Extension on precast concrete dwelling

acri

Morning.

I'm looking to get a small extension project underway in Dublin 12. The house is precast concrete and I recently had an interesting (read: eye-watering) conversation with an architect regarding the project.

The suggestion is that knocking the rear wall will require a steel "picture frame" setup where a simple RSJ won't do the job, as the concentrated load on the remaining rear corners would be too high for the current foundations. This means a steel frame would need to be installed across the top, sides, and across & into the foundations to spread the load, significantly increasing the cost of the build.

I'll be getting second opinions, but it opened my eyes to potential pitfalls in what I thought would be a relatively straight-forward job. Has anyone else done similar work on such a home? Was it a pain in the hole or am I getting bent out of shape over nothing? While it does no good for the stress levels, I'd rather be wise to what needs to be considered before beginning.

pawdee

How small is small?

Single story or two story extension?

When was the pre-cast house built?

What thickness are the walls?

What's access like?

Throw up an old photo there!

Obviously you'll need a structural engineer but it mightn't be as bad as the architect is making out.

Have you considered timber frame? Nice and warm, lightweight frame, clad in anything you like.

Good luck with it. Sounds like an interesting project.

pawdee

Don't know how that went up twice but it was worth repeating I suppose! 😁

pawdee

Strange. Fixed now.

acri

https://www.boards.ie/discussion/comment/121796397#Comment_121796397

How small is small? - 20sqm

Single story or two story extension? - Single (kitchen extension effectively)

When was the pre-cast house built? - 1950's

What thickness are the walls? - At a guess, I would say 200mm

What's access like? - Poor. Mid-terrace. Blocks carted through the house sort of deal

Have you considered timber frame? Nice and warm, lightweight frame, clad in anything you like. - I did bring this up, but what I was told was it all depends on who I hire for the job, as what happens is jobs get subcontracted and I'll end up paying more people under the umbrella of one project

Reading through some of the other threads, there seems to be debate on whom to listen to: builder vs. architect vs. engineer

Think I should probably have one of each on site and get all viewpoints

listermint

https://www.boards.ie/discussion/comment/121796451#Comment_121796451

Check with neighbours. That would be my first port of call. No doubt there's various extensions.

muffler

https://www.boards.ie/discussion/comment/121796415#Comment_121796415

The first reply to any new thread will always appear twice. It's one of the many issues in this Vanilla platform but a quick refresh of the page sorts it.

acri

https://www.boards.ie/discussion/comment/121796462#Comment_121796462

Aye, the neighbor on one side looks like they have a lovely extension so I'll probably knock in with a tin of biscuits.

pawdee

I built on a mid terrace, town centre, back kitchen / dining room extension two years ago. I was all set to go with lightweight timber frame with standing seam cladding as I didn't fancy pumping concrete and barrowing blocks through the house. Not to mention spoil from foundations going the other way. I got a quote for foundation screws and planned on having no concrete whatsoever on the build.

In the end, the owner of the land that my house backs onto very kindly let me have access (for free) so I went with traditional block work walls on strip foundations. The whole thing was still a pain in the arse but worth it in the end.

My only regret is that I put a flat roof (torch on) on it. If I was doing it again I'd have put a slate pitched roof. The flat roof comprises over 30% of the overall roof area so on that basis my insurance company rode me by doubling my premium. If I'd gone for a pitched roof I could have avoided that and had some extra attic storage or vaulted ceilings with rooflights. The extension is plenty bright without them so no big deal on that front.

acri

https://www.boards.ie/discussion/comment/121796529#Comment_121796529

The house itself will be undergoing a total refurb which I'll largely be carrying out myself, so bringing materials through the house isn't too much of a big deal save for the labour of it. Would much prefer to go for a pitched roof, but it all comes down to cost really. I'd like to try and get this done for €2500/sqm for builders finish if possible. I hadn't considered the insurance implications of a flat roof though, so that's defo something I'll need to take into account.

