Gus Potter

Hello all. A Client of mine asked me why I had only put reinforcing mesh in the top of the raft slab on XPS insulation and not in the bottom as they had seen many details where there was steel mesh in the bottom and top of the slab or just mesh in the bottom. One underlying reason for the good question was.."what kind of temporary construction load can I put on the slab? .. as I want to run a machine about on it" The following is a bit of an introduction to the theory on how you can place the reinforcing mesh (laymans terms).. the experience bit I'll leave mostly out for now. Please remember that all raft foundations need to be designed on a case by case basis. Also raft design (particularly on thick insulation) is a dark art in that we design them partly on theory, partly on experience and by adapting standard design guidance to suit the particular site and your (often self builder's) requirements.. Please note that non of this covers.. difficult ground, made ground, sloping sites and so on.. there are many types of raft foundations.. I'm just talking about mesh reinforcement here and assuming a fair wind (in terms of ground conditions) below the insulation layer. This design is a essentially a passive raft on XPS insulation on CLAY soil. I use capital letters her as when you look at any ground investigation report the dominant mater is captialised. The soil description is wriiten in order of priority. This particular raft has to take some point loads from a structural frame around the perimeter and a modest loading from the external walls that sit on the edge of the raft. This raft has an edge beam to carry the column loads to spread them about. Below is a rough drawing of the slab away from the edges, I've ommited the mesh, DMP etc for simplicity. In the case above there is a capping layer of 6F5 as the soil contains organic material, a bit of made ground and so on.. we kind of need to dig that out as if not we would have to have a much thickened and more heavily reinforced slab. We are aiming for economy in the round here. The capping layer is there essentially stop excessive settlement caused by say made ground and any organic material decomposing. Forgive the lack of brevity but the concrete slab does not know what it is resting on.. it just wants to know what is holding it up, how squashy it is and how strong it is. What we often like to do is to make sure that the type one and capping layer is stronger in terms of compression strength than the insulation and also settles less than the insulation. In other words the stuff below the insulation is less elastic. This lets us concentrate on how the insulation behaves. Ok lets look at a typical insulation, say an XPS. Now when you look up the manufacture's data tables the will often say our stuff is rated at 300Pa at 10% compression. Now 300 kPa is 30 tonnes a square metre! Now that is a great headline figure! But on a 300mm thickness of insulation that equates to 30mm of downwards movement.. just imagine what that is going to do to your doors and windows, drains, finishes etc. What we do is to say.. how much movement can we live with. Say we can live with 6.0mm of movement when the building is subject to it's full load. The sum to get a reasonable load bearing capacity of the insulation is: 300 * 6/30 = 60 kPa.. which still is a lot of bang for your buck if the loads are spread about evenly.. which often they are not. Anyway we now know if we put 60 kPa on the insulation how much it will squash down by and from that we can calculate its elasticity.. how squashy it is. On top of this insulation we put a concrete slab. Now plain unreinforced concrete has a small capacity to resist tension.. not a lot but it does do something if we have tension loads spread over a wide area.. it can hang in there! There is a design guide that is used by many of the big industry suppliers of software (TEDDS/ TEKLA for example) to the SE community is called TR34.. Concrete industrial ground floors. In this design guide is a brief mention of concrete slabs on insulation. What many folk like me and other designers / specialist Contractors do (the Americans pioneered this so I use their guides as well) is to take the basic equations and adapt them in a conservative way. To adapt the equations in principle is relatively simple. Below is a simple model of how an unreinforced slab fails away from the edges under a point load. One way of appraoching this is to assume that the first crack in the concrete happens in the bottom of the slab, now we are adapting conservatively the standard TR34 rules. But we need to know what distance "x" is. This is called the radius of relative stiffness. Simplistically the insulation under is pushing back up as hard as it can (remember it is elastic like a pencil rubber) and the concrete is bending down but it is less elastic than the insulation. We need to find the "balance point" where the insulation and the concrete have equal forces to maintain equilibrium. Like below and then we can calculate the tension in the concrete in the top of the slab where the circle is and check to see it is ok. You can see how the circle, the tension in the top of the slab, is relatively large and that is why we are allowed to take the tension properties of the concrete into account. Here is an important bit for many of the raft slabs you see on Build Hub. Steel mesh is often used for crack control. Say A142 mesh, but often there is just not enough mesh to turn the concrete slab into a fully qualifying reinforced slab. But intuitively we know it will be helping a bit wich add a bit of a safety net. In the round though. I've had a look at what folk are getting designed by their SE's on Build Hub, qualitatively. You can't take too much of a view as there is often no quality info. I've also had a look at the AFD ( Advanced Foundation technology) offering. Lots of it is good stuff with plenty detail. Hopefully the above will give you a bit of an insight into how they (AFT et al ) achieve their thin slabs. BUT.. Often in this passive raft or just any raft foundation design there is no free lunch! DO NOT make the mistake of trying to get you budget figures to work by shaving a slab down to 100 mm thick for example unless you are confident on the ground you are building on, know what kind of insulation you are going to use and you have no funny loads from the superstructure! These loads include uplift wind and point loadings. That's all for now folks.

