Porch Foundation (again, sorry)

[markblue777]

I think what I have to do is actually dig where the foundations will be and see the soil type I have. From what I read the raft foundation is an option for soft / shjte soil types

 

[RobinBHM]

It would be really helpful if you could post a very rough sketch of the drain run and position of porch to advise further.

Is the drain a private drain, IE is it just coming our of your property to the inspection chamber then onwards to a main sewer?

The disadvantage with a raft is that there is nothing to support the structure below the invert level of the drain runs or inspection chamber.

It all hinges on how good your soil is really, so I would suggest digging an inspection hole first and that will help with deciding on the best foundation. A raft sounds like less work in theory, but may not be in practice, it depends on the type of soil, depth of top soil etc. The position of the DPM in a raft is not that straightforward, its possible to get it wrong and let rain water travel inwards.

I realise this is a small porch and you dont want to over engineer, but at the same time you dont want problems that cant be rectified in the future.

 

[markblue777]

Hi Robin,

Here is a rough sketch of what i am trying to achieve. and from what I have seen it appears that the soil line runs as shown

As far as I am aware it is only our property and we feed into an rectangular inspection chamber that is in the middle of the communal access road (the type that need special rods to open the chamber)


Yeah I see what you mean Robin, I don't want to cowboy it, I want to do it right. Just lacking finding the information I need on foundations and bridging pipes etc (know of any good books?) so saw the slab as a possible way

cheers
Mark

EDIT
Just a note that if you open the image you will be take to the site I host my images and can see a few shots of the front of the house so it may give you a few more details that make it a bit clearer.

 

[RobinBHM]

Hi Robin,

Here is a rough sketch of what i am trying to achieve. and from what I have seen it appears that the soil line runs as shown

As far as I am aware it is only our property and we feed into an rectangular inspection chamber that is in the middle of the communal access road (the type that need special rods to open the chamber)


Yeah I see what you mean Robin, I don't want to cowboy it, I want to do it right. Just lacking finding the information I need on foundations and bridging pipes etc (know of any good books?) so saw the slab as a possible way

cheers
Mark

EDIT
Just a note that if you open the image you will be take to the site I host my images and can see a few shots of the front of the house so it may give you a few more details that make it a bit clearer.

Hi Mark,
That makes it clearer to know where the soil pipe run goes. Im guessing the soil pipe from your house comes from the side of the house and the run you are showing is on its way to the public sewer?

Given that you've got 2 pipes to lintel over and an inspection chamber very near to the edge of your porch, I would consider digging 2 holes at the front corners of the porch and concreting to form pads (you will probably need one at the rear RHS where the bin is positioned in the pic also). Then you could lay 3 steels (universal column section would probably work) and build off that. You will probably have to cast the steels in concrete to form a ground beam, but Im not a builder, so somebody else may be able to advise. The left hand side at the house, you could probably cut out a pocket and insert a beam, maybe sitting on a lintel. The floor of the porch would then want to be a suspended floor which is easily done with timber joists.

It looks like the house FFL is about 200mm above ground level, so you wont need to do much of a reduce dig. Generally you need about 150mm of air space under a suspended floor.

One thing that hasnt been mentioned is that permitted development is under 3 sq metres and max 3 metres high -from ground level.

cheers Robin

 

[markblue777]

Hey Robin,
Yeah the soil pipe comes down the side

I was actually looking at pillar system so I am assuming the pads is similar to doing this? do i need to dig 2 holes to under the depth of the soil pipe as before?

If I don't cut into the existing building I could just do a hole and pour a pad for that also, correct?

I think I will need to look at a few things to see some more information on what this method will be and how to go about doing it correctly. But, This seems like the simplest way of doing it really and with a lower possibility of going through the soil pipe.

Yeah the porch will be under 3 sq metres externally and match to the existing pitch roof at the front so should be under 3 meters tall (if not i will do a sum roof type of thing so it goes just to the allowed height)

Thanks for the help Robin

Cheers
Mark

 

[RobinBHM]

Hi Mark,
Yes the holes will need to be below the level of the soil pipe, but overall depth is determined by soil type.
Yes you can dig down the side of the house foundation and pour a pad instead of cutting out a pocket.

The usual advise stands: size and depth of pads and section of steels to be determined by structural engineers calcs.

There are some available online: http://quick-calcs.net/designs/index.php

If you are constructing a timber porch, which will start at DPC level with a cill, then you only need a single skin of brick work. That means the steels and pads will be centred on the line of the 100mm brick base.

Dont forget to make sure you know where the services run and be careful not to dig through gas, water, electric, telephone!

 

[markblue777]

Hey Robin, Thanks

When you talk about the single skin brick base, will this skin sit on the steel work and then I build off that with my timber frame?

Yeah i will be keeping an our out for all of them. I think i should avoid them (with a bit of luck)

Cheers
Mark

 

[markblue777]

Hi Robin

To ensure I email them the right information to get a quote for the calcs is the following what I need to find out

"I need to work out the size and depth of pad foundations to support a timber frame porch measuring 1500mm X 1700mm and no more than 3 sq meters in height with a clay tiled roof.

