Hello and foundation type opinion and cost?

[GaryChaplin]

On 15/05/2023 at 17:20, Alan Ambrose said:

 

Maybe @Gus Potter (an SE) and @GaryChaplin (an oak frame guy) will have time to comment.

 

Thanks for asking me to comment, though I've way to busy to read this whole thread. Sorry. 

 

I would always design an greeb oak frame to be within an insulation 'envelope'. This can be acheived with SIPS (expensive) or a continuous layer of PIR, taped at the seams.  The oak will shrink within this envelope and not give you issues with airtightness. 

 

I have dozens of examples of how this can be done. However, these sectional details may be of use. 

 

[greenqueen]

On 19/05/2023 at 00:15, Gus Potter said:

Glad you have found BH, you'll learn loads here (I do), get the benefit of all sorts of different views and if you stick at it.. will allow you to make informed decisons that suits you best and save money along the way

 

 

@greenqueen"In your opinion what foundation type is best for sandstone rock and sandy soil" The design process goes a bit like this. Rock / sand .. lets start from experience by looking at strip founds and then work up in levels of complexity = cost. Sandstone = good start. Now lets look at the type of sand, where the water table is and where it may be in the future, how thick the sand layer is and if the site is sloping.

 

Breaking this down.

 

The type of sand is important. Are all the grains of sand roughly the same size (poorly graded) if so they can act a bit like a pile of marbles so the bearing capacity is less or.. is the sand well graded where there are small particles of sand and big ones that all interact, have a larger area of surface contact and thus have a greater bearing capacity.

 

When the sand particles are submersed in water (where the water table is and where it may be in the future)  it reduces the effective density of each particle of sand by about half (Archimedes) thus as the bearing capacity of the sand relies mostly on intergranular friction we need to know about the water table. We also need to know about the water table is as when we dig a hole it may flood. You can dewater but one big risk is that you suck out the fine particles and make things worse Also you may have so much water that you can't easily pour concrete. If you have a high and mobile water table this may lead you to look at the raft option early on as you keep it out the water table. Of course if the water table is really high then you need to remember that EPS etc floats!

 

If the site is sloping then ideally we want to found the building on the same soil strata, not half on the rock and half on the sand as this leads to differential settlement which can "break the back" of a house.

 

To conclude this part. I think you need to get a handle on the ground and that will help you focus on the viable options that suit you. Post your ground investigation results if you wish for some feedback.

 

@greenqueen " If we are going to build with a combi oak and sips frame, how does/does this affect the foundation type? "

 

As other have said .. sips is structural @Alan Ambrose so why do two load bearing structures?

 

I think oak frames are brilliant. You mention that you may not want to use them everywhere. But if it was me I would want to have them on full show, no point in hiding any part of them. Yes they do move about a bit when seasoning and later during summer and winter. Funnily they tend to shrink in the winter as we put the heating on and can lower the humidity levels.. a bit counter intuitive? But in reality the movement is not that much different from a timber frame house, probably less so.

 

In summary I would price on standard stuff: strip founds, look at suspended floors or a simple 100- 125mm thick ground bearing insulated slab then add in your extras to mitigate thermal bridging ect to bring the house up to the thermal performance you are seeking. Go for the simple stupid first that local builders can handle.. you'll get more sensible quotes this way. That gives you a benchmark..  then it lets you test each of the bells and whistles options.

 

To try and price this up as a novice I would start on say a standard masonry cavity wall construction or a timber frame for the whole house. Then add in the oak frame as an extra over. Sounds odd but depending on the type of oak frame you go for; acting as a true arch or more portalised (which puts horizontal thrust into the founds of the ground floor or floor slab if a raft) then the hidden extras ( foundation detailng and mitigating cold bridging etc) will be covered by adopting the extra over approach. Later you can start to shave of cost by good design and detailing.

 

Have a look at these.. great idea which works.

 

Turning now to some technical aspects of raft foundations and EPS. This is not "new technology" we have been designing insulated rafts for ages. In the UK they crop up a lot on cold stores and in the USA.. well there is not a lot they don't know about this, and they have to also have deal with permafrost in the northern regions. Be glad we live where we do in the UK!