And why not have a bit of fun. I mess about with my own house and as you, some of the stuff I apply to the day job or.. not if I learn the hard way!

Help ma boab John, some set up. Browsing though your spread sheet diagram there are some clever touches in there. The bit that interests me most is that box that I think is the plate heat exchange to keep the air pump separate from the gas boiler. I like your style and approach.. so + 1 from me and also I appreciate you sharing your diagrams.. I know that these don't get knocked up in five minutes. In a funny way we are both trying to achieve the same end.. which is the most cost effective way of heating a house with say UF via say ASHP and Gas.. or anything else.. where we may differ is that I look at the cost and in particular the maintenance cost of a system over say 50 years.. with my commercial / professional hat on. With my hobby hat on (being mucking about with this stuff for at least 30 years) I love it! I go for the simple stupid, affordable, hopefully mass market and you are exploring the forefront of control systems and innovation. Is there a middle way that folk on BH can marry together what you are doing with my hard headed approach? OK folks with my SE / raft design day job experience.. with underfloor heating hat chucked in... Over the years I've adapted my raft designs to take account how you introduce under floor heating pipes into ground floor slabs. It took me along time to figure this out but I'm confident as an SE I have most of the answers. I also feel capable of explaining how, in laymans terms, this heating malarky is not as complex as it needs to be. There are many ways to skin this cat SE wise.. but often the approach and one I take is to say.. how are you going to use what is often an open plan space. Do you want some rugs on the floor, do you have big areas of glass where we may want to close up the spacing of the UF pipes? What kind of controls do we need to suit your way of life.. are you out all day or work from home, young family, kids grow up and then I think about how I might want to lay out and form the slab below, can I as an SE have my cake and eat it? sometimes yes.. But always..make it simple stupid and future proof. For me what I've learnt over the last 30 years of doing this is that UF pipe layout and systems is it's much more to do with buildability, ongoing maintenance cost and preserving your asset. l the size of the house qualitatively, the number of floors and look to see where the cold air comes from and design for that. That gives me a "feel" for what would work. It is really is a craft rather than calculation.. the real craft is explaining to your Client how it works as a "craft" so the house performs the way they want.. the calculations are often there to convince BC!

That's ok I just hope you would turn me into a Noble Gas xxx.

Have a chat with your SE, hopefully folk on BH have given you some pointers about what to ask and a bit more insight. All the best.

