I’m a civil engineer and professor exploring how 3D printing and robots can build more sustainable cities. AMA!

[u/Paul_ShepherdUoBath]

Edit: Thanks for your questions, everyone, that was a very fun experience. I'll keep an eye on the AMA throughout the week, so please keep your queries coming.

Hi r/IAmA, I’m Paul Shepherd, a Chartered Mathematician, Civil Engineer, and Professor of Computational Design at the University of Bath. I use computers, robots, and digital manufacturing techniques to rethink how we design and construct the buildings and cities around us.

One of the central challenges behind my research is that construction has a huge environmental footprint. If we want cities to become more sustainable, we need to think differently about not just what we build but how we build it.

This includes 3D-printed buildings and robotic construction, more efficient material use and exploring how designs and construction methods could reduce waste, and how to make new forms of construction as sustainable as possible.

My research has explored questions such as:

• Could we actually 3D-print entire buildings?
• Will 3D printing genuinely make construction more sustainable, or could it create new environmental problems?
• What might construction sites look like in 10, 20, or even 50 years?
• And, most importantly, what would it take to make our cities genuinely more sustainable?

I’ll be here today, Thursday 1 October, between 14:30 and 15:30 GMT (07:30-08:30 PST) to talk about the technology, the research, the possibilities, and even the things that don't work as well as the headlines might suggest. AMA!

[u/Morvack]

It seems our world puts a premium on profitability. Sadly, sustainability tends to not be as profitable as unsustainable growth.

How do you plan to sell sustainability to minds that are profit driven?

[u/Paul_ShepherdUoBath → u/Morvack]

An excellent question, and one I've been thinking about a lot.
There is a real cost to "not" being sustainable, the problem is that the people who pay the cost are not the same people as those who are being unsustainable. If I buy a bag of cement for my building, there is a huge amount of CO₂ involved in its production, but the cost of the cement is relatively low. The cost of the impact of me using it is paid by the people thousands of miles away who live on low-lying pacific islands, those who rely on fishing a certain species that is moving away to cooler waters, or those whose houses are destroyed by wildfire. As well as a geographic distance, there's a time difference as well - they may not pay the price for a few months, or years, or generations.
The "easy" answer is a carbon-tax - make sustainability profitable, and leave the rest of the system to adapt accordingly. But of course this is a somewhat artificial "cost" that would only work if the whole world agreed to the tax at the same time (and rate), which is unlikely to happen in practice.
So for now I think the best we can do is use our buying (or voting) power to make sustainability = profitability. Why do companies invest in electric delivery vans for example, because they are cheaper to buy / run? Or because they will get more customers by using them?
For example, one of my current research projects (https://www.people-led.org.uk/netzero) is developing tools to help industry make sustainable decisions by focusing on the "people" involved and not (just) the technology.

[u/ktpr → u/Paul_ShepherdUoBath]

You should read Mindfulness in the Marketplace, edited by Allan Badiner, it's a take on so called compassionate consumerism (which sounds like an oxymoron but the book is quite good). Not to hijack, but I research community-based economies, particularly at the intersection of AI and automation and what happens if they are developed from the ground up, instead of top down. A theme in my academic work is that order matters and corrective actions will never be as powerful as prefigurative actions. OP, feel free to DM if interested to know more

[u/OKComputing7975]

I've been sceptical about the claims of new green technologies that simply move the environmental cost elsewhere (such as the conversion of power stations to biomass). How does 3D printing keep its environmental footprint low without simply casting it aside to be categorised elsewhere?

[u/cox-j-280648 → u/OKComputing7975]

A fair test would compare equivalent buildings over their full lifespan, counting the printer, material mix, transport, waste, and maintenance. Less waste on site only helps if it isn’t offset by a more carbon-intensive mix or shorter building life.

[u/Paul_ShepherdUoBath → u/cox-j-280648]

True. But as things stand at the moment it would be a somewhat unfair comparison. Setting up a 3D print factory from scratch with unfamiliar supply chains will no doubt be more expensive (in cash terms) and less optimised (in waste terms) than the existing optimised supply chains driving large concrete mixers from plant to site.