Mellor

https://www.boards.ie/discussion/2058337206/extension-on-precast-concrete-dwelling

Get other opinions. More appropriately from a structural engineer not an architect. But a different architect would be good. FWIW, the unless I'm missing something, the suggestion does not sound tight to me.

The suggestion is that knocking the rear wall will require a steel "picture frame" setup where a simple RSJ won't do the job, as the concentrated load on the remaining rear corners would be too high for the current foundations. This means a steel frame would need to be installed across the top, sides, and across & into the foundations to spread the load,

This all sound very odd to me.

• Too high for the foundations based on what?
• A steel beam over, into the wall would partial spread into the foundations. Steel columns down from the beam is a point load, increased the force on the foundations.
• Unsure what "across & into the foundations" refers to. But if it's another beam at floor level, I wouldn't expect that to spread the load much

acri

https://www.boards.ie/discussion/comment/121796572#Comment_121796572

• Too high for the foundations based on what?
  • Experience?
• Unsure what "across & into the foundations" refers to. But if it's another beam at floor level, I wouldn't expect that to spread the load much
  • So beam across the top (standard RSJ-style), vertical beams either side to connect to a horizontal beam flush to the floor. The effect is even distribution of weight across the resulting frame.

It made sense to me at the time. But I tend to take the professionals at their word before looking into it further.

listermint

A beam at floor level... squints at ceiling trying to imagine what this guy is actually solving with that.

Is he selling steel ? 😃

acri

I imagine something like this but with a beam also across the bottom for weight dispersal.

Look, I don't know. It made sense at the time. Though to be fair I only really remember words like "complexity" and "viability" and "cost".

I've reached out to some structural engineers hoping to have a chat. I'm open to suggestions via PM, by the way. Would much prefer to engage off the back of positive experiences rather than throwing a dart at my monitor.

listermint

https://www.boards.ie/discussion/comment/121797850#Comment_121797850

The beam across the bottom is pointless. Traditionally if the foundation to take the point load isn't up to the load. You'd dig out the spots and put in footings, not throw a beam across.

Water John

Would simply cutting a doorway through the concrete wall be an option?

acri

I was looking to take our the wall as much as possible to make the most of the width of the house, which is quite narrow. Perhaps an opening the width of standard double-doors is an option which is, what, 1.6m? Sort of scuppers what we're after, though.

listermint

Engineer I think is money well spent.

acri

Just off the phone with one, as it happens. Booked in for a site visit next week. Already I see the language is different.

1. The house is not precast concrete as we do not do such dwelling construction in Ireland: it's poured in-situ

2. He thought the steel "picture frame" solution for the rear wall sounded odd, but it's hard to tell without seeing the building/site

Feeling a bit more optimistic about this consultation.

mountai

Be guided by an Engineer rather than an Architect in structural matters . Sounds like a house built in the 50s . No problems about taking the back wall down with structural pads for bearing steels on .

Mellor

https://www.boards.ie/discussion/comment/121796617#Comment_121796617

I'd question his experience in that case.

The beam across the floor would do little to spread the load, less that connecting to the precast walls at ceiling level. So he is actually increasing the point loads on the foundation as I describe above. If the foundation can't take increased load via the walls then it definitely won't take it with a "picture frame" - although there's no way he could know that without inspecting the foundations or the 1950s engineering drawings.

https://www.boards.ie/discussion/comment/121797850#Comment_121797850

In the case the columns (vertical RSJ type steel) are there because the wall arrangement is not there to connect to. In your case you are joining to a the side walls, so shouldn't be an issue in itself. That said, get an engineers view. There may be other potholes.

The one issue that comes to mind is the it's mid terrace, so presumable the connecting walls are party walls. So you will need to leave a pillar of wall to bear the steel. The structural engineer will look after all of that.

Lean on the engineer for the structural design, then let the architect resolved the design within the parameters the engineer provides.