GazJS Posted June 26, 2020 Share Posted June 26, 2020 Good Morning, We have a design to the Domestic Build Act, Scotland, and I would appreciate if anyone can relate a guideline found in NHBC to this document also, or advise if it’s only relatable to NHBC ... The said guideline requires any fill below the floor slab of greater than 600mm to dictate that a suspended slab is used instead of ground bearing. I see this rule commonly across the internet, but I need to directly relate it to the build act, Scotland if anyone can help? kind Regards, Link to comment Share on other sites More sharing options...
ProDave Posted June 26, 2020 Share Posted June 26, 2020 We used a suspended timber ground floor because of the site levels and amount we had to excavate to strip the top soil etc, we would have needed to build up the ground level about 900mm at the lowest point. As it was we had to import quite a lot of inert infill just to get high enough to pour the no structural oversite under the timber floor. So for us it was cost, not any "act", Our builder did try his best to persuade us to build up and pour a solid slab which suggests there would have been no reason not to do so. 1 Link to comment Share on other sites More sharing options...
DevilDamo Posted June 26, 2020 Share Posted June 26, 2020 It’s normally the ground conditions that dictate the ground floor construction as opposed to the amount of reduced/dig, which ultimately comes down to cost. Well that is the case down here in the south. Link to comment Share on other sites More sharing options...
GazJS Posted June 26, 2020 Author Share Posted June 26, 2020 Thanks for the replies ... some background on my question ... We had a design then SEPA set a floor level which required the structure to be elevated (the ground was already lower than the road) initially it was a standard build but the increased levels mean there is up-to 1.7m to finished floor level of which the top 150mm of a ground bearing slab using the 1.7ish of fill to bear its weight. I know NHBC restrict this to 600mm to remove the possibility of settlement on excessive fill which even if compacted properly will settle. Link to comment Share on other sites More sharing options...
ProDave Posted June 26, 2020 Share Posted June 26, 2020 I think the issue is if you are building up 1.7M above ground, then the wall up to that point would have to be designed as a retaining wall to hold back the compacted infill that you fill it up with. It sounds FAR easier to use some form of suspended floor. What is your resistance to using a suspended floor? Link to comment Share on other sites More sharing options...
the_r_sole Posted June 26, 2020 Share Posted June 26, 2020 What's the "domestic build act"? I'm not sure exactly your situation but normally SEPA would comment on planning applications and that's when they would dictate a finished floor level, following that you would usually start detailed design for the building warrant. Generally as long as you comply with the planning conditions and the floor design compiles with the regs (usually you'll have an SER certificate from a structural engineer) then you're fine, if you're using an nhbc warranty then you'll have to build using their details. The structural engineer is the only person that can certify a floor slab design so speak to them. Having said that, 1.7m is a substantial underbuild, usually planning departments dont like an under underbuild like that (in my experience of scottish planning authorities) I would be looking at some form of suspended slab for an underbuild that size unless you have a quarry on site! Link to comment Share on other sites More sharing options...
ProDave Posted June 26, 2020 Share Posted June 26, 2020 I assume if SEPA have specified that floor level it is because of a flood risk? Link to comment Share on other sites More sharing options...
eandg Posted June 26, 2020 Share Posted June 26, 2020 12 minutes ago, the_r_sole said: What's the "domestic build act"? Presumably the Building (Scotland) Act 2003 [and subsequent Building Regs]. Link to comment Share on other sites More sharing options...
GazJS Posted June 26, 2020 Author Share Posted June 26, 2020 1 hour ago, ProDave said: I think the issue is if you are building up 1.7M above ground, then the wall up to that point would have to be designed as a retaining wall to hold back the compacted infill that you fill it up with. It sounds FAR easier to use some form of suspended floor. What is your resistance to using a suspended floor? yeah, no retaining wall so it’s fill both sides in balance - costly costly. It’s sounds extreme in level but the house will site 660mm above the neighbouring road, so it won’t look out of place. we want to use a suspended floor, but the engineers have specified ground bearing and trying to show that it’s not suitable for a re design. 25 minutes ago, the_r_sole said: What's the "domestic build act"? I'm not sure exactly your situation but normally SEPA would comment on planning applications and that's when they would dictate a finished floor level, following that you would usually start detailed design for the building warrant. Generally as long as you comply with the planning conditions and the floor design compiles with the regs (usually you'll have an SER certificate from a structural engineer) then you're fine, if you're using an nhbc warranty then you'll have to build using their details. The structural engineer is the only person that can certify a floor slab design so speak to them. Having said that, 1.7m is a substantial underbuild, usually planning departments dont like an under underbuild like that (in my experience of scottish planning authorities) I would be looking at some form of suspended slab for an underbuild that size unless you have a quarry on site! SEPA commented and the engineers who designed the house didn’t design to the level. We noticed and pointed it out and the just extended the design to include for more fill. It’s got an SER certificate but I was hoping the 600mm rule had been missed and could get a redesign 18 minutes ago, ProDave said: I assume if SEPA have specified that floor level it is because of a flood risk? yeah, the corner of the road will flood 50mm after our study, and they want 660mm above that level 11 minutes ago, eandg said: Presumably the Building (Scotland) Act 2003 [and subsequent Building Regs]. that’s the one. I read it but it’s seems basic compared to others Link to comment Share on other sites More sharing options...
the_r_sole Posted June 26, 2020 Share Posted June 26, 2020 Who are your engineers? Sounds like they don't want to do the work for some reason? What's their reasoning for saying it's not possible to redesign it? Probably cheaper to get a new engineer and avoid the import of that amount of infill! Link to comment Share on other sites More sharing options...
