Gus Potter

Ok you say reasonably exposed. The vulnerable points are the eaves, verges and roof ridge. Say you don't live in the north of Scotland, Islands. Very quick sum and to cut a lot of the maths out. The wind load uplift will be about 100 - 150 kg/ m^2 (working load on a 50 year return period) at the exposed edges of the roof. But as you move towards the middle of the roof it will be less as an area average., the design codes call this roughly "non simulataneous action" as the wind is made up of vortices, big and small, that don't act on a roof evenly. The original part of the house is probably ok? How old is it, if it's been ok up till now then is that ok? Think about this another way. On new build stuff as an SE I design to the codes but when assessing existing historic houses we need to be pragmatic. If the wind gets up it often blows the tiles / slates off at the edges of the roof; verges, eaves or around chimney stacks. There used to be guidance on this in the old building codes. Yes, but the quality of roof tiling workmanship would have been good. The key here is to not let tiles get blown off in the first place as as soon as that happens you lose the dead weight resisting the wind uplift. And here the quality of the extension roof needs to be examined. This is obvious.. if you prevent the tiles / slates getting blown off at the edges of the roof and thus prevent progressive peal back of the tiles / slates then you, maintain the dead weight which resists the uplift. Your big purlins will not be contributing much unless the ends are well strapped down to the gables. Go back to basics and think.. what has been changed in terms of wind loading. The extension is the obvious thing.

Hiya. I get @Stu84's disappointment. But as others have said, and which I design for, is to be able to level things out with the masonry underbuilding. You can often go for a 20mm thick mortar bed with no detrimental effect. Thicker mortar beds can be ok but you thicken the wall base on the first couple of courses.. it's still economic. A few SE / Geotek comments for all. Trench fill is a rough business. Often we need to do it in sections, dig a hole and get the concrete in rapidly to stop the sides of the excavation falling in and, in sensitive clays, stop it from getting compromised once you expose it to air and let the pore water pressure change. There is no time to work to 10mm tolerances.. it's like trying to polish a jobby (shite) , what is under the skin is the vital thing. Pore water pressure? Some clay soils are quite happy to bear load until you change the water pressure, by digging a hole and disturbing them. Sometimes digging a deeper hole is counter productive. Now there are lots of photos on BH where folk are doing trench fill. very badly.. They worry and sometimes try and shutter the sides of the founds, the ground water is rising and it all turns into a mess. Think.. if we need to excavate for a rear extension and doing trench fill.. how do we get rid of the extra muck in a hurry? As an SE often what I want to see is you leaving the sides of the trench rough as we get a tacit beneficial key up the sides. Occasionally, if in made ground that may settle I'll say, drape some plasic down the sides so the settlement does not "drag down the founds", this is standard procedure in pile design. This sounds rough but it works and in the heat of battle on site when the weather is bad.. it's a pragmatic option. Now can I ask. Just say you had added the cost of a site visit to your SE design brief? Say that would top out at £500 quid. Usually that will be about £200 as for the SE what they charge as a headline rate ( graduate Engineer cost at "£90.00 an hour) you get back in recommendations.. it's good for business to provide "after sales service".

Do you fancy sharing your ground investigation results. Over lay that with your site boundary and then give us some section details so we can see what you want to build in the excavation. A few good and clear drawings are worth thousands of works. Once we know a bit more then you'll get more targetet advice and suggestions. You'll probably get the most impartial advice on BH. Yes, it may be that there is a way of executing your design with limited support but we can't comment properly unless we have access to all the information.