 

 

@IanR and @saveasteading You are both correct.. to be difficult I'll add my own interpretation.. all in the best taste.. but I'm going to try and simplify / expand for BH folk.. but may make matters worse.. !

 

There are many different types of rafts and @George can probably add to my list and chip in. I'll give this a go describing the common types, there are many and they all work in a different ways.

 

1/ A true raft. This is say a 250 - 350mm thick flat slab that is wrapped in EPS with a two / three story domestic  building on top, point loads or intermediate load bearing walls from the second storey and roof load coming down in the middle. By making the slab this thick we reduce the density of rebar which lowers the cost as rebar costs more than increasing the thickness of the concrete. Also by making the slab thick we reduce the punching shear stress that comes from the point loads say from columns to create a big open plan space on the ground floor. In summary you would lay the EPS, turn it up the sides, lay in some rebar and pour the concrete. If doing a basement you probably need starter bars and some stuff to sort out the corners and stop the water coming in. Main thing is that the thick slab reduces the rebar congestion which suits local builders.. keep it simple stupid. An important part is also think about slab shrinkage.. story for another day / later. BH folk.. don't get caught out by trying for the thinnest slab you can as the rebar cost etc can go up drastically and you invite other problems.

 

2/ A semi flexible raft.. still a flat slab as above. You see this cropping up on BH where the EPS extends beyond the inner load bearing leaf to say the outer wall. Now if you assume the load bearing wall is the inner leaf and the load from that spreads out at 45 degrees the EPS contributes a lot more to supporting the big structural load from the inner leaf. But you often find that the EPS can get over the line. Two main reasons are this: When you look up the EPS spec it gives you a compressive strength of say 200- 300 kPa at 10% compression just say for the sake of arguement. That means that the EPS needs to sink by 20- 30mm before you get your 200- 300 kPa resistance. Now 20- 30mm movement is not going to be acceptable, especially when the ground under may also be sinking.

 

What we do is to say let's limit the amount of load that the EPS can take. I'm not going to tell you this range of values as this is commercially sensitive. If you search about other designers they are not going to give away these values  until you cough up a design fee, also you need to know about the ground and the chosen / proposed insulation.

 

To design the semi flexible raft we work out what the EPS can carry and then use the concrete slab to carry the rest of the load. This extra load gets shed back into the slab which acts roughly as a cantilever. But for that to happen we need to design the slab to act like a big reinforced concrete slab. But if you turn the slab into a reinforced concrete slab you have to comply with the design codes that relate to that... and that is very difficult if not inpracticable on a 100 mm thick slab. It can sometimes work on paper until you need detail the rebar anchorage and deal with rebar congestion etc.

 

3/ The edge thickened raft. Here we have a concrete slab that is attached to what looks like a strip foundation. As above the same principles apply.. the strip found can only carry so much load so we shed the load back into the slab and often reinforce that as the top of the slab is in tension. The edge thickening is often used where we may have point loads from columns. The edge thickening acts as a concrete beam which spreads the load over a longer distance around the slab edge then the remiander back up into the slab.

 

I have to give up here on the various descriptions and way of doing all of this. Suffice to say you there are often loads of different options..

 

But to add to the complexity we need to consider how big your slab is and how much it will shrink and crack and where can we put movement joints?. When you edge thicken a slab it basically gets anchored at the edges. As the concrete cures it shrinks. A true flat sab can slide a bit but an edge thickened slab is bound into the ground at the edges and thus we often need more steel to resist the shrinkage cracking.

 

Well to finish. Probably the ground conditions are the thing to get your head around, it can be a lot of fun researching your local area, talking to the neighbours, did they build an extension what happened? That basic stuff can save you thousands! And when you move in you already know your neighbours.. that is a good start?

 

 

 

 

 

 

On 19/05/2023 at 00:15, Gus Potter said:

Glad you have found BH, you'll learn loads here (I do), get the benefit of all sorts of different views and if you stick at it.. will allow you to make informed decisons that suits you best and save money along the way

 

 

@greenqueen"In your opinion what foundation type is best for sandstone rock and sandy soil" The design process goes a bit like this. Rock / sand .. lets start from experience by looking at strip founds and then work up in levels of complexity = cost. Sandstone = good start. Now lets look at the type of sand, where the water table is and where it may be in the future, how thick the sand layer is and if the site is sloping.

