Extension : floor level 1 foot below ground level

[Ady J]

hi all


1 main question -does this plan seem sound below :

New project

Single storey 7mtrs x 3mtrs

*** inside floor level 300mm below outside ground level

Started groundwork for extension -actually classed as outbuilding ( but will be connected with small conservatory )

Raft extension
Cavity , block inner, brick outer

The actual finished internal floor level will be approx 300mm below the outer ground level & im a tad worried about damp etc

I’m hoping this will do the trick:

Damp proof courses as normal
-below inner floor screen/floor insulation, tucking in under both courses of bricks all around


Also plan :

Black engineer bricks coming up to & few above outer ground level

+ vertical damp proof course tucking under 1st bricks (within cavity ) going up to few above outer ground level

Will also fit x2 airbricks to outer brick ( insulation in cavity attached to inner block )

Will also put DPC plastic sheet against outside of building up to level of outer ground (again the inner floor is 300mm below this )


So I’m my mind it’s all protected + be ted

Is there anything I’m missing ?

Sone seem to put plastic sheet DPC under whole foundation I do not intend to do this ( as above mine will be just below the floor screen/floor insulation, vertical sheet also as explained above )

Any confirmation for those with more experience appreciated

Regards
Ady

Ps restricted to 2.5mtrs high & I want high ceiling inside

 

[^woody^]

You need a horizontal dpc to stop moisture coming up, a vertical dpc to stop moisture coming across, and a cavity tray above ground level to stop moisture coming down. They all need to over lap, as does the DPM. Plus enough insulation with no voids, to prevent condensation where the ground backs the wall.

I don't know what your airbricks are supposed to be doing, but you should not be ventilating a masonry wall cavity.

 

[Ady J]

Woody big big thanks for replying
Vertical & horizontal overlapping
-have that in plan

Going to lookup what this cavity tray thing is

Your very helpful massive thankyou

 

[Ady J]

You need a horizontal dpc to stop moisture coming up, a vertical dpc to stop moisture coming across, and a cavity tray above ground level to stop moisture coming down. They all need to over lap, as does the DPM. Plus enough insulation with no voids, to prevent condensation where the ground backs the wall.

I don't know what your airbricks are supposed to be doing, but you should not be ventilating a masonry wall cavity.

Hi

Just looked these up
Do I need these -I’ve not really seen these used anywhere

If so is it around the whole of the outhouse/extension? & just above the ground level ( its tad different each of the 4 sides of the building)

The inner block work will have insulation attached to it , these cavity trays press up to the surface of that ??

Also are cavity wall trays really just for above openings/lintels ?
( exterior windows & doors )

Sorry for all the questions

Help appreciated

 

[^woody^]

You would form a cavity tray with a roll of dpc. The idea would be to stop water running down the cavity and soaking the insulation.

Hasn't this been designed by whoever drew the plans?

There are a couple of other ways of dealing with this, but would involve draining the cavity, and this would depend on other factors.

I'm certain that there has been a detail drawing posted in here or the regulation forum reg regarding this. Maybe by Freddy Mercury's Twin, but not sure about who.

 

[Ady J]

I’m with you now I think

Run another DPC sheet down from Top pressing against Insulation
(That’s atyached to inner block)
Then tuck it under outer bricks at low level -but few bricks above actual ground level
& obviously put those little plastic vents every couple of bricks to drain anything out

Both DPC sheets meet
-the internal on running under inner floor screen
-the vertical one running down tucked under outer bricks (as discussed)

So internal room is completely shielded from potential damp including the inner block


I’m loweting earth so 2 sides of outer ground level will be about same as inner floor level
So it’s only the other 2 sides I’m concerned with

Ps it’s pretty decent site -water table must be very deep & the single story new build is fairly shielded by surrounding Victorian houses (this is going up in my back garden )

 

[Ady J]

You would form a cavity tray with a roll of dpc. The idea would be to stop water running down the cavity and soaking the insulation.

Hasn't this been designed by whoever drew the plans?

There are a couple of other ways of dealing with this, but would involve draining the cavity, and this would depend on other factors.

I'm certain that there has been a detail drawing posted in here or the regulation forum reg regarding this. Maybe by Freddy Mercury's Twin, but not sure about who.

No one drew the plans
Doing this under permitted development , it’s one room 7 meters by 3 meters for projector/sound room

Also there’s bi-fold doors down one side ( about 5 meters ) the building will be heavier on the other side ?
Am I overthinking this ?

 

[Footsoldier888]

No expert here but it is best to think in terms of structural stability than weight. Glass and steels are heavy too!

 

[^woody^]

No one drew the plans
Doing this under permitted development

But there is still building regulations. Even if you are doing this on a notice, the inspection of the dpc and dpm is a statutory inspection so you need to get your approval agreed beforehand.

