Foundations basics

[Andrew Welham]

Dear ALl

My request is about foundations for an extension I am planning, at this stage I am just looking for pointers so I can have a sensible chat with the building inspector, prior to creating any plans. Sorry if this post is a little long, but I need to explain my ideas A picture to illustrate this conversation can be found at the following address

am looking to build a single story extension in the shape of an L round my house. I want to build right up against my boundaries (which are shown by the thicker lines along the sides of each drawing

The picture on the left shown my ideas for foundations and the one on the right shown the room layout (excluding external entrances)

I want to do this building three phases - Room 1 first - The new garage - Room 2&3

This is for a number of reasons, including breaking the build down into three separate phases, and most importantly there is currently a garage where room 2 is going to be located.

I want to build right up again my boundaries and I understand that I can?t build using trench foundations as if I don?t build in the centre of the foundation they will rotate over time. For this reason I am considering slab foundations for the part closest to the house and then traditional trench foundations on the other side of the drain pipe. I understand that slab foundations are usually at ground level as the slab also forms the floor of the building, how ever in my case I want finish the slab at the same level as the top of the trench foundations, and I will them create a raided floor over the entire area. This is not only warmer, but also created a nice place to locate pipes and cables.

To bridge foundations (over the drain pipe) I am considering using lintels (The strongest possible). I am looking to building above building code where ever possible to ensure maximum strength in the building.

The 2nd Slab would form the garage floor with a single wall no insulated. I don?t intent to park a car in the garage, but instead use the space to store my tools etc. For this use is a garage the best designation for this type of building? There would also be short walls to form the back and front of the garage.

Finally comes slab 3 & trench 3 used to form room 2. By completing the wall in trench 3 then room 3 will automatically be built.

Now I have explained my ideas I have the following questions.

1. Is a garage the best designation for the use described above?
2. Confirmation I can build on the edge of a slab?
3. I am a little concerned especially with the garage ahat all the load will be on the wall opposite the house, is this still ok, ??? or will the slab rotate ?
4. Should I be using any additional additives in the concrete? I have heard of some fibres which are stronger than re bar, is this correct / advisable?
5. Can I mix slabs and trench foundations in the same build? (If so what do I need to consider) re bars????
6. Can I build a trench foundation in a number of distinct phases i.e. trench 1 & 3 which will eventually form a single trench, should I use re bar to ensure the concrete strength is retained along the join?
7. Is what I have described a recommended method? If not what are the alternatives
8. Is there any other way to build right up against a boundary without impacting the neighbours land?
9. Any concerns the building inspector would have when they see this??

Thanks

Andrew

[Phil L]

Have you recently won the lottery?

I don't understand this question.

By 'slab' I presume you mean raft? - why are yopu contemplating using a raft, then using block and beam floors over the top, coupled with trench foundations at one side?

If it ends up going on a raft, the raft will cover the entire area and will also incorporate a 'toe' - a huge concrete counterbalance on the opposite side.

The person who designs the raft will also design the steel that goes into it, it will need to be mechanically shaped, bent and sometimes welded into the shape of the concrete which is to be poured - all the concrete has to go in on the same day - you can't do it in sections.

see above

You can but they don't like it, it isn't as strong as one long trench and one continuous run of concrete.

Just about everything you have thought of so far is the most expensive, time consuming and outright unnecesary

Build on trench footings, you may lose six inches, but this raft will cost you at least 20 grand more than strips

He'll tell you to cover the drains which run through the proposed building with concrete, and he means they have to be completely encased in it...what would be a thousand times easier would be to dig your back trench twice the width and run a new plastic drain from one side to the other, then connect up to the two existing drains at either side thn the drains are not under your building at all, extra cost = about £150

[Owain]

And we don't have "building code" in this country; we have Building Regulations (or in Scotland, Building Standards).

They'll want to see your plans, calculations, concrete specifications, and result of trial pit excavations. They'll probably want an architect's or structural engineer's stamp on them as well.

Owain

[Peter Crosland]

I will let others answer the rest of the questions but you will need to consider the implications of the Party Wall Act since you want to build so close to your neighbour assuming his house is also close to the boundary. Details are here

effect of the Act is to ensure that any work you do is done a in satisfactory way and does not cause problems to your neighbour. You have to pay all the costs associated with the Act.

Peter Crosland

[Dave Fawthrop]

|Andrew Welham wrote: |> I am looking to building above |> building code where ever possible to ensure maximum strength in the |> building. | |Why? | |And we don't have "building code" in this country; we have Building |Regulations (or in Scotland, Building Standards). | |> 9. Any concerns the building inspector would have when they see this?? | |They'll want to see your plans, calculations, concrete specifications, |and result of trial pit excavations. They'll probably want an |architect's or structural engineer's stamp on them as well.

I remember finding a Building Inspector down a foundation trench, shoveling the soil which had the consistency of heavy cornflakes (shale of made up ground) I took his advice to put in a reinforced raft, like the rest of the house. Do not believe anything till you have a hole.