 

Breaking this down.

 

The type of sand is important. Are all the grains of sand roughly the same size (poorly graded) if so they can act a bit like a pile of marbles so the bearing capacity is less or.. is the sand well graded where there are small particles of sand and big ones that all interact, have a larger area of surface contact and thus have a greater bearing capacity.

 

When the sand particles are submersed in water (where the water table is and where it may be in the future)  it reduces the effective density of each particle of sand by about half (Archimedes) thus as the bearing capacity of the sand relies mostly on intergranular friction we need to know about the water table. We also need to know about the water table is as when we dig a hole it may flood. You can dewater but one big risk is that you suck out the fine particles and make things worse Also you may have so much water that you can't easily pour concrete. If you have a high and mobile water table this may lead you to look at the raft option early on as you keep it out the water table. Of course if the water table is really high then you need to remember that EPS etc floats!

 

If the site is sloping then ideally we want to found the building on the same soil strata, not half on the rock and half on the sand as this leads to differential settlement which can "break the back" of a house.

 

To conclude this part. I think you need to get a handle on the ground and that will help you focus on the viable options that suit you. Post your ground investigation results if you wish for some feedback.

 

@greenqueen " If we are going to build with a combi oak and sips frame, how does/does this affect the foundation type? "

 

As other have said .. sips is structural @Alan Ambrose so why do two load bearing structures?

 

I think oak frames are brilliant. You mention that you may not want to use them everywhere. But if it was me I would want to have them on full show, no point in hiding any part of them. Yes they do move about a bit when seasoning and later during summer and winter. Funnily they tend to shrink in the winter as we put the heating on and can lower the humidity levels.. a bit counter intuitive? But in reality the movement is not that much different from a timber frame house, probably less so.

 

In summary I would price on standard stuff: strip founds, look at suspended floors or a simple 100- 125mm thick ground bearing insulated slab then add in your extras to mitigate thermal bridging ect to bring the house up to the thermal performance you are seeking. Go for the simple stupid first that local builders can handle.. you'll get more sensible quotes this way. That gives you a benchmark..  then it lets you test each of the bells and whistles options.

 

To try and price this up as a novice I would start on say a standard masonry cavity wall construction or a timber frame for the whole house. Then add in the oak frame as an extra over. Sounds odd but depending on the type of oak frame you go for; acting as a true arch or more portalised (which puts horizontal thrust into the founds of the ground floor or floor slab if a raft) then the hidden extras ( foundation detailng and mitigating cold bridging etc) will be covered by adopting the extra over approach. Later you can start to shave of cost by good design and detailing.

 

Have a look at these.. great idea which works.

 

Turning now to some technical aspects of raft foundations and EPS. This is not "new technology" we have been designing insulated rafts for ages. In the UK they crop up a lot on cold stores and in the USA.. well there is not a lot they don't know about this, and they have to also have deal with permafrost in the northern regions. Be glad we live where we do in the UK!

 

 

@IanR and @saveasteading You are both correct.. to be difficult I'll add my own interpretation.. all in the best taste.. but I'm going to try and simplify / expand for BH folk.. but may make matters worse.. !

 

There are many different types of rafts and @George can probably add to my list and chip in. I'll give this a go describing the common types, there are many and they all work in a different ways.

 

1/ A true raft. This is say a 250 - 350mm thick flat slab that is wrapped in EPS with a two / three story domestic  building on top, point loads or intermediate load bearing walls from the second storey and roof load coming down in the middle. By making the slab this thick we reduce the density of rebar which lowers the cost as rebar costs more than increasing the thickness of the concrete. Also by making the slab thick we reduce the punching shear stress that comes from the point loads say from columns to create a big open plan space on the ground floor. In summary you would lay the EPS, turn it up the sides, lay in some rebar and pour the concrete. If doing a basement you probably need starter bars and some stuff to sort out the corners and stop the water coming in. Main thing is that the thick slab reduces the rebar congestion which suits local builders.. keep it simple stupid. An important part is also think about slab shrinkage.. story for another day / later. BH folk.. don't get caught out by trying for the thinnest slab you can as the rebar cost etc can go up drastically and you invite other problems.

