Foundations required for new build loft conversion

[spada01]

I'm looking to extend on two floors and like the idea of specifying large enough joists and a roofing structure that leaves the option open to convert the loft into a habitable space in the future.

Would I need to go beyond the standard foundation depth to allow for this possible future loft conversion?

 

[RonnyRaygun]

No, no need to go any deeper to allow for extra load. Would be sensible to make the footings at least 600mm wide to get a better load spread to the bearing strata.

Width is the issue where extra load is concerned. Depth is governed by the soil type and proximity to trees, so if you've got any trees nearby you may need to go deeper.

Ideally the footings should be checked for the proposed loads to ensure bearing pressures won't be exceeded, but I'd be pretty confident 600mm would be wide enough.

 

[geraint]

No, no need to go any deeper to allow for extra load. Would be sensible to make the footings at least 600mm wide to get a better load spread to the bearing strata.

Width is the issue where extra load is concerned. Depth is governed by the soil type and proximity to trees, so if you've got any trees nearby you may need to go deeper.

Ideally the footings should be checked for the proposed loads to ensure bearing pressures won't be exceeded, but I'd be pretty confident 600mm would be wide enough.

how can you say no deeper and then say wider.... to spread what load.... i have never heard of being told to change the foundations for a loft conversion..

surely its the depth... with the subsrate is important... not even the width, as you can build on the edge of the footing and it has no difference than building on the centre of it...

 

[freddiemercurystwin]

You really are a bellend geraint aren't you?

 

[RonnyRaygun]

Here comes Grrraint getting things wrong again...I hope I never have to step into a house you build...

For the benefit of you and the OP this is how footings work:

For the sake of argument, let's say the soil has a bearing capacity of 100kN/m2, and there's 50kN/m load at the base of the wall. A 450mm wide footing won't be enough as:
50kN/m / 0.45m = 111 kN/m2 which is more than the allowable 100kN/m2.
So if we make the footing 600mm wide:
50kN/m / 0.60m = 83 kN/m2 which is less than the allowable 100kN/m2.

If you leave the footing at 450mm wide and make it deeper, it's not going to make a blind bit of difference to the spread of load (except for allowing the load to spread under openings such as French Windows or Patio doors).

Footings are made deeper for a number of reasons, but it's rarely got anything to do with the load on them.
They can be deeper to ensure the base of the footing is below the tree zone of influence, or to prevent frost heave.
It's possible that digging deeper might get you to a better bearing strata but that wouldn't often be the reason for digging a deep foundation.

By the way, building on the edge of a footing will load the footing eccentrically and could cause an uneven distribution of forces at the base.

You probably could build a house with a loft conversion on a 450 wide footing but you'd want to be sure of the load distributions and the capacity of the soil or you might come unstuck...

 

[RonnyRaygun]

You really are a bellend geraint aren't you?

 

[Static]

how can you say no deeper and then say wider.... to spread what load.... i have never heard of being told to change the foundations for a loft conversion..

surely its the depth... with the subsrate is important... not even the width, as you can build on the edge of the footing and it has no difference than building on the centre of it...

I normally just chuckle over your posts but this ones SOoo out that it needs a few comments..
The post is about allowing for a possible attic room in the future when planning an extension.. Thus three storey loads onto foundations.. so width is the key.
Depth will depend on what ground conditions are, for all anyone knows the OP is bearing onto sandstone, or weak aluvium.. 9/10 the first 3 or so meters will be the same material but there are the odd occassions where there is a weaker band of material below the more solid upper crust.. so deeper isnt always better..
Anyways, the width Ronny has given is based on an assumed traditional build with no concrete upper floors, and bearing onto firm to stiff ground.. but as said before get it designed by a professional..

ps.. Geraint if you ever build a wall on a footing edge on one of my sites i would have you thrown off the project faster than you could say "but i thought.."

 

[^woody^]

I'm going to court controversy and partially agree with geraint

The OP should have no difficulty in fitting his loft conversion on the existing foundations of a two-storey property - the new loads are not too great

I'm not quite sure on the whole meaning of geraint's second paragraph, but it is possible to build on the edge of foundations given certain conditions

 

[RonnyRaygun]

I'm going to court controversy and partially agree with geraint

The OP should have no difficulty in fitting his loft conversion on the existing foundations of a two-storey property - the new loads are not too great

I agreed too, but I got the impression that the extension wasn't yet built and the OP was still at the design / planning stage, hence the suggestion of making the footings slightly wider than he may otherwise.

I'm not quite sure on the whole meaning of geraint's second paragraph, but it is possible to build on the edge of foundations given certain conditions

Agreed, but the eccentric loading will need to be designed for.

 

[^woody^]

Yes, but if the OP just places standard 600mm wide foundation strips, then that should do for his loft too

If he has unusual ground, then his foundations will be designed for the two-storey element alone and not specifically for the extra loft element

 

[geraint]

You really are a bellend geraint aren't you?

back to insults... with robin behind you batman...

 

[geraint]

Here comes Grrraint getting things wrong again...I hope I never have to step into a house you build...

For the benefit of you and the OP this is how footings work:

For the sake of argument, let's say the soil has a bearing capacity of 100kN/m2, and there's 50kN/m load at the base of the wall. A 450mm wide footing won't be enough as:
50kN/m / 0.45m = 111 kN/m2 which is more than the allowable 100kN/m2.
So if we make the footing 600mm wide:
50kN/m / 0.60m = 83 kN/m2 which is less than the allowable 100kN/m2.

If you leave the footing at 450mm wide and make it deeper, it's not going to make a blind bit of difference to the spread of load (except for allowing the load to spread under openings such as French Windows or Patio doors).

Footings are made deeper for a number of reasons, but it's rarely got anything to do with the load on them.
They can be deeper to ensure the base of the footing is below the tree zone of influence, or to prevent frost heave.
It's possible that digging deeper might get you to a better bearing strata but that wouldn't often be the reason for digging a deep foundation.

By the way, building on the edge of a footing will load the footing eccentrically and could cause an uneven distribution of forces at the base.

You probably could build a house with a loft conversion on a 450 wide footing but you'd want to be sure of the load distributions and the capacity of the soil or you might come unstuck...

what a load of *******s... you are desperate now to prove... any man in your game will laugh and hope they never come across someone with your amateur thoughts..

welll maybe your girlfriend might...

 

[freddiemercurystwin]

You really are a bellend geraint aren't you?

back to insults... with robin behind you batman...

That's cos your drivel isn't worth anything else.

 

[RonnyRaygun]

what a load of ****... you are desperate now to prove... any man in your game will laugh and hope they never come across someone with your amateur thoughts..

welll maybe your girlfriend might...

Grrraint, you're a

 

[spada01]

Thanks for all the replies and explanations. Just to clarify, we're using a structural engineer for the BCO drawings who has extended a house two doors along and the ground conditions were very good so he suspects we'll just need 1m depth. I think he has talked about 60cm wide foundations so good to hear that these should be fine if we decide to convert the loft into a habitable space.

Thanks again