[Owain]

Very true. I think foundations are the biggest liability in terms of "contingencies".

All it needs is a high pressure water or sewage main, a forgotten air raid shelter, remains that look suspiciously human, or some Roman pottery, dug up in a trial pit to blow the budget.

No substitute for a good 'ole.

Owain

[Andrew Welham]

First of all apologies for using the wrong terms, I have not been involved in this type of work before.

There are a number of reasons for my original (maybe flawed) thinking: The drain pipe runs behind all the houses in my road collecting sewage from each house as it goes. Therefore in order to redirect the pipe I need to dig up my garage and the neighbours patios on each side. My intention is to cause as little impact to my neighbours as possible & and not remove the old garage until later in the built. For the above reasons I don?t think the pipe can be re routed.

A statement was made that I will need to cover the entire pipes in concrete, does this mean I will need to lower the man holes an cover them as well ??? Basically meaning there will be no access to the pipes from my land in the future?

There are two reasons I am considering rafts.

1. To get maximum utilisation of space
2. More importantly, if I don?t use rafts the wall on the right hand side (from the house) of room one will join the house at the frame of the patio door, and I?m not sure how I would tie the wall to the house? Would I be allowed to extend the wall which once held the patio door to fill the gap. I would have thought this would have been a point of weakness?

I plan to do this work my self, and therefore would be interested to understand why using rafts would cost an additional 20K????

The questions about the designation of a garage. The reason for this question is that I believe if the building is designated as living area then I need to build cavity walls, but for a garage I only need a single layer wall. Also the roof designs are different warn vs. cold.

Ideally I would like to build a single layer wall (cold roof), but without all the additional requirements for a garage such as fire doors,

12 inch step to stop patrol fumes rising. As I won?t be storing a car in the building.

Lastly I didn?t win the lottery just learning about building.

Many Thanks

Andrew

[Phil L]

Aplogies for being 'terse' but my nobbies[1] are crippling me

A few questions here:

1) how do you know that the drain runs along this line? 2) how far from the house is it? 3) where is your existing garage

If there are manholes there, they will need to stay there (in the middle of your room(s), hence my advice to sling the lot and start from scratch, unless you want someone to start removing carpets and rodding sewage out of your house in years to come - manholes are there for one reason - to remove blockages) - even if you go down this route, the actual pipes will still require to be encased in concrete.

It's not worth it.

It doesn't matter where it joins onto the house, pillars will be built and tied into existing brickwork.

You don't mention the meterage of the area you want to build on, but a raft is a lot of work and a lot of muck has to come out, more importantly a lot of concrete has to go back in, even doing it in one big raft would be extremely costly but you want to do it in 3 stages, making it triply so.

For the amount you would save by not insulating the walls and roof, I wouldn't bother, and a single skinned wall will get damp eventually for the sake of a few hundred quids worth of blocks and fibreglass.

That can be avoided by not having a garage door on the front, then it's just a workshop

Don't approach the building inspector for a chat with these ideas, firstly he won't listen to you until your plans have been submitted, and secondly, you need to get proper plans drawn up by a competent person, but doing it in seperate stages will escalate the costs through the roof, you are basically proposing to build 3 seperate extensions

[Phil L]

Nobbies - Nobby Stiles = piles

[Andrew Welham]

all the neighbors have drains in similar positions and i have seen the drain on the council plans.

each of the man holes is approx 270CM from the house at their closest point, and are running with the long side of the man hole parallel to the house. I was considering having the wall parallel with the house at about that point, but unfortunately the drains are in the way so i was going to go just the other side of the man hole covers.

see the new link

thanks of that i was unsure if that would have the required strength.

I want to go about 3.5 M from the house and 8M across to form all 3 rooms. This excludes the garage which is currently the drive way, on a 1960s semi. Sorry don't have the exact measurements at the moment.

The reason to build room one first i the immediate need for space. then the garage to allow the old garage to be knocked down in stage 3.

thanks for that again i was just trying to save space internally

I agree i am not yet ready to approach the inspector. I do how ever want to plan end entire build prior to creating the plans to ensure that i don't draw plans which can't be built

I assume that i can apply for the building as a whole and then construction can be in a phased approach. Planning and building/inspection are two completely separate tasks?

>

[Phil L]

These drains are going to cause you a lot of headaches unless you A) build on the inside of them, or B) re-route them

Strength isn't an issue where two buildings join, neither is holding the other up, they are essentially sepereate buildings next to each other.

depending on where it is, don't expect change out of 70 grand.

It's cheaper (a lot!) to have it built as one.

The person who draws the plans will be up to speed on regulations, calculations and everything else and he will ensure that they will pass before submitting them on your behalf.

In some cases yes and no!

In this case I fear you would have to submit each plan as a seperate entity, it's highly unlikely that any kind of raft would be allowed to go in in stages....if you built the whole thing on strip footings, then I don't see a problem, except much higher costs and lengthy disruption but if you don't care about these then it's up to you.