Foundations

[David_Janet]

Is it neccessary to have 'proper' foundations for a single story concrete block garage, 8m  x  5m, or will the concrete floor base be sufficient? (It will be a simple box construction, with a tiled roof)

If it is neccessary, how deep should the foundations be?

And if not neccessary, how thick should the concrete base be?

Thanks for any input

Regards

David

 

 

 

[AnOther]

I think probably not but your best point of reference would be your Marie where you should (must) at least inform them of your garage plan even if you don't require specific permission to build it.

 

[Gluestick]

Much depends on the subsoil, drainage, siting (e.g. next to a steep hill) and so on.

Personally, I would never ever consider a brick/blockwork building without some form of foundation.

You could, of course, create a "Raft" from your slab. But rafts need piling (or substantial brick piers) and need to far thicker than a normal garage base and also need reinforcing mat or rebar within.

Foundations would be far cheaper.

 

[lostinfrance]

[quote user="Gluestick"    

 But rafts need piling (or substantial brick piers

Where did that come from.[8-)]

                                      It is a garage.not a block of flats.[8-)]

                                                                                      But the again it could be a belt and braces D.I.Y.job.[:D]

[Gluestick]

Please re-read what I stated.

If for example, there is danger of mud slip, or it's clay, then simple foundations are not enough. A simple raft doesn't have to be as demanding as a major construction project: it can substitute for peripheral foundations. It would, however, have to be both thicker than a normal garage floor and it would need some way of tying the thing together, so any shift was as one. Chicken wire might do: depends on the subsoil.

"Piling" might be as simple as burying concrete drain pipes, before it's poured.

We used simple rafts for the bases of pressure pumped system houses in Central and South America and West Africa. Proof to 7 Richter and also hurricane resistant. Seemed to work OK.

You assume that the subsoil is perfect: I don't.

8 metres by 5 metres with a tiled roof is a substantial building. The walls would need piers unless hugely thick to resist the natural tendency for the weight of the tiles and timber to push the walls outwards.

Over 8 metres a brick or blockwork wall with no real foundation will be inclined to push out of line, even with no roof: add the roof mass and you have an additional moment.

Many interesting properties have been built in Southern Spain over the past 20 years: without proper foundations; simple single storey villas.

[blink]

[lostinfrance]

[quote user="Gluestick"]

Please re-read what I stated.

If for example, there is danger of mud slip, or it's clay, then simple foundations are not enough. A simple raft doesn't have to be as demanding as a major construction project: it can substitute for peripheral foundations. It would, however, have to be both thicker than a normal garage floor and it would need some way of tying the thing together, so any shift was as one. Chicken wire might do: depends on the subsoil.

"Piling" might be as simple as burying concrete drain pipes, before it's poured.

We used simple rafts for the bases of pressure pumped system houses in Central and South America and West Africa. Proof to 7 Richter and also hurricane resistant. Seemed to work OK.

You assume that the subsoil is perfect: I don't.

8 metres by 5 metres with a tiled roof is a substantial building. The walls would need piers unless hugely thick to resist the natural tendency for the weight of the tiles and timber to push the walls outwards.

Over 8 metres a brick or blockwork wall with no real foundation will be inclined to push out of line, even with no roof: add the roof mass and you have an additional moment.

Many interesting properties have been built in Southern Spain over the past 20 years: without proper foundations; simple single storey villas.

[blink]

[/quote]

You please read the first post.

                                             Not make up your own to answer.

                                                                                               And please do-not try to teach your gran-mere to suck eggs.

[Charlotte3]

Thank goodness, lostinfrance,

 

I thought it was just me......personally, after all my years in France I've never needed anything built to withstand earthquakes.......maybe I'm just being irresponsible?

My advice?  Build as the French do, not as the Brits!

 

Good luck,

Aly

[Kitty]

What is wrong with Gluestick's answers?  They are sensible.  There are quite a few houses here (on South West coast of France) without foundations and there are even more cracked and dilapidated buildings.  [Www]

 

[Simon]

You'll need foundations.  Dig a trench 600mm wide down to the hard, ie remove all soil and vegetation, and then pour a concrete foundation of at least 200mm deep. Don't scrimp on foundations, they'll be the cheapest and most important part of your building.