I also need the calculations for the sections of steels that need to be used to go from pad to pad to support the structure and suspended floor"

Am i missing anything that they need to let me know the size of etc

Cheers
Mark

 

[markblue777]

One more note Just got a call back from building control and they have said that the soil type in the area is generally sandy clay over ballast does that make a difference on the type of footing that I should be able to have if I do have this type of soil?

Cheers
Mark

 

[Lons]

Mark
Just my personal opinion but as a builder with my own company for the past 18 years.

You're over complicating it though I fully understand why. What I would do if it was mine is very simple.

Take off the manhole cover to determine depth of pipes, then assuming the depth isn't' stupidly deep, Dig a couple of exploratory holes on the line of the proposed founds. If all ok then I'd dig out a full found trench 600 wide (assuming cavity walls), x 900 deep or to bottom of pipe if lower. Pour either a 200 strip found or partial / full trench fill as you wish making sure that pipes are boxed out / lintoled over as necessary and ideally reinforced.
If a strip found and below pipe then can easily use conc blocks as support and std cheap conc lintels from builders merchants over. Compressable material or pea gravel around the pipes. Nothing to stop you stepping the found eithe as long as overlapped and reinforced. It's not rocket science, will outlast you and fully acceptable to building control who aren't involved anyway.
If you use conc blocks to ground level make sure they are 7n blocks or use purpose made trench blocks. Engineering bricks are overkill and will cost a lot more in time and mortar but do need to be used from ground to dpc level.
Cheers
Bob

 

[markblue777]

Hi Bob,

When I spoke to building regs today they said that although I don't need to listen to them they suggest matching the existing house foundation as thats what they would say normally. (obviously going deeper to get under soil pipe if needed)

The issue I have is that the foundation width on the side of manhole cover could only be 300mm max as that is all the room I have with the brick centered on that as well. Would 300mm be ok all around? From looking at data sheets it seems to be ok.

When it comes to lintelling over the pipes how does this actually happen, I know the basic of box round the pipe, fill with pea shingle then lintel over but what I dont get is how you then match the lintel height with the block work or am i missing something really obvious here? do you have a diagram or a resource that shows whats what perhaps?

Cheers
Mark

 

[blackrodd]

I would keep it simple, as Lons has said, also at some stage in the future you may wish to sell you're house.
Solicitors now need to see evidence of competance of works you have done, or had done and I believe you have to sign a document for this.
This is all the more reason for keeping the job well documented and comply with regs and not have to get very involved in the future, form filling.
I would say, keeping away from the drain, which may be shared, would be a very good idea.
Where is you're kitchen/bathroom stack pipe? It look like a shared sewer to me, going by the manhole angle you have drawn.
Regards Rodders

 

[markblue777]

Hi rod,
The soil stack is at the rear of the property. The manhole has 4 entry points but only one is in use from my property. The other 3 points in there are cappped off

Building regs said to take pics of the work as I'm doing it for proof of job done right basically. And when I get extension they can sign off on it to as long is all good with pics

I'm not sure how I could avoid the drain though as its position is going to be next to the foundation

 

[markblue777]

When putting the lintels down would I be correct in assuming that I put it down with a bit of mortar to set it in place over the gap which is packed with pea shingle. And then I would have a course of engineered brick or block to get the level all the same then I just make the brick course work up to the needed level

Cheers
Mark

 

[RobinBHM]

A concrete lintel is 65mm thick, the same as a metric brick. A block is equal to 3 bricks high, as you say you make up the gap with engineerings.

 

[markblue777]

Thanks robin, its good to get some confirmation.

 

[blackrodd]

A concrete lintel is 65mm thick, the same as a metric brick. A block is equal to 3 bricks high, as you say you make up the gap with engineerings.

It may be useful to remember that in each case, this includes a 10mm mortar bed.
A good point about the pics etc for the building regs.
Regards Rodders

 

[Lons]

A concrete lintel is 65mm thick, the same as a metric brick. A block is equal to 3 bricks high, as you say you make up the gap with engineerings.

That and what Rod said is it in a nutshell. If using conc blocks or trench blocks you could of course just cut blocks to the size you want with a bolster or stilsaw / angle grinder.

It's simply like building in a lintol over a window or door, brick / block support each side, lintols over or if using trench fill, just shutter it to give the same support.

When I built my own large extensions a few years ago, with full planning and building regs approvals i was forced to re route soil pipes, extend them and install an additional system. This meant pipes running through in 3 places, not a problem, I fully trench filled where I could and stepped the founds where beneficial, I also needed to go down to 1 metre an one area due to ground conditions though the BI said it wasn't required, I thought it was!

Take your time, do it properly and take the pics, keep receipts and you'll be right.

Bob

ps
Rather than me doing some sketches, just google the subject and also stepped founds and you'll see what I'm talking about.

 

[markblue777]

Yeah been googling but I can't seem to find anything with enough info unless I'm just missing something.

I have ordered a couple of books on home extensions so hopefully they will go into some detail. Also found another book that is used for teaching which looks good as well. So prob be getting that too

Yeah I will look over the step foundations as they may be usefull depending on depths I have to go.

Cheers
Mark

 

[Lons]

Loads of images Mark

I haven't time to look at the minute but just very quickly typed " Lintol over pipes in foundations" clicked images and lots of self explanatory pics, try it.

Bob