Basement floor construction with finite depth

[GlanMenai]

Hello,

Photos of the footings - Imgur Link

Would someone be able to help with suggesting a build up of concrete floor and insulation that I could fit into the basement level of our home? In the link above, I've posted two images - the pink string-line is the ideal finished floor level for the house, and I've dug down to find just the bottom footings for the wall.

These are just two random holes I've just and the first shows the depth of 25 cm, and the second image a depth of 30cm - from the pink string, to the bottom of the footings. It's a very old (1890) Victorian House, so levels aren't going to be perfect.

On this basement level, we would love to put some water based underfloor heating in there, so we're going to be looking at installing the whole system, please correct me if I'm wrong, or if there are better alternates out there, from my understanding and research we would need, from the soil up, all of these to fit in there:

• Soil

• Slate waste

• Sharp Sand

• DMP Course (thicker)

• Concrete

• DMP (thinner)

• Insulation

• Underfloor Heating and Screed

• Flooring mat

• Engineered hardwood (finished floor)

As I'm dealing with a finite depth here (30cm-ish), can anyone suggest a system based on the technology we have today that would get the best out of everything? The more insulation I have the better etc, but by putting more insulation, it means my concrete slab would be thinner.

Second question; if I did think about digging lower than my house footings, is this possible? In order to just get a bit more depth to maximize the insulation I could put in there?

Third question: If I wanted to get a professional over to advice on this, who would that be? (We're UK based)

Thank you so much!

[saveasteading]

Is  there already a floor? does it ever get wet?

[Conor]

Insulated raft - 200mm EPS and 100mm power floated concrete (Ufh etc within).

 

You'll need to consult a structural engineer regarding the impact of excavating that close to existing footings, water proofing etc etc.

[markc]

If the basement is going to be classed as habitable then you will need to include flood prevention … unless you are on a very steep slope and the basement is self draining.

assuming it is underground then a sump with pumps and alarms, this will also have bearing on the floor make up

[saveasteading]

Basement construction is very serious Engineering. as comments above, it needs a lot of attention to prevent flooding, and thermal insulation is the least concern.

Can you advise the current situation? Is it just a void under the building or a proper, accessible usable cellar?

[Pocster]

Internal perimeter drain ??

[Gus Potter]

10 hours ago, GlanMenai said:

Hello,

Photos of the footings - Imgur Link

Would someone be able to help with suggesting a build up of concrete floor and insulation that I could fit into the basement level of our home? In the link above, I've posted two images - the pink string-line is the ideal finished floor level for the house, and I've dug down to find just the bottom footings for the wall.

These are just two random holes I've just and the first shows the depth of 25 cm, and the second image a depth of 30cm - from the pink string, to the bottom of the footings. It's a very old (1890) Victorian House, so levels aren't going to be perfect.

On this basement level, we would love to put some water based underfloor heating in there, so we're going to be looking at installing the whole system, please correct me if I'm wrong, or if there are better alternates out there, from my understanding and research we would need, from the soil up, all of these to fit in there:

• Soil

• Slate waste

• Sharp Sand

• DMP Course (thicker)

• Concrete

• DMP (thinner)

• Insulation

• Underfloor Heating and Screed

• Flooring mat

• Engineered hardwood (finished floor)

As I'm dealing with a finite depth here (30cm-ish), can anyone suggest a system based on the technology we have today that would get the best out of everything? The more insulation I have the better etc, but by putting more insulation, it means my concrete slab would be thinner.

Second question; if I did think about digging lower than my house footings, is this possible? In order to just get a bit more depth to maximize the insulation I could put in there?

Third question: If I wanted to get a professional over to advice on this, who would that be? (We're UK based)

Thank you so much!

Hello GlanMenai.

 

You're off to a good start with your questions and research. Hope this helps a bit.

 

To progress.. it's a good time to seek some professional advice. This could be from an experienced Architect (basements) or a specialist basement contractor if you have some contacts. Most often an SE is the best place to start as you'll need them anyway at some point so best to get them in early. I'm assuming you have an idea of what you want to achieve layout wise so this is more of a "structural" undertaking as you have a defined footprint.