 

[Ady J]

This is going to be really great

But there is still building regulations. Even if you are doing this on a notice, the inspection of the dpc and dpm is a statutory inspection so you need to get your approval agreed beforehand.

I rang the council & they tried to sell me this ( about £600 )
But I established it wasn’t legally required , especially for a building within permitted development rights

I do want to do the very best & most through job though to avoid any probs in future obviously

I really don’t feel I need to pay someone £600 to show up to look at something then know their head then buzz off again

A complete & thorough seal (overlapped & sealed at any joins) below floor screen & down inner cavity meeting & tucked between outer bricks with those plastic weep vents seems a good way to go ( vertical one also a acting as a continuous cavity tray )

Still obviously checking/reading up etc

“Doing on a notice” -not sure what this means no one’s gave me notice for anything , this is being done under & with criteria of permitted development rights

 

[garyo]

Permitted development only relates to planning permission. Building regs are a separate issue. However for a garden office / conservatory you're exempt from building regs if the floor area is < 30m^2

Just watch out that your 'connecting' conservatory structure will probably be included in that area.

 

[Ady J]

Permitted development only relates to planning permission. Building regs are a separate issue. However for a garden office / conservatory you're exempt from building regs if the floor area is < 30m^2

Just watch out that your 'connecting' conservatory structure will probably be included in that area.

Gary yes your right
-this is just under the 30^2

The small conservatory between will fall short by 1” so it will not connect except for an industrial compressed rubber strip that can be easily removed , it’s really a weather guard
The doors will be secure/external from house into conservatory & conservatory into extension/outhouse , good plan I think

Preparation on groundwork is making me strong too!
3 hrs a day with tad more at weekends aiming for foundations to be complete next springtime

 

[Footsoldier888]

It sounds good. I get your plan to avoid unneccessary expense but what you are planning is not a garage type structure but a fully formed building. I guess a lot of it boils down to what sort of ground conditions you have and what the risk level is.

If you ever want to sell it will affect the saleable value of the house if you have this outbuilding done to a good spec but no completion certificate.

For example I knew this guy who built an orangery without plans and building control but he then sold a couple of years later at which point the buyer requested the completion certificate. BC turned up to check it and said he had inadequate foundations and he had to underpin it costing a few thousand. The point is if you don't get the certificate you are always at risk of falling foul of the regs if you come to sell.

If you are lowering earth might you need a retaining wall say??? You need to be careful about this stuff if it is near a boundary.

 

[Ady J]

It sounds good. I get your plan to avoid unneccessary expense but what you are planning is not a garage type structure but a fully formed building. I guess a lot of it boils down to what sort of ground conditions you have and what the risk level is.

If you ever want to sell it will affect the saleable value of the house if you have this outbuilding done to a good spec but no completion certificate.

For example I knew this guy who built an orangery without plans and building control but he then sold a couple of years later at which point the buyer requested the completion certificate. BC turned up to check it and said he had inadequate foundations and he had to underpin it costing a few thousand. The point is if you don't get the certificate you are always at risk of falling foul of the regs if you come to sell.

If you are lowering earth might you need a retaining wall say??? You need to be careful about this stuff if it is near a boundary.

Cheers for good points
I heard this about resale etc
If this ever happens I’d be happy to dig down to show the foundation level + pics of build stages , although this is not to profit it’s for personal use

The building will look good
Reclaimed Victorian bricks (to match)
& 4mtr bi-fold doors along 1 side
High spec inside

1 side of the building is replacing the wall between me & neigbour
Height wise it only adds about 3 bricks in height extra (compared to current wall ) they are totally fine with this , everything completely sealed up to few bricks above the relevant outer ground level as I’m paranoid about damp

How high above internal level does outside difference need to be to cause the need for a retaining wall? ( retaining section to the sides of new building )
I guess this would have to be done as part of foundation using wooden formes with steel reinforcement ?

The outer ground level is different on each side ( doh! ) so base foundation with steel bar sticking out to required lengths then forme (wood box) work on each side to required height & separate concrete pour-in for each side
300mm wide is total thickness of each side
( 100mm brick , 100mm cavity , 100mm block ) bricks go top of this & will start above ground level

Am I thinking along correct lines :

To explain better
Foundation as normal EXCEPT
Iron bar columns sticking up to correct length for each side within a 300mm width all round

After a week or so
Make wooden long rectangle box 300mm wide to relevant height for base each of the 4 sides
(Rising just above outer earth level ) with iron bar at corners sticking out of box to enter new box for joining side to tie all together
& repeat each week for each side to relevant level

 

[freddiemercurystwin]

Sounds like it'll be a complete pile of old crap to me.

pilsbury that's not you again is it?