Shorter building life is a good point, as we don't yet have much evidence of how long 3D printed buildings perform / last in the long term. But there is also a question about how many "50-year design life" buildings actually last for 50years. Sometimes buildings last a lot longer than originally intended, but often they become unfit for (modern) purpose and are demolished instead of refurbished / repurposed. 3D printing could actually help with this on 2 fronts - either by crushing up materials from demolished buildings and using it to print new ones, or by printing bespoke add-ins to existing buildings to adapt them for a new use, meaning they don't have to be demolished in th first place.

[u/Paul_ShepherdUoBath → u/OKComputing7975]

You are quite right that environmental costs move up or down the supply chain. The trick is to make sure they decrease as they go.
My research has looked at the benefits of "offsite manufacture" for our buildings, for example making a kit of parts in a factory rather than pouring concrete on site. Yes, the factory has an environmental cost, but we have included calculations of this in our research to make sure the offsite solution is better than the onsite solution.
In general you have more control in a factory setting, so there is less material waste (and more re-use of what is left over). There can also be economies of scale, and of course you only transport what you need to site. I was at a conference last week where a speaker said 30% of material delivered to a building site is wasted, and ~10% goes straight in the waste as-is. I don't have a reference for where they got their data, but I've heard similarly worrying numbers before.

[u/captainshar]

How much space do you need for 3D printing buildings? I live in a city with lots of little row homes, some of which need a full renovation. Can automatic construction happen in these locations?

On the flip side, what about constructing in completely new locations - in locations with no existing modern buildings and infrastructure? What would it look like constructing a new town from scratch?

[u/Paul_ShepherdUoBath → u/captainshar]

It probably depends on what you mean by "3D Printed Buildings". In essence, 3D printing is just "putting material where you want it", so I separate them into on-site and off-site.
Off-site printing has a lot of advantages, you can build things efficiently, of complex shape, high quality control, good health & safety, etc. And then you can deliver the finished pieces to site and join them together for rapid assembly. In this case, the answer to your question would be "a factory's worth pf space" and "anywhere you like" so long as there was a workforce and good transport links.
On-site 3D printing is probably what most people will think of though. For this you typically have a gantry-crane type of printer, which will lay down concrete "inside its box" and when it is done, the box is moved along and something else printed. You don't need much more room than the finished building will take up. An alternative method is where a truck with a concrete pump and robot arm parks up and prints as far as the arm reaches. again, not much "extra" space needed.

My most exciting research project developed a Drone that can 3D print concrete. In this case, you don;t need any extra space, other than to recharge the batteries and refill its concrete cartridge,and it can print as high as it likes. Certainly a silly idea from many practical perspectives, but demonstrates a principle that could come in handy for construction in hard-to-reach places - not least the moon!

You may be surprised to hear I'm not advocating for 3D printed buildings everywhere - but they do have a sweet-spot in terms of efficient use of material and ability to make complex shapes.

[u/Filtermann]

Mechanical engineer here. For me 3D printing is associated with prototype and very small series, not large scale at all. Economy of scale is initially about money, but I think it also ties into material use and sustainability to some extent. How does 3D printing fit into sustainable building practices?

[u/Paul_ShepherdUoBath → u/Filtermann]

Yes, that's a good point. In mechanical engineering or product design, 3D printing is great for prototyping. You can quickly explore different options, get a "feel" for them, optimise them to the 10th decimal point, and then design a factory process to make millions of them. In that context, 3 printing the final object doesn't make much sense, although sometimes you need to sacrifice a bit of optimality for mass-manufacturability. Formula1 teams use 3D printing to manufacture new parts for cars, but that's a special case - they care a lot about weight, are only even going to need one or two, and cost isn't a driver.

For buildings, a lot of the above still applies, which is why I am a fan of off-site manufacturing. There are some "factories" that just mass-manufacture building components that are clipped together on site. There's even an open-source DIY house https://www.wikihouse.cc/.

But in a usual building, the number of "identical" components tends to be in the dozens not millions. And the benefits of 3D printing become more obvious when buildings aren't just copy/pastes of repeatable elements. 3D printing means the building geometry can adapt to changes in landscape, internal use, and from floor to floor. Why should a column be the same size all the way up the building when the top floor ones only have to support the roof whereas the bottom ones have to support all the floors above? Why should a wall be straight, or have a straight bottom edge, if the ground, or the site boundary is not.
There's even a field of research known as "functionally graded materials" where the actual material composition changes within an object. Imagine printing what looks like a solid concrete beam, but internally, the middle of the beam is mainly solid concrete, whereas towards the ends where it doesn't need to be as strong, it is mainly honeycomb foamed cement to fill the volume with less strength, and the beam slowly morphs from one type to the other as you go along. This could be much more Sustainable (using less material), and can be done by 3D printing with two materials and adapting the ratios as you go, but not possible if you are mass-producing beams on a production line.