Clay soils are probably one of most unpredictable materials that we encounter on self builds. The soil mechanics books we commonly use as designers refer to undrained and drained shear strengths, the drained shear strength is commonly used by CE's like @saveasteading for earth works design when say designing a dam. However, I have a pal that did his PHD and he developed, and I think got a patent, on how you measure the "latent cohesion" of clays when drained. In my earlier post I tried to explain the difference between drained and undrained strength of clay. Although my pal was an Engineer he clocked that the ceramic manufacture's that make toilet bowls/ sinks etc were having a high failure rate in manufacture.. and thus he got his research funding on partly this basis. Basically (I think) it works like this. Even if you dry out a clay it still binds together due to the behavoir of the minerals and the molecular interaction. @saveasteading the brick like shape I think is driven by the way the clay is deposited in layers coupled with the above. Incedentally my pal now runs a specialist Geotech Company and gets most of his work from the major UK developers designing ground improvement and lime stabalisation schemes. I can see his work as on the outskits of where I live, he has desinged the groundworks for a few thousand houses in tricky ground.

Hiya. Have commented, impartially of course. My view is that on many self builds now a days you don't make a massive, if any profit, if you value your time at market rates and go for say MBC / raft foundation packages etc. But what you can't put a value on is building a home, a fortress for you and you family, you learn loads of skills that also compliment our day job. I think you have taken a completely different hands on approach which shows that self build is still more than viable and there is money to be made if prepared to work hard and put yourself through what can be an steep learning curve. From my experience doing this as a day job.. you often end up with a much better built and durable house, even if you are a bit crap at it compared with a lot of the rubbish build by developers. My logic is this.. if you have the where with all to raise the funds, buy a site, get planning and put some building plans together you can't be that stupid. I do and have benefited from your advice, thanks. This is an important figure and realistic in my view. If you have included this in you sq m price then it really makes your figures valid. I have Clients a bit like yourself and they also factor in a cost for their time. This shows that if you are able and willing to work hard you will reap the rewards. Now Nod is not daft but at the same time he is probably just like you and I. Yes ok he has a contracting business and has a "trade advantage" but even if you say build for 1.5 k per sqm the first time that is still going to work out very well. I don't need to as you have shown that self building is still viable and profitable. Well done sir!

Enjoy my numbers that put some flesh on the bone of your steel option. They will help you sleep!

I'm sure Mrs Nod is more than capable. Seriously though, self building can make or break a relationship. It's not for the faint hearted.

My sums are just qualitative and intended to give hubbers an insight into how we quickly determine if something has got a chance of working. There are so many things to think about and ideas you have when self building that often, just seeing how folk like me take a view on things can prevent a lot of runs down blind alleys. The steel option is the next port of call. My approach is to say.. lets see if we can design and fabricate the stair and the wall as one steel assembly, bring it to site and fit it in. Then later, once the dirty work is done,fit the treads. Ok allow me to digress. An SE and sometimes CE's are descibed as people who practice the art and craft of Engineering design. Part of the craft (the really fun part for me) of design is often to be able to design within a budget and still make it look Actitecturally appealing and intelligent. But of course appearance is all in the eye of the beholder. Now if (and it's a big if) you have bought into that then next time you are in B & Q in one of their portal frame buildings then look at the rafter to column connection. The tapered haunch is usually fabricated often from a steel I beam cut diagonally.. so you get two tapered bits out of one cut. If you could live with this then what you would see on the underside of your floating tread is a tapered steel section. What you are doing here is marrying up the industrial look with a cantilevered tread. Now here is the clever bit! If you sit the trapered section closer to the front of the tread and detail it you make any gap less than 100mm so it is now BC compliant. Many of the stairs you see on the web are non BC compliant. But if you do it this way then you do get some twisting (torsion) in the treads so you also use the tapered section and fixings to resist that small amount of twisting. My guess is that you can cut diagonally a 152 x 89 UB steel which would give you less than the BC 100 gap. It depends on your choice of tread and material as any fixings need an edge and end distance from the tread. Using a tapered section means that it has greater depth at the wall end and thus the connection here is simplified. So now the idea of fabricating the wall and stair as one work package becomes easier to get your head round. This may not be for you but often design is saying.. hey we looked at that and ruled out. There is no getting away from the fact that if you want any kind of cantilever stair like this it comes at a premium and you have to decide "is this right for us or should we just introduce a stringer to the stair and go for a better quality glass and tread? As someone who does this as a day job then I would, if my own house, on the budget you have, go for a stringer and better quality treads and glass fixings. If you have kids / a dog now or in the furture then go for the most durable tread you can afford as in a few years time, if the treads suffer and start to look bad then any cantilever becomes a moot point.