[Nearly Retired (I am now)]

David_Janet,

If you want a proper answer without paying for professional advice I suggest you post a photo of the proposed site. Tell us if it slopes and if there are any trees about. Dig a hole up to say 1m deep or if not, as far as you can with a spade, and report back with your findings. The Brains Trust will give you as good info as they can.

[Gluestick]

[quote user="Cathy"]

What is wrong with Gluestick's answers?  They are sensible.  There are quite a few houses here (on South West coast of France) without foundations and there are even more cracked and dilapidated buildings.  [Www]

 

[/quote]

Thanks, Cathy. [kiss]

Still, at least we now know who not to buy a house from!

For the OP who sought guidance, have a look here:

First:

Second:

Some basic knowledge to commence with.

Really, it's all about sub-strata.

If that is solid massive rock, then probably, you won't need any foundations! [:D]

If it isn't..........................................

 

 

[thunderhorse]

[quote user="lostinfrance"]...

You please read the first post.

                                             Not make up your own to answer.

                                                                                               And please do-not try to teach your gran-mere to suck eggs.

[/quote]

What on Earth are you babbling on about? Gluestick made reference to the existing concrete base which could be used as a raft. He then issued the caveat, but... and followed it with foundations would be far cheaper.

You are the one who should be reading posts more carefully.

[UlsterRugby1999]

Mother of God - do you know what surprises me about some of the folks who respond to serious threads seeking serious responses to serious needs on this forum

NOTHING

A question was asked and Gluestick gave a seriously well thought out and balanced response and numbnuts go AWOL with crass responses - why for dear sake.

Lostinfrance - get a feckin life - wouldnt it be better to prevent problems now by doing the job properly (as the French do so now) rather than waiting for 10 years and having to underpin [:@]

[quote]D.I.Y.job.[/quote]

Based on your response this is all you would appear to be capable of

Charlotte3  not sure where you are based but here all new houses and other such constructions are being given properly formed and calibrated footings/foundations. Maybe before you slag off a serious poster with serious intent - you take the time to wind your neck in and quit having a go at someone who clearly knows what they are talking about.

David-Janet - Prevention rather than cure is always the maxim. The cost of placing footings now is much less than the cost of having to excavate and underpin later. I wish you good luck and I'm sorry that you met wingnuts along the way.

 

[lostinfrance]

[quote user="Charlotte3"]

Thank goodness, lostinfrance,

 

I thought it was just me......personally, after all my years in France I've never needed anything built to withstand earthquakes.......maybe I'm just being irresponsible?

My advice?  Build as the French do, not as the Brits!

 

Good luck,

Aly

[/quote] 

No it is not you  Charlotte.

                                        I think is that we have some well intentional Barry Buchnells on the site.

                                                                                                                                                           And they could put people off ,with some of there answers.

[red]

Know nothing about foundations, but!! not too sure that you don't need a ' permis de construire ' for

a building of the size stated, in which case advice  ' may ' given by the relevant authorities.

[red]

the 'be' following the 'may ' was censored

[BIG MAC]

Single storey simple construction on low plasticity soils I guess a strip footing 400 x 400 containing some French reinforcement (Aready nicely wired up) set in c30 concrete at a depth to bottom of footing of 1 metre should be more than adequate. Piles etc are overkill unless you are on dodgy ground (shrinkable clays or sand) Piles are used to achieve either a frictional resistance or to transmit loads down into better soils or strata.

Long walls will want expansion jointing and piers if 1/2 brick thick.

Roof spread talked about elsewhere as roofs pushing walls over is scaremongering simply make sure your cut roof or trusses are adequately restrained in the lower chord ie properly triangulated.

Ground bearing floor slab wants to be on well consolidated hard core blinded with sand and a DPM installed albeit French concrete seems impervious anyhoo.

In all likelyhood what I suggest here is probs overkill for what the Maire will want but I hate building things twice.

Claires Google drawing thing on anthor post in here is good fun if you are knocking up a drawing to show Officials.

Good luck.

[Clair]

[quote user="BIG MAC"]Claires Google drawing thing on anthor post in here is good fun if you are knocking up a drawing to show Officials.[/quote]

http://www.completefrance.com/cs/forums/1260405/ShowPost.aspx

[David_Janet]

WOW! What can I say!

