Building a cheeky porch...but foundations on sand?

[kingandy2nd]

I’ve over-estimated the number of blocks I needed for my extension and will have nearly half a pallet of blocks and thermalites left.

To make use of them I thought about a porch on the front of the property. I’m only calling it ‘cheeky’ as it’s not on the plans, but I understand if I keep it under 3m2 it’s a permitted development and doesn’t need building control.

However, my house is built on sand and the new extension has to be piled down to 12m in some places. Of course I don’t want to go to those sorts of costs for the porch.

I know that porch foundations have caused some squabbles on here recently, so sorry for being up a sore subject for some, but do you think I could get away with a raft foundation for the porch?

As you will see from the picture/diagram the house already has an overhanging roof (which I’m going to replace anyway). So it’s a case of that actually having some walls to sit on on the outside too, rather than just hanging over. I don’t actually plan to have that much in the way of blockwork, as the glazing will be full height, so hopefully not too much weight on such a foundation.

Do you think this sound ok or am I heading for disaster?

thanks
Andy

 

[^woody^]

What foundation is the house on?

 

[kingandy2nd]

What foundation is the house on?

The house is all piled Woody.

 

[johnny2007]

Get a sheet of aquapanel, cut it to size, rest it on the sand and build from there.
The best builder in the country do this all the time and never had a problem.
He's here helping other members, so you're in good hands, just wait for him...

 

[HERTS P&D]

Johnny, you naughty boy.



Andy

 

[^woody^]

Whilst loading is part of it, another significant factor is that the ground is liable to movement and that movement could move a slab just as much as anything else.

Basically it's a risk, and it may be that to mitigate the risk you don't have a lot of blockwork in the footings.

That existing porch slab may give some indication of whether a new porch slab will move much.

 

[kingandy2nd]

Get a sheet of aquapanel, cut it to size, rest it on the sand and build from there.
The best builder in the country do this all the time and never had a problem.
He's here helping other members, so you're in good hands, just wait for him...

Johnny, you give some good advice on this forum but you gotta let this one go

Whilst loading is part of it, another significant factor is that the ground is liable to movement and that movement could move a slab just as much as anything else.

Basically it's a risk, and it may be that to mitigate the risk you don't have a lot of blockwork in the footings.

That existing porch slab may give some indication of whether a new porch slab will move much.

Yeah, I guess it’s a risk. I was going to dig down around the existing porch slab which is original to the house and see that’s going on there.

I retiled it a few years back and there wasn’t any signs of cracking/movement I remember.

 

[Notch7]

eah, I guess it’s a risk. I was going to dig down around the existing porch slab which is original to the house and see that’s going on there

Build it with a reinforced raft, it needs to be strong enough so that if it moves it will stay in one piece.

What you risk is the porch floating around on the sand differently to the house. I would suggest putting a timber packer on the house wall and screw it on with compriband tape on it, then cover in and out with trims.

 

[johnny2007]

Johnny, you give some good advice on this forum but you gotta let this one go

Sorry, I will try to resist in future...

 

[kingandy2nd]

Build it with a reinforced raft, it needs to be strong enough so that if it moves it will stay in one piece.

What you risk is the porch floating around on the sand differently to the house. I would suggest putting a timber packer on the house wall and screw it on with compriband tape on it, then cover in and out with trims.

Yep, I have a sheet of 7mm mesh left so was planning on using that.

That's interesting about doing the way you suggested, I was thinking of heavy duty ties to connect the new porch to the house to help keep it in position, but I think what your suggesting is to actually allow the two to move. I was planning on rendering the outside of the porch - so sounds like that should be done as separate thing with some end strips...

 

[wessex101]

Yep, I have a sheet of 7mm mesh left so was planning on using that.

That's interesting about doing the way you suggested, I was thinking of heavy duty ties to connect the new porch to the house to help keep it in position

No! The absolute opposite. You need to allow for the differential movement so a detail at the junction between porch and house that will allow for the vertical movement between the two separate structures.