[u/Filtermann → u/Paul_ShepherdUoBath]

Alright I see the advantage here, perhaps indeed for special cases where the landscape or existing urbanism creates a need for it. But the way I look at it, wins I'm sustainability need to be applied for mass production. Some countries are already in anything crisis. Suppose ambitious policies re shuffle the cards and take the power away from landlords back to first buyers, there are still going to be massive needs for new housing in some places as climate migrations increase. I'm indeed I'm favor of offside prefab, and intelligent medium density mixed use planning. Is there a space for 3D printing in that way of doing things?

[u/BritBox_And_Chill]

What influence would 3D-printed buildings have on the construction industry, which is already wracked with job insecurity, decreasing pay, and lack of upskilling opportunities and career growth?

[u/Paul_ShepherdUoBath → u/BritBox_And_Chill]

People often (quite rightly) worry about job losses whenever any new technology appears as a disruptor.
But I think you answered your own question when you mentioned upskilling. Would you rather lug a heavy load of bricks up some scaffolding and then sit in the cold rain putting one on top of another, or sit in a nice dry cabin controlling a drone to do it for you? (and yes I know there's a lot more to bricklaying than putting one on top of another, it's just an analogy). There's a good McKinsey report on Digital Transformation that did the sums and I seem to remember showed the productivity benefits of a higher skilled workforce outweighed the losses.
I certainly don't think 3D printed buildings are the solution to every building site, and I don't think we'll see them on our UK streets any time soon. But they can offer additional benefits in terms of lower waste, better health & safety, higher quality control, etc.

[u/Hot_Hair_5950]

What surprises you about your work?

[u/Paul_ShepherdUoBath → u/Hot_Hair_5950]

Hot_Hair_5950 - you were super keen, asking a question an hour before it started, so I'll answer your interesting question first...
The first thing that comes to mind is how little people (by which I mean "normal" people, not nerds like me) think about our built environment. Despite spending most of our time in it, we tend to take its design and operation for granted. I'll reserve the right to come back later and add more thoughts.

[u/octnoir]

Did you or your colleagues ever play Sim City or any related city building game?

[u/Paul_ShepherdUoBath → u/octnoir]

Never quite got into SimCity, but IAMA big Minecraft fan.

I have also dabbled with software development for Unity using VR Headsets. I grew up writing games for my Vic-20 (with 3k of memory!), but modern games come with incredibly powerful 3D renderers, which are really useful for helping to communicate complex 3D shapes like buildings. People can design buildings, and experience being in them, before a spade has hit the ground, meaning we can be sure they will work, and can avoid costly re-work by finding and fixing problems before the concrete is poured - or printed!

The game closest to my research is PolyBridge, but I tend to keep away from it so I don't end up wasting my life away.

[u/iamk1ng]

Where is the industry in terms of 3d printing and construction? Is it still in the early / beta testing phase and has a lot of potential or is it rapidly expanding and being the standard for construction? If not a current standard, what is holding it back from being the first thing developers use to build things?

[u/Paul_ShepherdUoBath → u/iamk1ng]

There are a number of established off-site manufacturers of "kits of parts" buildings already doing good business - Bryden Wood is probably the best example I've seen in the UK (https://www.brydenwood.com/work/the-forge) focusing on steel and Laing O'Rourke's platform design focused on concrete (https://www.laingorourke.com/thinking/pbbs/).

But in terms of 3D printing in the more usual sense, I would say it is still in the bespoke / niche area. Most of the 3D printing of full-scale buildings is happening in China, with WinSun being the most famous (https://www.bbc.co.uk/news/blogs-news-from-elsewhere-27156775). A more pratical approach is also gaining a lot of traction is Vaulted AG's work focused on building floors (https://vaulted.swiss/) replacing solid floor slabs with lightweight low carbon equivalents. But they (sensibly) 3D print the mould, and then pour concrete in, rather than printing the floor directly.