Thank you everyone, for such informative, useful - even entertaining - replies. [:D]

A lot of stuff to digest, but I think it's pretty obvious that 'proper' foundations are the way to go.

I do enjoy this forum.....

Thanks again

David

 

[lostinfrance]

[quote user="David_Janet"]

WOW! What can I say!

Thank you everyone, for such informative, useful - even entertaining - replies. [:D]

A lot of stuff to digest, but I think it's pretty obvious that 'proper' foundations are the way to go.

I do enjoy this forum.....

Thanks again

David

 

[/quote]

Please do come back in time and tell us wich route you took.[8-)]

[BIG MAC]

That's a good Idea what about a forum or thread broken into Building (roofs, fenestration, insulation, masonry, carpentry, groundworks, drainage, decorating etc) and M&E (Wiring,plumbing heating, ventilation, lighting etc) Where we see actual solutions employed and approved ie. someone has gone out and done something with the approval of their Mayor lets record the precedent as likely very handy for others?...so not questions pseudo answers and waffle (Boy are we good at that) the place for that would be in the forums as per usual. The new section would be what happened after.....simply this was the problem....this is what we did.

Any takers and can it be done?

[Chancer]

Regarding approval of the Mayor (sic), well given the lack of any building control in France you may as well seek his rather than the approval of the almighty.

I once naively asked my Maire if a proposed method of construction would be acceptable, his reply was (allowing for my translation) "my dear fellow you can build your property out of cereal packets for all I or anyone else cares so long as it resembles what is on the permis de construire".

Judging by some of the "baracques" in my village the above is long established custom and practice.

My own property (re)built just after WW1 using reclaimed bombardment bricks (most people would call them hardcore) and river mud mixed with chaux cement in proportions so weak as to be non existant, has no foundations and no structural cracks.

There has for sure been movement over the years where the roof loading has pushed out the retaining walls (the original slates having been replaced by heavier tiles) but critically the "cracks" were manifested as the mortar joints opening between the bricks as can be seen by the irregular localised repointing, no bricks had cracked.

When I have created openings it has been the devils job to break through the solid brick walls but once the weight above had been removed the bricks can simply be lifted out by hand, the chimney I recently removed  was exactly the same after the concrete capping had been taken off.

And just to add the icing on the cake the terre battu cellar floor has been lowered undermining the bottom row of bricks which are not even stepped out to form a foundation.

The loadbearing sub-soil in my area is 100% chalk, the only houses that I see with cracks are those built from parpaing or newer brick buildings, both using cement mortar.

[Gluestick]

Interestingly, JR, I was reading recently some construction details concerning the Chunnel.

Apparently, the native chalk strata which was bored, is quite capable of withstanding the immense crust mass above: as well as the water pressure loading.

All that was needed was a coating of Shotcrete. The concrete liners were used purely for cosmetic purposes and to aid periodic cleaning.

The vast cellars in the Champagne, of course, were simply dug out of the native chalk by the Romans originally.

They still seem to be OK.......................

 

 

[lostinfrance]

[quote user="Gluestick"]

Interestingly, JR, I was reading recently some construction details concerning the Chunnel.

Apparently, the native chalk strata which was bored, is quite capable of withstanding the immense crust mass above: as well as the water pressure loading.

All that was needed was a coating of Shotcrete. The concrete liners were used purely for cosmetic purposes and to aid periodic cleaning.

The vast cellars in the Champagne, of course, were simply dug out of the native chalk by the Romans originally.

They still seem to be OK.......................

 

 

[/quote]

I think there is a little difference to the wine cellers and the( chunnel. )[:'(] N`est ce pas ?

[wiskersnatch]

First off it is unwise to build anything without proper foundations.  If you don’t know for sure what it there you must assume nothing. 

The depth and type of foundation will depend mainly on the proposed loading and the ground conditions.  A strip foundation for example for a garage may be a certain size but useless if not buried deep enough to avoid frost heave (in a clay area).  The permutations are vast but some general principles apply always. 

If the ground is very soft you must dig until you reach firm ground and if that is over a 2 m you will need to think about piles or a raft.  As you will know there are various types of foundation. 