 

Most SE's say follow a process which is akin to this...

 

1/ Introduction.. meet.. discuss what you want to achieve, your "soft requirements".. warmth, light , bathrooms (this is to get a handle on drainage) and so on. You may want to create a lighwell outside to introduce natural light into a basement and this will involve making a hole in the outside wall

 

2/ Walk round the outside and examine the condition of the visible structure.. looking for and at; the topgraphy of the site (sloping or flattish),  cracking, settlement, walls out of plumb, type of construction.. long list.

 

3/ Examine the roof layout and condition.. chimney locations and other features that could be heavy or become potentially unstable if some movement occurs when messing about forming the basement. Understand where the roof loads appear to be supported from the outside.

 

4/ Next go inside and up into the attic. Look at how the roof is formed and if the loads are evenly spread or are there for example large purlins transferring load to the walls in certain locations. Look for movement where the roof timbers join together. Often this can give an indication of how a building has settled over time as you can see how the joints may have separated / cracked over time. You compare what you can see inside with what you have seen outside.

 

5/ I'll assume the building is two storeys. Go to the first floor and determine which walls are the spine walls..usually brick. These are the walls that help tie the outside walls together, help resist the wind loading pushing the building sideways and maybe provide a bit of stability to walls containing chimney flues that need a "bit of extra help" to stop them moving sideways.

It's also important to look for walls that may have been removed to enable a change in layout of the first floor. If the original layout has changed this is material as you may have lost a bit of stablity and tying for example.

 

6/ Go down to the ground floor and do the same. Here you often find that folk have been widening out openings in walls, putting in beams and this introduces point loadings which were not there when the house was built. All the time looking for signs of movement and "odd stuff"

 

7/ Lastly look under the ground floor. Look at in particular the areas where you think there may be point loading (that is why you start at the top and work down) examine the condition of; the mortar / the bonding of the masonry/ the quality of the original workmanship.

 

8/ There are are few expert plumbers and sparks on BH who know not to knock out large holes in the underbuilding..but often you find that a "cowboy" has smashed large holes in the underbuilding with a club hammer. In some cases you can see that the building is starting to protest but has not got to the point where this is visible in the living space. But one more "knock".. what then?

 

9/ Lastlyish examine the solum. Is it earth and what is it made up of? Can you see evidence of water rising, lots of salt deposits, signs of flooding.. again it's a long list.

 

Now you have done your "Columbo" bit .. digest.

 

GlanMenai.. you have gathered some info on the founds, well done. The next stage say for the SE is to have a look at the soil. Soil types are a massive subject so I'll leave out info on all the different types, already in this post folk are alluding to this and there is plenty on BH about clay / chalks / rock an so on and how they behave in different ways.

 

To cut to the chase the SE will probably already know what generic soil type is likely prevelant in your area. A thing they will always be keen to determine is what the ground water conditions might be. This is fundamental to making head way as water can have a big impact on the capacity of some types of soil to resist load, remain structurally stable during construction (sometimes called stand up time and water management) when you are forming a basement (doing the work) and after completion. Clearly you need to know where the water goes as this will drive the design of the water proofing / drainage. Remember that insulation floats so you need to make sure you don't create a boat.. your floor could lift!

 

Once you have gathered the above info you can then get to the exciting bit where you can explore the different insulation options, UF heating solutions and how much depth you need to fit it all in. It's stating the obvious but the deeper you go the more risk you run in certain types of ground. On the other hand there are some types of ground where you can pretty much dig to you hearts content.. within reason.

 

Lastly, even if you are just intending to dig no lower than the level of the existing founds please get your proposals checked out. What you can do is remove what we call the "surcharge" which is the weight of the soil above foundation formation level.  This apparently small weight can have a significant effect on the founds if removed, especially on old buildings that may have been mucked about with. Also, you can change the way the water drains under the house and in some cases this can result in unwanted movement.

 

If you follow this process you should get a good understanding of how your house works and this could open up other avenues to explore that you may have not yet thought of! You may also save some money!

 

All the best with the project.