GazJS Posted June 26, 2020 Author Share Posted June 26, 2020 1 hour ago, the_r_sole said: Who are your engineers? Sounds like they don't want to do the work for some reason? What's their reasoning for saying it's not possible to redesign it? Probably cheaper to get a new engineer and avoid the import of that amount of infill! They won’t have any problem doing it, but for a cost and no doubt go through the whole process again, I was/am trying to find something that it’s poor practice for example to design it that way in the first place ... it was the setting out engineer mentioned the 600mm, even if it’s not breaking any regulations under the Act, you would surely think best practice would come into it ... the potential for settlement must be considered very high Link to comment Share on other sites More sharing options...
the_r_sole Posted June 26, 2020 Share Posted June 26, 2020 So you're trying to force the engineer to redesign for free? If it's got an SER then its been designed and double checked so I don't think you'll ever convince them that they have given you a design that's not compliant... What did you ask them to design in the first place? Did they design it before you knew the finished floor level and then move it up or did they design it with the full knowledge of finished levels? Link to comment Share on other sites More sharing options...
GazJS Posted June 27, 2020 Author Share Posted June 27, 2020 14 hours ago, the_r_sole said: So you're trying to force the engineer to redesign for free? If it's got an SER then its been designed and double checked so I don't think you'll ever convince them that they have given you a design that's not compliant... What did you ask them to design in the first place? Did they design it before you knew the finished floor level and then move it up or did they design it with the full knowledge of finished levels? Not necessarily no .... if the rule is only applicable to Checkmate and NHBC etc then fair enough. I am looking to see if there is any such equivalent in the Building Act. I find it strange why 2 companies would enforce and not another. we gave them the planning pack, they produced the drawing which showed a standard build with a suitable foundation. on checking the drawings we realised the floor level had not been fixed by the engineers in which we were given options to resolve. We chose to fill, which I have absolutely no problem with, as we would need to fill the exterior anyway. The only issue I have is NHBC, Checkmate and several google searches all prohibit fill of more than 600mm for a ground bearing slab, and I want to understand why the design standard of our design allows it and if I am going to experience excessive settlement in my floor or we need to change the design to be suspended off the inner block work and be less reliant on the fill. ideally someone with experience can say yes the build act has the same rule, or no it doesn’t and here is why. Link to comment Share on other sites More sharing options...
the_r_sole Posted June 27, 2020 Share Posted June 27, 2020 They aren't all equal, nhbc, checkmate etc are voluntary organisations who have preferred details and methods of construction that you have to follow to get their guarantees or warranty- the building regulations are mandatory standards which apply to all buildings and are enforced by local authorities. If your design meets the building regs, it meets the building regs, applying third party recommendations has no bearing on the suitability of the design. Your SER certifier has double checked your engineers design as structurally sound and this is covered by their professional liability Link to comment Share on other sites More sharing options...
GazJS Posted June 28, 2020 Author Share Posted June 28, 2020 19 hours ago, the_r_sole said: They aren't all equal, nhbc, checkmate etc are voluntary organisations who have preferred details and methods of construction that you have to follow to get their guarantees or warranty- the building regulations are mandatory standards which apply to all buildings and are enforced by local authorities. If your design meets the building regs, it meets the building regs, applying third party recommendations has no bearing on the suitability of the design. Your SER certifier has double checked your engineers design as structurally sound and this is covered by their professional liability Thanks for the input, I will contact the designer and advise my concerns and take there advice/move forward from there. Link to comment Share on other sites More sharing options...
DragsterDriver Posted June 28, 2021 Share Posted June 28, 2021 On 26/06/2020 at 12:05, ProDave said: We used a suspended timber ground floor because of the site levels and amount we had to excavate to strip the top soil etc, we would have needed to build up the ground level about 900mm at the lowest point. As it was we had to import quite a lot of inert infill just to get high enough to pour the no structural oversite under the timber floor. So for us it was cost, not any "act", Our builder did try his best to persuade us to build up and pour a solid slab which suggests there would have been no reason not to do so. Evening- Ideally i need to build a timber ground floor and raise my build on piles- have you got any build up detail or pics of yours? thanks! Link to comment Share on other sites More sharing options...
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