Hi Mike. I'll respectfully bow out on your offer of comparing scars! I've done a few but I'm still very much in the learning stage as technology moves as fast as I learn. To explain: For all on BH. When I started out as a novice 30 years ago we were not really aware of air tightness, how you layer up insulation and fix it, had no access to modern software and so on. We just flew by the seat of our pants! We liked what the Scandinavians were doing and thought how do we do similar in the UK at a sensible price? As self builder we were just as hard up as many of you are now. Back then I was a builder in the Scottish Borders and often the only way folk could get a bigger house for an expanding family was to self build. Plots were still affordable for folk on a small income at that time. Back then plastic plumbing pipes were still relatively .. "modern" and a plastic barrier pipe (to prevent oxygen / gas transfer) was something we thought astronauts used to go to the toilet. What I've learnt over the years is that for a UF system to work well it has to be designed for the lifespan of the house. You can mince about as much as you like with the controls but you absolutely must future proof anything that is buried in the floors and make these parts easily accessible at the tails so you can flush the system and so on. You must always consider if and when you come to sell the house. You need to protect your asset so the UF and controls do not become a liability... think of some surveyor / valuer who has got out of bed the wrong way. I think Mike strikes a good balance here between my "in your face" comments and having a bit of fun with controls. @MikeSharp01 ta. By all means have fun with the controls, make an app if you want.. but treat them as bells and whistles you have fun / hobby with, like you buy a car that needs love and attention. The next owner of your house and valuer may not see it the same way as you do..

Happy to do so. I pick just one easy target below. I know a bit about how you design a UF system that is buildable, will last for 25 -50 years and how you maintain all of that so you hold the value of your asset. I've done UF sytems on my own houses so have real life experience. I'm not making this up, I have been there and worn the tee shirt. Here is a particular example for self builders. If anyone wants to have a punt at my approach then please pipe up so new folk on BH can see how we can deal with technology. But the same rule apply to say security ststems.. it's not just keepimg the software up to date its the hidden cost of updates that just add to the day to day cost of living. Young folk with families just can't afford this.. I just don't want to expose my young clients in particular to these hidden risks. OK For the cynics. @Nickfromwales I have spent 40 years building my reputation in the construction industry. Nick is out of the same mould as myself. There is a good living to be made by being honest and reliable. For all to provide a bit of context. I was a builder for the first 20 years of my carear, then did a self build that had underfloor heating, went to uni to become an SE.. after a few years I am now an SE/ come Architectural designer. I'm into Passive house stuff, raft slabs.. anything that make my design life interesting. I have said this on BH since the day I joined. I have been designing basic UF sytems when lots of folk on BH were still in short trousers. If any of you have designed a UF system 30 years ago then let me know. That is when I did my first one. Cut me a bit of slack folks and just listen to what I know from experience. You don't need to take my advice.. but you are all big boys and girls. Most folk on BH try to use automated controls, fancy loop systems, weather compensation and the like. I'll tell you folks.. it's mostly pish. If you want to have a go at automating a UF system then you need to have at least two proven designs on you own house under your belt, delivered properly before you try and even atempt this. Avoid anything that has complex controls: because your software will get outdated, then pipe sizes and flow valves etc stop performing after a few years. Try to sell the house and a surveyor will look at the hard facts and may actually say to their Client the heating system is a pup so lets take many thousands off the value. The boiler degrgates, the pumps. The Scandinavians have UF as ON and OFF! simple stupid. Folks this is tough love I'm giving you. I know you are keen to try and automate say your underfloor heating/ security and so on but as a old crusty bastard I just know that ten years down the line it could be a massive liability that will impact on the resal value of your property. It's like buying a modified car from a boy racer! As a designer who does a lot of stuff for young folk on a budget, wanting to improve the value of their property but extend and alter it a bit I think about this deeply. How can I make alterations to a property and maintain their asset value. Personally I don't run about designing stuff that is detrimental to my Client's interest. There is a balance to be struck here, by all means be innovative. I was designing insulated industrial concrete slabs long before MBC and Advance Foundation Technology and the like got on the self build band wagon. Now in the round it's a good thing if MBC and AFD et all are marketing this. Yes I've expounded that view since the days I joined BH. For all, also the folk that are just starting out say renovating a first flat. Try and learn as much as you can. Don't loose you shirt and enthusiasm, even a small profit is a gain.. sets you up for for the next project. I'm Gus and grumpy at times. To add bit. Yes we want to do a self build or extend but we also want to make money and if we can do that and have some fun in the process then.. goooood result!

