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Hi everyone, I seen this forum when searching about raft foundations. Who knew how expensive they would be, I've been quoted £9k and £7.5k so far so I've looked at doing some myself and splitting it down to individual jobs, e.g pay a guy to dig it out, get someone in to show me how to build the metal work, get someone in to do the concrete. Obviously all checked at stages by building control. What are your thoughts on this people? Raft is 8.5m x 4m. 600 wide edging down to 1.2m, slab is 200. It's for a single storey extension on my house. Or if anyone is based in North East England give me a shout.

Hello, I am new so apologies in advance if I accidentally do not follow protocol... I am looking for some help in "signing off" my plan for the foundations of a detached 6m x 4m SIP garden room workshop. The structure will use SIPs for walls and roof, which will also include some reinforcing steel in order to cater for a green roof using sedum roof tiles (65kg/sqm saturated weight). I wanted to go down the route of concrete raft due to clay soil and wanting to be belt-and-braces in satisfying a classification of being constructed of substantially non combustable materials...due to being sited < 1m from boundary fence. The structure will also be a woodworking shop with machinery installed. I also just like concrete. I should also note that I have already done the majority of sub grade excavations... One of the main questions I have is whether the intention to use a 7N dense concrete block mortared into a rebate in the concrete raft is a sound idea. Although I am using the room as a workshop, I wanted to make sure that future use could be various and as such I am trying to achieve a high grade of airtightness, soundproofing and insulation. For simplicity in the drawing I have left off the vapour barrier and house wrap, as well as internal battens + combo of cement fibreboard/ply and external battens + combo of cement fibre weatherboard + cedar cladding. Any pointers on where I have gone wrong or other suggestions will be greatly appreciated. Thanks for your interest.

I've been back and forth on what foundations I would like to use on our plot and I am really keen on a passive insulated raft. I had originally liked the idea, but then went off it as assumed too expensive, then read more and believe it should theoretically should be cheaper/ less hassle but I have never quanitified this. Hence I am going do a cost vs benefit analysis to see what the actual costs are and from there assess the benefit. E.g it might still be more expensive, but if it means less labour and less delays then still works out better. Before I start did anyone else do this? and have any existing excel sheets or similar or input they want to add? My thoughts are it might be a higher upfront cost but it has to work out similar in costs by the time you factor in all the stages required to do a normal type strip foundation with ground bearing slab and then adding insulation, membrane, UFH, screed etc. I understand every house and site will be different, and ground conditions will have a factor, But I have a pretty large open plot, lots of space to store muck and use later on, it is pretty much flat and there are no major trees or any ground issues which are currently known and it would seem unlikely there is anything going to come out of left field and surprise me.

Is there anyone who can help me with information about thermal expansion cracking of the slab or walls in a passive house? I have a technical concern regarding passive raft foundation and (ICF) concrete walls for a passive basement construction (part of a passive dwelling). In technical terms: Any solution to accommodate anticipated differential movement between the raft foundation concrete with an underfloor heating system and the concrete walls that sits on the raft foundation? My concern is regarding the thermal expansion of the concrete in a raft foundation with an underfloor heating system. The temperature of the concrete in the passive raft foundation may fluctuate with 8°C / 15°F or more, compared to the temperature of the ICF concrete basement wall, depending on the season and the heat from the underfloor heating system. With the size of the raft foundation plat, this will induce a calculated expansion of the foundation slab concrete with 2-3 mm. The ICF concrete walls, supported by the foundation system, does not expand, as the temperature do not fluctuate similar. Perhaps we should made expansion joints between the raft foundation plate and the ICF wall, allowing them to slide the anticipated 2-3 mm on each other? However, to make the basement construction strong, we would want to connect the raft foundation system concrete and the ICF concrete wall with reinforced steel. Have anyone heard of this problem and any solution or guidance on how to solve it? Are there any experience about this potential problem anywhere? Expansion cracks in a concrete slab can potentially damage technical installations in the slab, including the heating pipes in the floor/foundation plate. I would very much appreciate any tips / advice. :-)

The raft foundation gang have been in today, loads of them and great progress. I heard a cuss as one of the gang had broken the upstand off a piece of the EPS which I was assured would be fixed. When I returned to site the fix was in place, but as you'll see from the pics the fix, using a bit of spare EPS, the depth of the upstand is reduced from the original. We have GRP angles that will sit on the upstand and be keyed to the slab. With the 'fix' there will be a small void between the GRP and that thinner bit of upstand. Should I insist on a replacement piece of EPS that matches the original or will it bit ok with the answer I'm expect when I see them tomorrow, that they will pack it out and everything will be ok? Difficult to see in the pictures, but the depth of the upstand in the replacement piece of EPS is probably 20mm less than the original.