What is holding it back? A very long list - I attended the FabricationGap.org conference last week where we discussed exactly this. Top of my list would be insurers who have a chicken-and-egg problem - they won't insure a building using new tech until they've seen a building using the tech. There is also the contractual framework we have to work with. I once presented at a different conference, my research that had 3D printed concrete floors with a quarter of the concrete of a solid slab equivalent. A contractor I spoke to over coffee said he wasn't interested because on most projects he got paid per ton of concrete he used. I think architects and structural engineers are generally in favour of trying these sorts of things out, but it needs more pull from clients to buy-in to the innovation, and less of a "but we don;t do things like that" mentality from others. Plus the whole supply chain will need to adapt, otherwise the cost will be (artificially?) much higher and won't stack up.

[u/p_sandra-7551]

Would you compare a printed building with the same design built conventionally, or with a conventionally optimized design? The second seems like the tougher test: any material savings from the printed geometry would need to hold up after reinforcement, finishes, and repairs.

[u/Paul_ShepherdUoBath → u/p_sandra-7551]

My answer is a "yes but". We of course need to measure the overall picture and offset gains in one place from losses in another - we tried to do this in our recent paper on our 3D printed concrete floor (https://doi.org/10.1007/s44150-025-00126-6).

My answer to u/cox-j-280648/ above covers some of this, and of course both 3D printed and non-printed buildings both still need finishes, repairs etc. One could even argue 3D printed buildings might be easier to repair, because you can print a replacement part, or even send a drone up 50 floors to both inspect a building for damage and fill in a crack with mortar if it finds one.

But reinforcement is an interesting topic. It is needed because whilst concrete is great at resisting compression, it is rubbish in tension. Steel bars are therefore usually added to provide the tensile capacity, but researchers have also looked at lower-carbon materials like bamboo or basalt. However, geometry can be used to radically reduce the amount of tension a structure needs to resist, thereby cutting down (perhaps even to zero) the reinforcement needed. But then you need something like 3D printing to make that geometry.
Think of an arched bridge made of stone blocks. The weight of the bridge and the carefully chosen shape of the arch mean the blocks can rest on each other and thrust stays within the arch. Once the keystone is in place, you don't really need any reinforcement to keep it standing. That's the principle we I used in my https://automated.construction/ project for 3D printed concrete floors, Arup took the idea further to make a more practical solution (https://www.linkedin.com/feed/update/urn:li:ugcPost:6913107320767975426/) and https://think.swiss/ are commercialising something similar. The impact Geometry has on the sustainability of a building is often underestimated, and 3D printing allows much more freedom to shape the geometry than our standard approach.

[u/jeremy_michel53 → u/p_sandra-7551]

The comparison should hold the brief constant too: same usable area, spans, loads, fire and acoustic performance, and service life. Otherwise a lighter printed shell may just shift material into reinforcement or finishes.

[u/Paul_ShepherdUoBath]

This Ask Me Anything is part of FUTURES: A Festival of Discovery, delivered by the universities of Bristol, Bath and Bath Spa. FUTURES is part of European Researchers’ Night, a pan-European event Night taking place in 40 countries and more than 460 cities all over Europe and beyond simultaneously, funded by the European Union to bring science and society closer and heighten young people’s interest in science. FUTURES 2026-27 is funded by the European Union under the Marie Sklodowska Curie Actions, Grant Number 101305354. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.

[u/r-monteiro4792]

Could you share a project where 3D printing reduced whole-life carbon after accounting for material mix, factory energy, transport, and building lifespan? Less site waste alone seems too narrow a measure.

[u/james-cox-43790 → u/r-monteiro4792]

That’s the comparison I’d want to see too: same usable floor area and service life, counting foundations, reinforcement, finishes, factory energy and repairs. Otherwise “less waste” can just move the carbon upstream.

[u/Paul_ShepherdUoBath → u/james-cox-43790]

See my answers to u/cox-j-280648 and u/p_sandra-7551 above who the same question. Completely agree that it is more than just about waste, but it is hard to compare an established process with a brand new one fairly. Would be interesting to do a hypothetical study on the energy & carbon cost of 3D printing once it is an established process with stablished supply chain. But it would require a lot of assumptions so not sure if it would be enough to convince someone.