Well done NOD. When I was younger I did a self build, I was a builder anyway at that time. We built roughly 180m sq of 1.5 story house. I built the garage first with a kitchen, shower room and we slept in a caravan so the planners did not boot us out. Wife went to work and I got up every day and built a bit of the house. From memory I think I spent about 4000 man hours doing the house, garage excluded, septic tank, services went with the garage and so on. About 100 weeks on the tools went on the house if you base that on 40 hours a week ~ 2 years. But then I had to manage stuff / source material and at the end of the day it took about 2.5 - 3 years. Now all that labour was tax free. When we came to sell the house we made a tidy profit. In terms of your cost per sq metre have you an idea of a value for the time you spent and how many hours that has required. If you take your figure of under 1.0k per sq metre then the idea of taking time off work to realise that, or doing extra hours outside the day job becomes very attractive.

For a bit of fun and some sums. The above won't fly as others have pointed out, but let's see if a floating stair like below might work conceptually in a 100mm thick concrete block wall 2.4m high with no significant compressive load from above. Say the wall has a door opening and thus the wall is only able to span vertically top to bottom. Before we do any detailed design we want to look to see if is it in anyway possible. Say the stair is 0.9m wide and its a domestic stair. The imposed (live load) is 1.5 kN/m squared, from the design codes. That roughly covers you say taking a big family photo when everyone gathers on the stair. Now this is an area load so let's convert that to a line load applied up the centreline of the treads. 1.5 kN/m^2 * 0.9m = 1.35 kN/m per metre run horizontally. The line load acts at a distance of 0.9m / 2 from the wall = 0.45m. Now we can calculate the bending force that is applied to the wall, say half way up the stair by going 1.35 x 0.45 = 0.61 kNm per metre run of wall. And now lets apply the Eurocode factor of safety to that of 1.5. The design moment is 1.5 x 0.61 = 0.915 kNm which is the factored bending force applied to the wall. Now say the wall is built from 100mm dense 7.0 N/mm sq blocks in an M4 mortar. Once you do a few calculations you see the bending strength of the wall is about 0.175 kNm per metre run which is a lot less than 0.915kNm . We stop there as there is no point in calculating the self weight of the stair and glass balustrade as this just adds to the bending force. What happens if we make the wall out of block on the flat so the wall is 215mm thick? Well the resistance of the wall jumps to about 0.6 kNm still short of our 0.915 kNm, no self weight etc has yet been included. Now if we add another story of wall above this increases the compressive load in the wall which stops tension developing at the mortar beds which increases the bending strength of the wall (up to a point). Let's add another 2.4m of 215mm thick wall above. So that's a line load on the top of the wall = wall density x thickness x height = ~20 * 0.215 * 2.4 = 10.3 kN per metre run of wall. If we recalculate the bending strength of the wall we get about 0.95 kNm which is more than our 0.915kNm. Ok we are closer but once we take the detailing into account, self weight of the stair, balustrade etc we are on the wrong side of the equation again. In summary, you can see if we do a qualitative analysis like this we are going to have to do a lot of work to get a blockwork wall even in a two story dwelling to work and then we have to detail for not least shrinkage, build, course and bond it all properly. Even if we change the tread material is still does not get rid of the bending force in the wall. Now what happens if we change from 7.0 N concrete block to Engineering brick 215 mm thick 2.4m high with no wall above. Well, now things look more promising. A wall like this has a bending strength of around 1.45 kNm per metre run > our 0.915 kNm so now we maybe want to look to see if we want to do a bit more work by calculating the self weights , stair tread deflection and so on. It's going to be close and and may not work, but even if it does then you still have to build it and bond in the treads somehow. All this kind of leads you towards not using masonryin this kind of construction and to consider a steel based option for the wall as @Nickfromwales suggested.