That is a good starting point. Take it as what it is, your Architect has given you a first impression" on what might work. I do this as the day job. Post your GI results if you want a bit of free advice on foundations. I'm not the only one floating about here on BH that knows a bit about this stuff... you can take our different views into account and weigh them up. For all.. To get the best pro bono SE / Geotechnical advice post some drawings of what you want the foundations to support. Don't try and make us second guess or you will just get some generic answers, answers that are not logical or commonly folk saying .. I did this but with no supporting evidence on how it may apply to your site. Your post is good in that I think.. here is someone who has an open mind who is pragmatic. Your text is short, but states the facts.

Maybe but at that length of duct run BC may ask you for calculations that show that the length of duct delivers the required extraction rate. If you get asked to provide duct calculations you need to take into account not least the bends in the ducts. This can be problematic! One reason for the problem is that the extractor hood manufacturing folk don't give you the data to do the vent pipe calculations! So if you ask me to do them for you I'll charge you for the time it takes me to say get in touch with the Germans and translate all that info so that I might be able to do some calculations on and then.. into something BC can pass. Now if you take my rate at £50.00 an hour the sums don't stack up unless you really.. really want a down draft hood! Now the Germans ain't going to bend over backwards for some self one off builder in the UK.. If you don't believe me then just ask about. If you can forget the down draft extractors. There are often other more effective options. Post some drawing and folk will help you out.

Hita Pappa. I can see you have put a lot of effort into this and there is lots of good ideas getting floated. Take all these and jot them down. It's time to pay some money and get an SE in.

For me I think of district heating.. common thing to do in plenty parts of the world. Main thing for me is keep it simple and stupid. You might need an extra automatic air vent here and there, other than that it should be fairly straight forward. Agree. But this works up to a point. Simplistically when the main circulating pump (sending the water to and from the house) is off the static water head is equal in the flow and return. For reference 1 bar = atmospheric pressure = 30 feet (~10 m) of a static water column. A good domestic Grundfos pump can pump a 6.0m head and if it gets a good start it might just create a negative below atmospheric pressure in the return pipe before it gets going and settles down. You kind of want to avoid this as it causes cavitation in the pump and that can shorten it's life span. Therefore any automatic air vents should be located on the feed (pressure side) if the upper floors are say 4 - 5 m above the ASHP. That's my qualitative assumption which assumes you don't have too many bends etc in the pipes.reality the best thing to do is give it your best shot and have a strategy.. if it does not work then have leave access to add in extra bits!

Hiya @Iceverge and all on BH. This is the thing I love about this site. Ok you may not be an SE/ Architect etc as the day job but you are sharing your experience and you are clearly not daft. Make no mistake. My day job as an SE / designer.. I do day to day stuff, knocking holes in walls and putting in beams.. oh and raft slabs and all that puts bread on my table. I also learn loads here as there are plenty folk with innovative ideas and many with decades of construction experience. For me it's a two way street.. I try and explain some SE stuff here but in return I get a lot back from all the other members efforts. I've also ended up getting to know personally some members and that in it's self makes it worth while for me. Mind you my spelling and grammer has not improved despite many giving me a helping hand. BH folk are at the front end of the design curve as you are trying to make self building affordable and unique. Be proud of what you are aiming to achieve.

