Porch Foundation (again, sorry)

[markblue777]

Hi all,
Second post related to a porch foundation but I think it is best to be in a different thread.

I have been thinking and if I am just making a wood framed porch (with either soft wood or pvc cladding) could I just use like a concrete slab for a foundation for the frame to sit on?

My thinking goes along the lines that in the post below that is having quite a bit of weight put on it so surly if I lay a slab that is about 150mm thick it could cope with a less than 3m2 wood frame porch?

https://www.ukworkshop.co.uk/forums/bike-tool-shed-t89083-15.html

Am I just well off missing something obvious and I should just face the fact that I need dig the foundations as normal or is this line of attack OK for a porch?

Cheers
Mark

 

[Mar_mite]

Could anyone advise on this? I'm very curious myself as I'm thinking of putting a timber porch on my house.

 

[n0legs]

I would say no, use a proper foundation.
The bike/tool shed is a stand alone structure, if there is any movement hopefully it will all move as one.
The porch will be an abutment to the house, if it starts to move it may collapse and at the minimum the weather resistance could be compromised.
Just my 2 cents worth.

 

[RobinBHM]

Foundation design is based on 2 things:

1. Stability, which is mostly to do with soil type and influence of trees.

2. Ability to support load of superstructure

Most people tend to think foundations are to do with ability to support weight, but mostly they are to with stability. Modern materials like cement mortars, celcon type blocks etc will crack with just a mm or so of movement.

The basic minimum std for a conventional trench foundation to building regs is 450mm wide by 1000mm deep and usually about 650mm of concrete. A trench foundation is by far the most common system and is usually accepted by a building inspector. A slab of concrete as you propose is not quite so straightforward as building inspectors will only accept a structural engineers design for a raft

In practice, you could probably get away with simple foundations with a timber structure as it is more forgiving with movement. But you need to dig down to solid ground, which is generally well into the sub soil layer and provided it is good ground with no trees too close (10 or so metres). A raft is possible but you would need a bit of a skirt and reinforcing mesh and bar. A raft for a small porch could be made pretty stiff so it doesnt flex, but it could move differently to the house so slight cracking could occur at the join, again not necessarily a problem with a timber structure.

Dont forget although it is a timber porch, it still needs to sit on a brick wall up to dpc level, so you may find a trench foundation less work. You could also opt for a strip foundation, which is a small concrete pour and concrete blocks and engineering bricks laid up to just below ground level -if you do the blockwork yourself, other a mass fill with concrete is cheaper than the labour cost.

If you want a timber floor then you could do a suspended timber floor with joists. If tiles, you will want oversite and screed ideally.

It all comes done to the type of soil you have, thickness of top soil and trees. You dont need building regs, so you need to satisfy yourself that your happy with the size of footing.

 

[markblue777]

Oh right OK, its just my Grandad lives just around the corner from me and he done his porch on a slab foundation and that is half brick half PVC. It's been up for quite a while and has lasted well. I will see what the soil is like outside and see if I could achieve it.

 

[Lons]

Oh right OK, its just my Grandad lives just around the corner from me and he done his porch on a slab foundation and that is half brick half PVC. It's been up for quite a while and has lasted well. I will see what the soil is like outside and see if I could achieve it.

Hi Mark

What Robin has said is basically true. A trench fill found is almost always prefered by builders as when factoring in the labour cost it is always cheaper, but strip founds are perfectly acceptable and fall within regs so cheaper if doing it yourself. I personally would always include reinforcement in a strip found btw.
As a builder, I make sure the founds are bombproof as the consequences don't bear thinking about but have applied the same principle to my own property for the same reasons.

All that said, from memory, your porch is not subject to building regs and therefore you can do what you like. You should however be aware that if you don't do it properly, it can actually reduce the perceived value of your house and cause more problems than it's worth.
The cost and inconvenience of the founds failing is considerable so get it right, you only want to do it once! You can easily fix above ground, not so easy below!
I suggest you dig down to see what's there before you make a final decision.

There are many thousands of buildings and extensions still standing without major problems built on little or no founds and building control regularly move the goalposts and impose a one size fits all and I've worked on grade 2 listed buildings more than 200 years old with just sandstone flags as foundations. There are also many examples of subsidence damage, a factor which affects all our insurance premiums. Your situation might well be different to your Granddad.

cheers
Bob

 

[markblue777]

Hi bob,
OK I get ya. I do have a manhole cover next to where the foundation will run in parrelle with. Does it make a difference to depth of foundation.

Some pics of front of house so you can see

cheers mark

 

[RobinBHM]

The usual guide is 150mm below invert level. This is so the load exerts pressure below the drain and wont affect it.

 

[MMUK]

Assuming the drains run across the front of the house your footings will need to be at least 150mm below this level. I would recommend you shutter a 200mm square box around the pipe(s) with concrete Lintols over the top and pack the void afterwards with pea shingle to accommodate any movement.

Depending on how deep the pipes are it may be best to trench along under where your slab will be and Lintol/pea shingle the whole length. You can use old 50mm paving slabs as a cover as long as the shingle is packed fairly tight.

 

[Lons]

Hi Mark
What the other guys said!

Assuming it doesn't cross your new found then 150 mm below invert otherwisw definitely lintol over as stated.
cheers
Bob

 

[markblue777]

Hey, yeah I will be litoling over as it goes through the foundation. What about the depth I need to go down so it does not interfere with the inspection chamber. I suppose if going under the soil pipe it will prob be enough to go under the inspection chamber

Cheers
Mark

 

[Mcluma]

when you start digging for the foundation and you can re-rout the pipe - than do that.

 

[markblue777]

That would be a hell of a lot more digging to sort that out and add unneeded bends to the pipe flow. I though they needed to be left as straight as possible.

 

[blackrodd]

Moving a sewer pipe, that's a lot of work to go through for a porch, do you have a pic of what you would like?
What have you're neighbours put up around you?
How big did you anticipate going for the porch?
Regards Rodders

 

[markblue777]

Yeah not really wanting to move the line for a porch. Bridging over it is not a problem.

Direct neighbours don't have porches, but people just around the corner have small ones under the 3m2 limit so no planning needed

I'm looking at being under the 3m2 prob about 2.6 ish so no planning is needed

Cheers
Mark

 

[markblue777]

I'm gonna look at designing the porch on sketch up maybe tonight so I'll put up here when done so you can see what I'm talking about a little more clearly

 

[Lons]

Don't even think about re-routing the foul drain Mark. You aren't allowed to do that without building reg approval for which you need to pay a sizeable fee and it has to be inspected. can open a whole can of worms!

cheers
Bob

 

[Mcluma]

I agree with Bob, but i also do not like putting pipes under foundation. I would avoid that as much as possible

 

[markblue777]

Would a raft foundation be sufficient for the foundation for it with rebar reinforcement. Or could a slab foundation with rebar reinforcement be OK if the soil is correct for it (like a firm or solid clay, I think that's what we have)

Cheers
Mark

 

[MMUK]

As far as building is concerned, clay soil is worse than sandy soil. You need much deeper footings for a house built on clay soil.