Very impressive! Look forward to the point when you start installing the structure.. going up instead of down. Fascinating looking project.

Hang fire for now before you incurr professional cost until more folk from BH chip in. You can do a lot to help yourself by providing a bit more info if you feel able. Main thing is BH is not a competition, there is no such thing as a daft question! Don't worry about abbreviations, the lingo or that. You can look at it this way.. you provide info, some folk on BH can chip in for free and have fun sharing what they know. I like to teach so enjoy trying to turn what can be very dry SE stuff into something that folk can benefit from. When I did my self build the internet did not exist! Can you imagine, it was onl;y 30 years ago roughly. BH members get to learn from your journey, then you are more informed and can ask you SE / Goetech folk the right questions.. everyone is a winner. Now I post on and off on BH but there are some clever Geotec guys lurcking and a good few retired CE's SEs and Qs's .. if you spark their interest it would be great! Then @ETC can chip in with 20- 30 years experience of Architectural input... FFS you have one a watch! Now the seasoned self builders chip in and contractors.. you literally have access to thousands of years of man hours experience on BH. So gather some thoughts and then see if your SE / Achitect can take all this in their stride, you are just digging a hole after all. That said you are doing a basement @Alan Ambrose is a good thread to follow.

This is a great question. If you search about on BH you'll find posts by Geotech Engineers, but someone who is much active is @saveasteading But here is a rough rundown on things, I'll put myself in the shoe of acting for a Contractor designing their temporary works.. we want to dig a big hole, build something in it and back fill without anyone getting hurt. We refer to an old addage called "stand up time". Which means how long can we keep the excavation open for before the sides etc start falling in. Now in say chalk the stand up time is often good. Chalk is graded a bit like a rock where we look at fractures and where water might be getting in that loosens the chalk. This is important as we dont want big slabs falling into the excavation. @saveasteading knows a lot about this so I'll stop there. In summary though, we can often dig a big hole in a Chalk soil, do what we need to do and go home, provided we control the ground water. Clay soils are a different animal. Lots of soils are a mixture of things but let's just stick to a pure Clay soil. Now we know from gardening that clay is "sticky". But if you dry it out it can turn to dust, almost like a sand and it fractures, starts to fall apart.. like your garden soil cracking. When we dig a hole in a sticky clay soil you'll often see that the sides don't fall in right away. But after a few days/ months the sides do fall in. As Engineers we might call this the "undrained soil strength" (right after digging the hole) and the "drained strength" once the clay starts to dry and loses its stickyness, called the cohesion. In summary what the Contractor Engineers do is assess all of this and find a way of executing the work safely at minimal cost.. That is why you see batters at different angles and weird looking stuff. Each site is different.

Yes you will find folk that promise you this. But then if you presented them with a specification and said Gus is going to check their work and hold them to the specification.. they will either run a mile or infate the price. Do your slab and then put a 50mm screed on top. I've done hundreds of raft slab designs, well probably (by defualt) thousands as I was a development SE that wrote the calculations for one of the biggest suppliers in the UK of raft slabs, long before MBC/ Advance Foundation Technology etc came on the scene, and the economy comes from keeping it simple stupid, buldable and allowing you to use local contractors as opposed to specialists. Part of the stuff I wrote for the software included practical tolerance levels for flatness and level of the slabs. There is no free lunch here, trying to save on your structrual slab by introducing tighter tolerances is false economy when a self builder. In summary it might sound counter intuitive but the extra thickness of concrete is small beer.

As a cursory inspection that won't get past a UK SE. The wall fixings into your proposed blockwork will fail drastically and the wall stringer is too flexible. The diagonal tread bracing is a straw man. Keep up with the ideas, something that suits your budget will come to light. How much have you allocated in your budget for the stair?