strip foundation query

chilledout

hi all,

building a single story extension onto back of my house, and want to build it the complete width of the house. boundry wall will be up against neighbours extension boundry wall. have exposed his strip foundation and it is 350 mm on my side of his wall, and 300 deep i presume ,have only excavated level to the top of it, also the ground is good and solid level to the top of his strip. Question is,
can i pour my foundation which will be 800 from his wall and 300 deep, on top of his existing foundation and build my cavity in the centre, this means that there will be 350mm of my strip sitting on his exposed 350mm and the remaining 450mm sitting on good ground or do i have to keep away from his strip altogether to avoid settlement problems down the line. or is there another solution? dont want to lose any ground if at all possible

cheers in advance:cool:

godtabh

wont there be en eccentric load on his foundation then? Or am I picturing it wrong?

sprinkles

I'd avoid having the foundation straddling the top of another foundation and virgin ground tbh. As if is only a single storey extension there shouldn't be a huge load acting on the foundation. Personally I'd excavate down to the same level as the other foundation and pour your foundation up against it. Not having the cavity wall centered shouldn't be an issue so you can keep the external leaf up to 100 from the edge of your neighbours foundation.

chilledout

to answer your question godtabh i suppose there would be an eccentric load
on his strip, my foundation would effectively be resting half on his and half on virgin soil however compact the soil is. wasnt sure if i could get away with it ,but dont want to cause any problems, just wanted to maximize width of build.

sprinkles suggestion did cross my mind but the digger is gone back off hire and it means a lot more excavation. i think it seems that no matter what i do i have to keep off his strip altogether. should i put something between his strip and mine before i pour it, to stop adhesion and allow for movement/settlement?


cheers:cool:

dan_d

I'm having trouble picturing exactly what you're describing from what you've written....but personally I'd excavate down beside his foundation and pour a new strip foundation tight to the existing one. Use flexcel or something similar between the 2. And leave a cavity between his wall and yours, to allow the roof to overhang slightly on top.

sprinkles

You should keep the 2 separate - I've used compressible insulation before on some jobs but if you had any scrap timber or anything it would do. You're not talking a huge load here but the ground will settle and could lead to cracks in your neighbours wall.

chilledout

cheers for all replies

how does the same apply for gable wall i will be extending in 3rd picture as u can see i have removed path and will be bringing this wall out another 2.4 metres, do i pour new strip the same level as the existing one which is bout 300mm from wall , and presume its ok to start building on top of this first before i commence on new foundation?

heres some pictures to illustrate what ive been waffleing about

sprinkles

chilledout wrote: »

cheers for all replies

how does the same apply for gable wall i will be extending in 3rd picture as u can see i have removed path and will be bringing this wall out another 2.4 metres, do i pour new strip the same level as the existing one which is bout 300mm from wall , and presume its ok to start building on top of this first before i commence on new foundation?

heres some pictures to illustrate what ive been waffleing about

Is the neighbours wall part of an extension or just a garden wall?

If it's part of an extension I would be a lot more comfortable loading it by pouring your foundation on top of it. However I would still dig the earth out from beside the existing pad as this soil has not been subjected to the same loads as the soil beneath the pad and could settle a little - even a small amount will lead to cracks in your plaster/finish. Most of your load will be coming down on the inner leaf which will be straddling the virgin soil and the existing pad. Make sure you get your concrete down to the same level as the existing pad - then pour a 300mm thick pad on top of it.

TBH the most that will happen is cracks in the wall/plaster if there is an issue.

chilledout

as you suggest sprinkles, neighbours wall is gable of his kitchen extension as is seen in photo, i will be replacing my kitchen extension of the same size, with one the width of the house and another 2.4 metres into the garden. also what is the requirment for pouring strip perpendicular from back wall, at patio doors, and at the corner of extension on far side, is it sufficent to cover with a buttress? over the 300mm of exposed strip. thanks again for replies..

Soil Mechanic

Sorry, just for clarification...

the problem seems to be you are concerned about building on your neignbours extension strip foundation, which appears from the photos to be on your side of the houses boundary ownership dividing line, yes/no?

1. Just to be clear -In spite of the similarity of design for both houses its not- these kitchen constructions you've caled "extensions" are both non-original features, yes/no?

2. both property dwellings appear semi-detached, so not just who does the strip foundation belong too, but also which property is that strip foundation in question supporting:
- your neighbours dwelling;
- your neighbours extension;
- or in fact both dwellings?

3. Where exactly is the boundary division?


These need clarified.

Why? Because you are concerend about problems, but you must divide this into LEGAL and ENGINEERING settlement risk.

Well as your planning permission (?...) doesn't appear to have laid out in crystal clear terms what you can -and cannot- do, you are right to be slightly concerned.......
...but as been said here, don't stay up late worrying too much about it.

Unless your planning on installing some kind of industrial-weight kitchen equipment, any long-term settlement should be minimal.

What kind of soil have you exposed so far?

SM

chilledout

hi soil mechanic

the house is end of terrace and is 4 bed, next door is 3 bed. all houses in the estate have these half width kitchen extensions built onto the back, effectively its not an extension as such, as it was part of the original build. so i presume the strip foundation for the main house and the extension were all poured together, i know this because i was living here as they were still being built.
as you say my neighbours extension strip foundation is encroaching into my side by 350mm as im sure all the rest in the estate are, which is fine, as it was not an issue until i decided to start building.
to clarify point 2. the strip for the house and the extension, as it was poured together, is supporting house and back extension. boundry was probably not the right term to use earlier on in posts, basicly his extension gable wall divides both properties and a wooden fence further down the garden.

have no planning permission as my build is only about 20 sq metres.
wont be anything heavy going into room as it will only be a play room for the kids .
soil seems to be well compacted ,not sure on type ,looks like sandstone..

cheers
:cool:

Soil Mechanic

Hello Chilledout.
sorry for the late reply.

At this juncture I'd just to stress that what follows is in no way able to constitute a legal opinion, whether by myself, or by boards.ie or anypart thereoff.
(You never know these days....)

In terms of advice would also like to recommend in the strongest possible terms that you consult with a suitable Chartered professional (e..g architect or engineer -preferably an engineer, it'll be cheaper & they will be more experienced with what your talking about. ) and get an written opinion, it's worth it for peace of mind alone if someone else (& their indemnity ) signs off on whatever design you propose.

Here is part A of the Building Regulation down south, not sure just how much they differ from their BRE UK counterpart up here... but worth a read anyway.
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1639,en.pdf
This should provide you will all the answers to your requirements, however not the design for the eccentric loading.

From the excavated spoil in photo DSCN0882 it looks like you have some form of very Granular cobbley/gravelly soil -whether or not it's Fill/in-situ is not clear.
Whilst I would again advise that you get a professional written opinion to cover yourself, the risk you would be taking if there were problems down the line -whilst probably minimal- would leave you open to legal action.

To echo the points made here, the ground is good for the orignal extension loading, but you just don't know what could happen with the greater load imposed by your larger structure.

If it was me, AFTER consulting with an engineer and seeing what they have to say, Spinkles 7 dan_d method seems best, if you are concerned about the cost of plant hire, could you not use the excavator to aid the demolition of the existing extension, then deposit the debris in your garden or immediately skip it & then do the extra excavation work beside your 'neignbours' 350mm strip, before your foundation pour?

At least that way you would get your moneys worth out of the hire.

Do let us know how you get on.

SM

chilledout

hi soil mechanic
think ill get a engineer to take a look for peace of mind, have soil excavated beside neighbours strip down to his depth and about 600mm wide and ready to pour once i get the all clear.

thanks again for replies will keep ye posted:cool: