New Extension Build

RaymoS2

Hi, I'm new to Boards but I have used it already answer 100 questions already for my new extension so I won't fill up space here asking them again.

I am building a 26 feet long x 15 feet wide single storey extension to the rear of my existing dormer but every stage I plan I hit another question and hope you guys can help.

My first question is I have a footpath running at the rear of my house the 15 feet width I propose and 3 feet wide. It is 9" below what my finished floor will be. I know I must obviously remove the foundation sides of it which will be 900mm each but can I just leave the remainder under the floor as its structurally sound and will save on hardcore and sand in this area?

Also, I intend to do a 900mm deep foundation by 900mm wide with a 300mm concrete base with the rebar 25mm from the bottom. Is this over the top for a single storey. It will take 3 rows of blocks to get it 150mm above ground level then for DPC.

Thanks in advance.

Penn

RaymoS2 wrote: »

Hi, I'm new to Boards but I have used it already answer 100 questions already for my new extension so I won't fill up space here asking them again.

I am building a 26 feet long x 15 feet wide single storey extension to the rear of my existing dormer but every stage I plan I hit another question and hope you guys can help.

My first question is I have a footpath running at the rear of my house the 15 feet width I propose and 3 feet wide. It is 9" below what my finished floor will be. I know I must obviously remove the foundation sides of it which will be 900mm each but can I just leave the remainder under the floor as its structurally sound and will save on hardcore and sand in this area?

Also, I intend to do a 900mm deep foundation by 900mm wide with a 300mm concrete base with the rebar 25mm from the bottom. Is this over the top for a single storey. It will take 3 rows of blocks to get it 150mm above ground level then for DPC.

Thanks in advance.

You can't use the footpath under the floor of the extension.

Rebar should be at least 50mm off the bottom of the excavation. And although you may plan to go 900mm below ground level for the bottom of your foundation, the ground may not be suitable. You may need to go further. You may even not need to go that deep.

You need an engineer/architect/surveyor/arch-tech. At the very least you're going to need someone to sign off the works as being compliant with building regulations at the end (otherwise you'll be in a world of pain if you ever try to sell). Engage a professional for design and certification.

RaymoS2

Thanks for that advice. Interesting on the rebar because a lot of other forums suggested 25mm so at least that's clarified. Hopefully the foundation doesn't need to go that deep in the end or deeper.........in relation to the DPC having to be at least 150mm above ground level and the fact that my new floor like my existing house is 230mm already above the ground level, is it ok to put the DPC 150mm above ground level on the outside row of blocks and higher on the inside row in order to have it above the new floor level inside or can the inside row of blocks have the DPC below the new floor level or alternatively it ok to just have the DPC on both sides 250mm above ground level on outside and approx. 20mm above new floor lelel.

RaymoS2

Can I just clarify the issue with the footpath. Why if the old footpath will be over 230mm below the new floor and the fact that you can still easily put in the insulation, radon barrier and then 150mm of floor can it not be left in place from a building perspective. It will be stronger than 100mm of hardcore and 50mm of sand?

Thanks.

Penn

RaymoS2 wrote: »

Thanks for that advice. Interesting on the rebar because a lot of other forums suggested 25mm so at least that's clarified. Hopefully the foundation doesn't need to go that deep in the end or deeper.........in relation to the DPC having to be at least 150mm above ground level and the fact that my new floor like my existing house is 230mm already above the ground level, is it ok to put the DPC 150mm above ground level on the outside row of blocks and higher on the inside row in order to have it above the new floor level inside or can the inside row of blocks have the DPC below the new floor level or alternatively it ok to just have the DPC on both sides 250mm above ground level on outside and approx. 20mm above new floor lelel.

You should engage a professional to design these details for you.

RaymoS2 wrote: »

Can I just clarify the issue with the footpath. Why if the old footpath will be over 230mm below the new floor and the fact that you can still easily put in the insulation, radon barrier and then 150mm of floor can it not be left in place from a building perspective. It will be stronger than 100mm of hardcore and 50mm of sand?

Thanks.

What makes you think it'll be stronger than the hardcore? What strength of concrete was used? How much fill material is under it? Was it properly compacted? What type of fill was used under it? Can you say with certainty there's no pyritic material in the fill used below the footpath?

For the sake of breaking out a bit of footpath (parts of which you're going to be breaking out anyway to dig the foundations and the damage that might occur to same during those works), remove it and use proper, certified, well-compacted material. Again, I recommend engaging a professional to specify the type of material required (hardcore is a very loose term, there are many different grades/types of fill material depending on the requirements).