@Nickfromwales there are a few options.. but often the rub is getting an adequate bearing without a window lintel on the floor below throwing a spanner in the works. Honestly it's the bain of my life.. I come up with an attic concept design that can look great..you get a stair to fit and I feel quite clever.. I put my SE hat on and my bubble gets burst! At that span to shove the loads where they can be easily dealt with.. no posi joist or timber solution is going to be economic unless you can significantly increase the floor thickness. To increase the floor thickness you invite a pile of other problems. Yes, technically you can reduce a timber joist spacing to less than 300mm but it becomes unbuildable. I totally get how folk on BH are contributing their knowledge in terms of say posi or solid timber floor joist construction and then how you insulate and fire protect all of that.. but when it comes to attic conversions there are many other things like the founds, old wall stability that comes into play. You need to nut that out first before you get onto an insulation / thermal performance strategy. @Nickfromwales and all. Some of my bread an butter work as a designer ( SE and architectural designer) comes from builders that I work with locally. I kind of operate a "hot line" where they can say to their Clients, hey I'll give my tame SE / designer a call. To be clear folks I operate completely independantly and all the builders have to tender competetively for the job. I've spent many years proving my integrity so you have to be absolutely fair. But I want the folk I work with to do well as if they are getting on so am I. It's in my intrerest to make sure they do well. Now I say to my Cleints.. here is a list of builders I know who are trustworthy.. but I always insist they get quotes under their own steam from other builders as this protects me from any accusation of a conflict of interest and lets the the builders that I work with know that they can't walk their prices up on my jobs... as that would impact on my business. For all on BH always get an independant quote as a sense check. Now I can see how folk on BH are tending to go for a timber type solution.. but his may not be the best way forward. Often as a designer you do a few options.. you kind of know that some are not the best but this lets you say to a Client.. I looked at this, you are informing your Client and letting them make informed decisions. And that kind of sums it up on a self build or extension. It's your money and you just need someone who will say..here are your choices.. you decide and if you do that as a Contractor nine times out of ten you will win the job and make fair profit!

Interesting thread this and @saveasteading makes a great point offering up the cold formed steel joist option. They can be remarkably cost effective when you take into account the labour element. Good set of diagrams, well done. Having had a very quick glance at this I can see that the primary span that needs navigation is (5.995m) say 6.1 m over the centre of the supports. I assume there will be some Velux, maybe a dormer so you have some point loads here from trimmers. To go back a bit to let folk on BH know what may be going on the in the murky mind of an SE and what drives their design / risk decisions. I think.. ok it's a mid terrace as reference is made to a party wall. Do I know if there are alterations next door, can I get access? Probably not. Can I put any significant load onto the party wall. Any load I put on it is going to be off the centre of gravity of the wall (called an eccentric load) and that will cause a bending force in the party wall which could shave a lot of it's load bearing capacity and introduce a stability risk. As @saveasteading , @ETC et all have mentioned this is akin to taking down one side of a chimney breast on a party wall.. here reducing loads actually perversely often causes a problem in a brick wall. What if one of the neighbours has already been messing about and they have put a beam right where we want to put one? If so then we have a fire protection problem to add to the complexity and cost risk. A few folk have suggested ideas on trimming steels etc. But you need to be careful here before you implement this type of solution, you need to know where the heavy loads from main trimming beams go as you work your way down the building. Often at attic level it looks great, then you find the point loads are over an existing window lintel on the floor below! Now you end up chasing your tail and it all get out of hand / cost rises rapidly. I think you need to bite the bullet here and get an SE. BC will probably ask about the founds. But to make this more manageable in proving the existing founds will work you want to spread the loads as much as possible down through the load bearing walls. It an old house so you want to spread loads as eavenly as possible. I'm going to lead you down the route of considering cold formed steel joists here. Now cold formed steel joist design often entails some horrific calculations and unless you have (as an SE) a good knowledge of this and software / spreads sheets it's not your bag. For me this is in my comfort zone so it goes in my toolbox of attic / barn conversion tricks and design methods. Incedentally there are a few companies that use "extendable " cold formed (cf) joists as a solution. Essentially you sister the existing timbers with cf joists and avoid disrupting the noggings holding the ceilings below. If I was doing this I would check early that I can get a compliant stair in and headroom in the attic, see where the point loads are, say from roof trimmings. Look to see if I can increase the structural floor thickness (but then you may need that one extra step in the stair that mucks up everything!) and check the existing eaves detail to make sure I can get good support, ideally spreading the loads.. Depending on where you are in the country you have different arrangements of structural eaves detail and it is essential to get your head round this. Have a look on the internet on how a pole plate roof works and a 1/3 rafter / joist eaves works. Now many posters have suggested timber type solutions but attic conversion are tricky as it's the detail that is often the issue. I can see how many posters on BH are suggesting ideas but attic conversion are a different animal.. as not least they are conversions. Hope this helps a bit.

