revit so could be a modelling error. 

 

Quote

Curious about whether this is structurally sound? The architect we hired for this reviewed our planning drawings and very confidently told us our house was straightforward and that we would not need a structural engineer, and that he was comfortable/happy to sign-off on the structural elements.

 

Ehhhh. I mean.... sort of. Architects don't 'sign off' - that is the job of Building Control. However if they design it to be fully compliant with Part A of the Building Regulations and they're suitably experienced (that is, they understand it) then yes, you do not need a structural engineer because the architect can just point to Part A as their structural justification.

 

More broadly, if they go for things like prefabricated trusses then the engineers who work for them will look after their bit and so on. So you do get some engineer input even if it isn't direct involvement.

 

HOWEVER...

It is quite easy to fall outside the constraints of Part A and my advice then would be to get a structural engineer. I have in the past done reviews for architects and just pointed out where they can tweak layouts to fit Part A. But Engineers should be able to save some costs as well - you have pretty major foundations although I suppose there is a concrete upper floor. 

 

I don't have any specific concerns on the cross section - although they have cited prefab trusses but have drawn something more akin to a vaulted roof with king post trusses - how much of that is artistic licence? 

 

[Gaf]

@Conor Yeah I touched base with two local companies and both said 100mm was fine. Both had been previously sent the construction drawings for quoting and developing a slab layout.

 

@Mr Blobby Thanks for the advice and photos. I love getting to see the actual work as I find reading about it not as useful. When you mention big slabs, do you mean wide spans or just thick/heavy slabs? Two slab companies have reviewed our construction drawings and both came back specifying 150mm hollow core and our maximum span width is 5m. When I was chatting with them again today I asked about sealing the ends and, exactly as you said, they could do this in house no hassle at all and they listed exactly what you suggested (and one said a few people fill the ends with mineral wool, but I'm not sure that's airtight...).

 

@George Yeah I think by 'sign-off' he meant 'do the construction drawings including structural elements without needing an SE'. With the SE, yeah I take your point, we contacted SEs at the early stages and they all said their construction drawings would not include design elements, electrical layouts, insulation plans, airtightness, etc and all advised we had to go to an architect for these. Then the architect said the house was simple enough that he was comfortable including the foundation, lintel, steel, details without an SE. The SEs were asking €5k+VAT so at the time the architect seemed like the best to go through since the SEs all said we'd need one anyway. Hopefully that doesn't bite us in the ass.

 

I'm considering asking/paying an SE to review the drawings just to ensure there's nothing of concern from their perspective and then, if something is noticed, biting the bullet and paying for the full SE input.

[Gaf]

3 hours ago, George said:

Should be OK if fully sat on the 100mm blockwork, I would defer to the slab supplier. The 50% is just a bad drawing - although appear it was done in Revit so could be a modelling error. 

Sorry, bit wrecked from the day and realised I didn't really respond to some of your points.

 

Both manufacturers confirmed 100mm blocks would be no issue for them. As you said, maybe an oversight in the drawing programme, and they do state that shop drawings are required for the slabs.

 

3 hours ago, George said:

More broadly, if they go for things like prefabricated trusses then the engineers who work for them will look after their bit and so on. So you do get some engineer input even if it isn't direct involvement.

 

I don't have any specific concerns on the cross section - although they have cited prefab trusses but have drawn something more akin to a vaulted roof with king post trusses - how much of that is artistic licence? 

That's good to know at least. So the engineers working for the prefab company will be doing a level of approval / sign-off (not building reg signoff) on the trusses. From a Google there, it looks like they're raised tie roof trusses? Sorry if that's completely wrong!

 

3 hours ago, George said:

HOWEVER...

It is quite easy to fall outside the constraints of Part A and my advice then would be to get a structural engineer. I have in the past done reviews for architects and just pointed out where they can tweak layouts to fit Part A. But Engineers should be able to save some costs as well - you have pretty major foundations although I suppose there is a concrete upper floor.

Only thing that has made me wonder about the architect's approach is the builder coming back to me saying there were no measurements given for the lintels or steel, and he wanted to get the architect to review the foundations to be 100% sure on them so there would be no issues on site. The builder similarly called it "unusual" to have an architect specifying the steel, lintels, and foundations.

 

 

[Mr Blobby]

14 hours ago, Gaf said:

When you mention big slabs, do you mean wide spans or just thick/heavy slabs? Two slab companies have reviewed our construction drawings and both came back specifying 150mm hollow core and our maximum span width is 5m. When I was chatting with them again today I asked about sealing the ends and, exactly as you said, they could do this in house no hassle at all and they listed exactly what you suggested (and one said a few people fill the ends with mineral wool, but I'm not sure that's airtight...).

 

We have 200mm slabs and the longest is 5.4 metres. 

 

The hollow core slab suppliers will provide a calculation report after measuring up the walls built up to first floor.

 

Something to check when you start building, is keep an eye on the lintels over openings.  If the opening is in a wall supporting the slabs then it may need to be beefier than lintels used elsewhere.  Your architect/SE will supply you with a lintel schedule and there will be an assortment of lintels on site to choose from.  Don't rely on the bricklayers to study the SE diagrams too closely. Same with padstones.

 

Regarding SE costs, we paid only about 2K and our build is way more complex (architects 🙄) than I would choose if I were starting over again. 

 

Edited February 1 by Mr Blobby