The most common on domestic dwellings is the ‘Strip’ foundation. For a house, in firm ground with no clay,  a convenient way to calculate the size is to take the wall thickness and measure out and down from each side at a 45 degree angle.  This will automatically give you an approximate size.  (As the loading bears down at 45 degrees).   The top of the foundation should be a minimum of 600mm from GL (in firm ground).  If you have clay and frost the distance must be NLT 900mm to avoid frost heave.  The concrete should be C30 as a minimum (just ask for 30 Newton concrete).  Lots of builders use C20 but frankly the cost difference is so small it is better to go with 30.  By the way, I have used concrete in various places and have to say French concrete is as good as I have ever come across. 

As a minimum your foundation should run 150mm each side of the wall (so a 250mm wall would have a foundation 550mm wide, round up to 600mm).  This is enough to support a domestic dwelling of 2 stories.  It will certainly be enough for a Garage.

 A raft foundation is basically a big slab, most of the slab should be the thickness of about 150mm to 200mm (depends on loads) and the edges, which shall bear the major loads should be deeper (imagine a waffle inverted).  The problem you may have is that the base, although solid is unlikely to be reinforced and a raft needs to be reinforced.  It is not a good idea to treat any slab as a raft because a raft is designed to float and has specific areas designated for heavy loads (most often its perimeter but also other areas sometimes). 

Your slab is likely to be a floor and not designed for huge point loads i.e. walls supporting a roof.  It may well support cars and tractors but when you introduce walls and roofs and wind loads and snow loads etc you are in another world altogether.   If you introduce new heavy loads to the edges you are likely to crack it, unless it is extremely thick or reinforced or both.

Typically Rafts are used in areas where ground conditions vary, so if there is any subsidence the whole structure moves, sinks etc and does not crack. Rafts more often than not do not have piles, they are meant to float (that is a relative term)  when they do have piles they are not, strictly speaking, raft foundations. They are piled foundations.

Remember,  concrete is most excellent in compression but less than useless in tension and varying ground conditions gives areas of tension and compression.  Any point you have the ground pushing up and weight (walls) pushing down will be a point of tension.  Reinforcing (metal) is great in tension, which is why it is used.

Of course a raft may be the same thickness all over but that is a waste of concrete.

Piles are another form of foundation.  These can be used in extreme soil conditions such as marsh ground.  Each pile or set of piles (they are often grouped) has a Reinforced concrete pile cap and these can support reinforced beams or a reinforced raft (Not really a raft as it can work without even touching the ground, a raft is deisgned to float). 

Piles are costly and not often used on domestic dwellings. Piles can be driven (a big hammer) or poured.  If poured a hole is bored with an auger, a steel reinforcing frame usually dropped in (not always) and concrete poured in and vibrated.  Piles can be driven till they meet hard ground or can, where hard ground is not available at a reasonable depth, can rely on friction, even in very soft ground. After driving or pouring a pile should be loaded with a known load and its movement can be measured.  There are tables for the movement allowed, you just need to know the imposed and superimposed loads, the safety factors are built into the tables.  Needles to say piling is an expert job.  I mention it here as you may find it of interest, I don't suggest you pile.

I hope that even if you don’t find this useful you find it interesting.  It is a vast area and many people tend not to give it due consideration.  My guess is that this is because foundations are hidden.

But back to your question.  I have a suggestion which may be helpful. Instead of using the slab as a foundation why not dig around it and drop in a strip.  I would recommend also installing about 4 x 8mm reinforcing bars.  In a row, evenly spaced. These need to be covered in concrete so use spacers to support them before you pour the concrete.  (Spacer you can buy at any builders' merchant).   The cost is low and the added strength is huge. For real 'belt and braces' install bars in the bottom and the top (50mm cover).  If you do this you will be sure to have a very strong base to build upon. Also, try digging down beside the slab to see its depth.

Did you know that concrete gains 50% of its strength in the first 7 days and continues to get stronger for about 50 years.  There after it losses strength very, very slowly.  When poured in huge mass concrete does ot go off quickly.  Apparently inside the Hover Dam there are areas where the concrete is still soft!

 

Regards

 

CY