You may have some recourse but you'll have to put a lot of reading into this and thought. Be very careful not to lose your shirt over this! 1/ There is a gov manual called "designing streets", get that and read. 2/ The highways department for your area will have a standard document on visibility displays. Key points are check the speed limit on the highway and whether each access relied on shared visibilty.. Visibility displays change a lot depending if it is a 30 mph or 20mph! 3/ Check to see if there is a covenant put in place by the original developer. if you living in an estate then often these apply where you can only grow a hedge up to 3 feet high beyond the front line of the principle elevation of the houses. Check to see if the council have the power to override any covenant, sometimes they do. 4/ The boundary may be hard to establish. I looks bad and can see how you are less than happy! This could make your life misreable. Would you consider cladding your side in timber to compliment your house and live with the loss of visibility. Often neighbours are not there forever!

Here is a different approach which can work based on my experience. This is a shortish summary. 1/ Read many of the recommended books above, skim read at times. 2/ Identify how you want to live, where and why. Write that down and summarise. 3/ Establish your top outlay.. what you could afford if push comes to shove. . Knock off 20% for contingency. Yes 20%! as you will eat into this remarkably quickly at the concept stage if you are not experienced. To provide context. If you buy a site and go to a builder for a turnkey option they will add on at least 10% for management and another 10% for general risk.. which includes you being a dodgy Client. One key is to start building relationships early on so folk trust you. Many of us do this in our day to day work.. but suddenly forget how good we are at it, building trust asking for help, also applies when dealing with a builder, while at the same time sniffing any shite in the air. 4/ Write all that down. 5/ Find a designer that has a lot of experience (30-40 years) with loads of local contacts and who does this as a day job. Pay them £500 quid to come round on two or three evenings to give you impartial advice. If they know their stuff it will be the best £ 500 quid you ever spend as they will explain to you how it all works on the ground and how you navigate the regs, the tender process, the contract options ( varies from a handshake with the ground worker up to a full blown industry contract for example) and how you can make a self build pay.

Agree. Use a flexible exanding foam AFTER you have got the pipes finally positioned. Sometimes we have to have pops up really close to the raft slab edge so we need to make sure we can weave that though the main rebar, especially if we have point loads from Oak frames or the like. Anyway an MBC type raft sits above the main insulation layer so it's on the warm side anyway.

You can never have enough books.

Your comments are valid. When I was a building contractor.. to provide a bit of context. I employed 7 - 8 men full time and many of them were long standing and loyal.. but I paid them well. I also pulled in sub contractors. Depending on the seasons I could have my core guys and then up to 20 folk working.. but I was still small fry, just a local rural builder (Scottish Borders) that had contacts. But make no mistake.. I could have bought in a TF from say the modern equivalent of MBC, Pascills et all for one off jobs but the service was not that great, maybe because they view self builders as a one off and generally inexperienced Clients. They add an extra profit margin. For all.. when I was a builder I could have knocked up a TF on site faster than saying going to a TF company, many of the builders I now work with do so I was not that special! I made that happen by starting the kit as the brickies were putting in the under building. The great thing is that all the guys worked together so the joiners on the TF would check the brickies work and eat their lunch together in the site hut. Sometimes if the roof was simple we would get prefabricated roof trusses and make sure they fitted on the walls. Some of the kit roof designs I see on BH are just not suited for the prefabricated application and I can see that a stick built roof is the best and most economic solution. I can be done this stick build thing and often more smoothly and in you time frame! But for all on BH the main barrier to this is that you often don't get the right design team in place and spend enough on professional fees and taking advantage of their contacts and are willing to pay for a few panel drawings for what is a one off project. The thing that irks me is that folk expect Builders to price jobs for free. Sometimes I say to the builders I work with when we have say a tricky renovation / refurb job.. hey the Client will pay you to sit down and work out a real price!