 

2/ A semi flexible raft.. still a flat slab as above. You see this cropping up on BH where the EPS extends beyond the inner load bearing leaf to say the outer wall. Now if you assume the load bearing wall is the inner leaf and the load from that spreads out at 45 degrees the EPS contributes a lot more to supporting the big structural load from the inner leaf. But you often find that the EPS can get over the line. Two main reasons are this: When you look up the EPS spec it gives you a compressive strength of say 200- 300 kPa at 10% compression just say for the sake of arguement. That means that the EPS needs to sink by 20- 30mm before you get your 200- 300 kPa resistance. Now 20- 30mm movement is not going to be acceptable, especially when the ground under may also be sinking.

 

What we do is to say let's limit the amount of load that the EPS can take. I'm not going to tell you this range of values as this is commercially sensitive. If you search about other designers they are not going to give away these values  until you cough up a design fee, also you need to know about the ground and the chosen / proposed insulation.

 

To design the semi flexible raft we work out what the EPS can carry and then use the concrete slab to carry the rest of the load. This extra load gets shed back into the slab which acts roughly as a cantilever. But for that to happen we need to design the slab to act like a big reinforced concrete slab. But if you turn the slab into a reinforced concrete slab you have to comply with the design codes that relate to that... and that is very difficult if not inpracticable on a 100 mm thick slab. It can sometimes work on paper until you need detail the rebar anchorage and deal with rebar congestion etc.

 

3/ The edge thickened raft. Here we have a concrete slab that is attached to what looks like a strip foundation. As above the same principles apply.. the strip found can only carry so much load so we shed the load back into the slab and often reinforce that as the top of the slab is in tension. The edge thickening is often used where we may have point loads from columns. The edge thickening acts as a concrete beam which spreads the load over a longer distance around the slab edge then the remiander back up into the slab.

 

I have to give up here on the various descriptions and way of doing all of this. Suffice to say you there are often loads of different options..

 

But to add to the complexity we need to consider how big your slab is and how much it will shrink and crack and where can we put movement joints?. When you edge thicken a slab it basically gets anchored at the edges. As the concrete cures it shrinks. A true flat sab can slide a bit but an edge thickened slab is bound into the ground at the edges and thus we often need more steel to resist the shrinkage cracking.

 

Well to finish. Probably the ground conditions are the thing to get your head around, it can be a lot of fun researching your local area, talking to the neighbours, did they build an extension what happened? That basic stuff can save you thousands! And when you move in you already know your neighbours.. that is a good start?

 

 

 

 

Thanks so much for such a detailed and clear response that has really helped me understand what my options are and what to do next, I will focus on the ground conditions and that should inform the rest, oak is a stretch for our budget but it is beautiful. Pretty much all of the oak suppliers seem to offer an oak frame encased with SIPs , welsh oak offer an oak and timber frame with cellulose insulation. I had no idea there were so many variations and options for foundation, I'll price up the basics and then add on if we can afford it, good tip. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Budd]

Hi there,

Any news on soil testing or ground conditions results please.

We seem to be on a similar journey.

 

We will be getting a soil survey done ASAP .The only information we've got atm on soil type is from BGS Geology viewer which describes the soil as Sandstone-

"Red, brown and purple fluvial sandstones with red mudstone interbeds"

Don't know if this is any use to us at this point ? 

 

Cheers

 

Dean

 

On 23/05/2023 at 16:40, greenqueen said:

Thanks so much for such a detailed and clear response that has really helped me understand what my options are and what to do next, I will focus on the ground conditions and that should inform the rest, oak is a stretch for our budget but it is beautiful. Pretty much all of the oak suppliers seem to offer an oak frame encased with SIPs , welsh oak offer an oak and timber frame with cellulose insulation. I had no idea there were so many variations and options for foundation, I'll price up the basics and then add on if we can afford it, good tip.

 

Edited January 4 by Budd
adding more information

[steady]

what did you end up doing@greenqueen this is something were exploring now [oak frame v normal build ]in fact i could of written the thread 😀