Rear extension - party wall

callie16v

Hello all,
I am planning on doing a single story rear extension soon. I have been told by several builders that it is possible to build a 9' wall on the flat on the boundary line between ourselves and nextdoor (half on each side), so that both ourselves and nextdoor can use it as a structural wall. My neighbour dosen't like the idea and thinks that you would need to build 2 independant walls if we both decided to build an extension.
Can anyone provide some assistance on this please?

sinnerboy

your builder is right . however if the neighbour does not agree and cant be persuaded - then best to entertain him . a 100/100/100 block / cavity/block wall is ok too .

Mellor

It is possible, this is what is inbetween the two houses (most likely)

But this is only if you are both extending at the same time, if your neighbour is planning on waiting then a regular wall will be needed (expect in some cases where it will not affect insulation)

callie16v

Thanks for the advice,
the neighbour has no plans to extend at the moment, but i was thinking that the wall would be there for him to use if he decided to extend in the future? by 'regular wall' do you mean a 9' solid on the flat?

Mellor wrote: »

It is possible, this is what is inbetween the two houses (most likely)

But this is only if you are both extending at the same time, if your neighbour is planning on waiting then a regular wall will be needed (expect in some cases where it will not affect insulation)

smashey

I think he means a cavity wall if it's going to be an external wall and your neighbour isn't going to use it yet.

Mellor

Exactly smashey,
If you were both builder together then a 215mm wall would work, and maximise floor area. This would be an uninsulated wall as the delta temp across the wall is zero.

As the neighbour is not building yet you have to insulate the wall, you also have to consider the roof. I'm not usre how you propose to do this while still keeping the 215 wall availble for the neighbour. Its most likely going to be a 300 cavity wall inside the boundary

sinnerboy

agree . if neighbour is not building for a time build an insulated cavity wall . but look to have it located on the centreline / boundary line between you . get the most foot square you can

ircoha

Just to add to good points already given

To allow the OP build the wall up to the line, but not on the line the neighbour will need to agree as even if it is a parapet wall ( no overhanging facias etc) some of the foundations will need to be in the neighbours garden.

As noted earlier the best solution is to build a parapet wall with his consent and have the middle of the cavity on the boundary line and then both of you are free to do what you want.
Needless to say nothing can vent through this wall or overhang

callie16v

Thanks All,
Was talking to the builder last night, we are going to go with an insulated cavity wall built up to the boundary. is it a good idea to get an engineer involved to make sure everything is built to spec?

muffler

callie16v wrote: »

Thanks All,
Was talking to the builder last night, we are going to go with an insulated cavity wall built up to the boundary. is it a good idea to get an engineer involved to make sure everything is built to spec?

Yes - highly recommended. He can ensure that everything is done properly and issue you with the relevant certification at the end of the project which you will need if you have a mortgage or should you ever decide to sell at a later date.

callie16v

Final Question!
I have an extended utility room with w/c with access from the kitchen. the utility room is being knocked to make way for the new extension, the w/c is remaining as is.
The question is: is it ok to use the existing floor for the utility & w/c (take down existing walls) and pour the remainder of the slab for the extension around it or is it better to take up the floor in the utility and pour one complete slab for the new extension as one slab?
howe this in not too confusing!

callie16v

The builder has suggested building up the new walls, putting on the roof and then taking out the existing utility floor and walls, then pour the new floor as one slab.
is this a better idea?

sydthebeat

callie16v wrote: »

The builder has suggested building up the new walls, putting on the roof and then taking out the existing utility floor and walls, then pour the new floor as one slab.
is this a better idea?

no,

take out the floors now and build the rising walls first, then new floor then continue with the blockwork.
This way you can be ensured of proper damp proofing as your DPM and DPC will overlap.
His way doesnt allow for that..... although it does allow him to work with a roof over his head.

callie16v

Thanks for all the good advice. we have decided to rip out the existing floor and pour a new slab as one, then the blockwork.

sydthebeat wrote: »

no,

take out the floors now and build the rising walls first, then new floor then continue with the blockwork.
This way you can be ensured of proper damp proofing as your DPM and DPC will overlap.
His way doesnt allow for that..... although it does allow him to work with a roof over his head.

muffler

callie16v wrote: »

Thanks for all the good advice. we have decided to rip out the existing floor and pour a new slab as one, then the blockwork.

As advised by syd that is the correct approach.

td18

Mellor wrote: »

Exactly smashey,
If you were both builder together then a 215mm wall would work, and maximise floor area. This would be an uninsulated wall as the delta temp across the wall is zero.

As the neighbour is not building yet you have to insulate the wall, you also have to consider the roof. I'm not usre how you propose to do this while still keeping the 215 wall availble for the neighbour. Its most likely going to be a 300 cavity wall inside the boundary

Could you not build a 215mm wall and insulate internally with drylining ?

Even though this is not a party wall yet, if your neighbour extends in the future and it becomes a party wall i thought it has to be 215mm thick

Mellor

Strange thread to dig up but i'll bite,

You can't build a 215 wall and insulate internally unless the whole thing is insulated internally (which may not be an option depending on the construction of the existing house)


As for the party wall being 215, yes IF it is a party wall it is 215, but unless the neighbour is building an extension now then it can't (almost always) be a party wall and must be a 300 wall inside the boundary

callie16v

Mellor wrote: »

Strange thread to dig up but i'll bite,

You can't build a 215 wall and insulate internally unless the whole thing is insulated internally (which may not be an option depending on the construction of the existing house)


As for the party wall being 215, yes IF it is a party wall it is 215, but unless the neighbour is building an extension now then it can't (almost always) be a party wall and must be a 300 wall inside the boundary

Bit late now the extension is built;)
We went with a 215 cavity block parapet wall inside the boundary, insulating this with 70mm boards internally.
What do you mean by "whole thing"

sydthebeat

callie16v wrote: »

Bit late now the extension is built;)
We went with a 215 cavity block parapet wall inside the boundary, insulating this with 70mm boards internally.
What do you mean by "whole thing"

is the whole construction 215 and insulated internally???

if not, the junction between your inner leaf and the 215 'outer wall' is one big huge cold bridge and is considered bad construction.

callie16v

sydthebeat wrote: »

is the whole construction 215 and insulated internally???

if not, the junction between your inner leaf and the 215 'outer wall' is one big huge cold bridge and is considered bad construction.

Not sure if i completly understand, but yes, the whole construction is 215, all of the external walls and the ceiling are insulated.

sydthebeat

then there shouldnt be an issue... its not great consturction but it is acceptable by homebond.... its a method of construction very indigenous to the dublin region.....

Slig

can you tell me why its not a good construction method?

sydthebeat

the condensation point 'can' occur within the building..... leading to a build up of mound fungus etc...

have a read of this thread..
http://www.constructireland.ie/vb/showthread.php?p=2016