What a great post Nick. I spent the first 20 years of my carear stick building TF's before I became a designer. To knock up TF studs on site you need to set up a good saw bench. We used to make one from timbers that suited 4.8 and 6.0 m timber lengths. The chop saw (a top quality one and expensive) got mounted each day on the bench and taken home at night. To cut all the stud lengths / noggings you use stops / marks on the bench. The other main tools were a passlode nail gun for the 90mm long framing nails and I used a air driven nail gun with a compressor for nailing the sheeting when making the panels on the ground for the first floors. For two story houses you adapt the panels to make them manually liftable.. and as you say no need for a crane! I work with a lot of builders (do the TF panel drawings) and we use this method as they win jobs! It's not just that they win jobs.. they get less hassle as they have much more control over the process / delivery and that leads to management savings and that less hassle makes them more competetive. The more competetive they are the more bread and butter work flows my way. Yes you do, in fact you can often achieve a better build as you can account for the variation in the underbuilding and things being off the square. The savings can be significant in terms of buying the timber. I understand that @saveasteading has done a deal with a merchant to basically buy a pallet /s or so of timber off the boat.. cutting out many of the middle men. Ok set that advantage aside. As a self builder cash flow is everything at times. Stick building can smooth the cash flow way. Yes agree. Larsen trusses as well can be knocked up on site. You can do it the slow way using nail plates from say Tool Station or the quick way using ply gussets nailed with a nail gun. You can also make the Larsen trusses bear on the edge of a raft foundation with a bit of diagonal bracing. Again here an experienced joiner will "smooth" out any minor dimensional errors that may have manifested in the underbuilding. A TF fabircator cuts you no slack in this respect, they are not that flexible if you get your found a bit off the square or level or want to move a window or door late in the day. First the upsides.. 1/ You don't have to stump up a big wad of cash up front and hope the TF turns up when you hope it will. You pay for the timber and once it is on site it belongs to you. Then you look after it, dry it slowly until you are ready to use it.. this reduces the shrinkage and cracking you get later on, that has hidden cost benefit. 2/ You don't have to worry about the TF company going bust. 3/ Self building should be an exciting journey, at times you change you mind.. well quite often. Stick building is much more flexible in this respect. 4/ If you can find the right joiner to do the TF then they will often introduce you to all their pals who do all the other trades. Down sides.. 5/ You need someone to do a TF panel drawing and coordinate the main services that might clash with the TF. I do this as an SE / Designer / Ex builder.. but it comes at a cost.. it's a fair bit of work to think it all through, there is no getting away from this cost unless you are able to design and draw a timber frame with the panel drawings and connections economically.. That's it for now but hopefully you can see how I think @Nick and I may be singing off the same page. Mind you we don't always agree at times! In summary. I would always want to compare a manufactured frame with a stick build for domestic one off jobs. Don't forget that we are doing one off builds (extensions or houses) the TF manufactures need to sell mass market to known and big spending Clients who they work with day in and day out. They have invested millions of pound in ther factories.. your self build / extension is the cream on the cake profit wise. Where they really make their money is selling you insulation and the "service" etc. Now if you look at @Nickfromwales.. it seems to me that here is someone with loads of experience who may have something to offer.. I suggest PM him?

Posts over lapping. Lucky you can get to both sides! Good tip, also go and create an account for youself on the British Gypsum website and use their deisng tools, you can print off the calcs and so on. To make this relevant you'll need to download the White Book, it has good details and laymans terms guidance etc.. Remember you need to fire protect the perimeter joints around the wall edges so have a look at that too as to how you detail that out.

50 years expererience talking here from @saveasteading, not to be disregarded.

Hi @Mike DC Are you trying to fire protect the party wall in an attic conversion? Can you provide more detail as we are all guessing here. If you want to get a bit of free help then you need to provide more detailed info.

Agree, this often works. The word goes about you are good folk to work for and folk will be helpfull, especially in a rural area.

This can be easyish or horrible in terms of builability. Do you or can you get access from the neighbours side? If you have access from both sides then it can be pretty straight forward as @nod says. If you only have access from your side..it can be a challenge to understate!