getting levels right

[climber101]

Hi I have been sketching out a cross section of my planned timber frame single storey extension.

The attached “ext2” shows the overall idea and “ext1” a closer detail on the floor joist to existing wall.

I would be much obliged for anything you see wrong on either sketch please plus answers on the following please

1) referring to “ext2” the new floor joists I am planning are deeper than my existing floor joists (145mm as opposed 117mm). This means under joist point AA (rhs of drawing) I am going to have to put in a fair thick soleplate - approx 67mm say with a 5mm mortar bed - this seems like a very thick soleplate to me so I’m wondering is that OK or another way

2) referring to ”ext2” I’m thinking concrete slab of 100mm – I have read you can get away with 50mm. Is a slab of 100m OK?

3) referring to “ext 2” I’m thinking about 300mm of hardcore under the slab- OK?

4) referring to “ext1” the top of my intended wall plate on which the joist sits will be near enough on the level of existing DPC. Im thinking I will wrap it in DPC material whether I need to or not but my concern is I will be blocking off the air vent bricks with this wall plate which lie just under the DPC… what to do??

5) the concrete strip founds specified are 600mm x 150mm. this 150mm sounds a bit too thin for my liking – can anyone tell me how common 150mm is as I’m tempted to go up the height of additional block e.g. 150+225 = 375mm at least

Thanks again

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[noseall]

why have you referred to the measurements in fractions of a metre and not in mm?

 

[climber101]

Hi noseall thanks for looking the package i used for the sketches (Googlesketchup) displays sizes in metres e.g. 145mm appears as 0.145m or 2mm appears as 0.002m! (It was on a thread here that i saw someone saying Googlesketchup was good and at first i didnt think it was very good until i started working out how to use it now i think it's excellent)

 

[^woody^]

mm units can be set from the "model info" dialogue box from the "Window" tab

 

[Shytalkz]

Another Sketchup convert - I think it's a fantastic bit of software, all for free; and the Pro version is only £240 anyhow. I often use it to bang out scrap details for calcs in 3d, looks wizzy, as well as imparting the info and saving someone having to trawl through the screed of smoke and mirrors that makes up the output of an SE .

As Woodster says, you can easily set the default units - and precision too.

 

[climber101]

thanks noseall, woody, shytalkz, i'll try using that mm units option ! - (i like the fact you can dimension things really quickly in sketchup - it's also a lot warmer planning it on sketchup than starting work outside at the moment )

any thoughts on my queries please as i know you'll know !!

 

[noseall]

typical single story foundations are 225mm (minimum) thick and laid at 1m depth.

the rest is a blur i'm sorry.

 

[climber101]

thanks noseall on the subject of founds i dug a test hole last year at edge of house a day before i went on holiday (attached here) to see how far to the top of my founds (some 5 block down). I then stupidly ran out of time and filled it back in before i could dig at the earth at side of found to see how deep it was.. my extension founds will be like a big u shape starting at this wall. I'm guessing i have to take a grinder to these existing founds cut them flush to the wall? and then new founds as deep as i want up to the top of these existing founds at least - whilst i do so i'm concerned that i'll be temporarirly undermining a tiny section at the edge of existing founds if i plan to go slightly deeper with my extension founds than existing founds.?

PS i happened to see a certain photo of some rather deep founds on here that you were doing - impressive !


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[^woody^]

Climber, the lines in your drawings are not straight. If you don't follow the horizontal and vertical axis in sketchup, then you may introduce inaccuracies, and things wont work out how that should.

Also, this is building and not engineering, so don't plan on working to mm accuracy such as 67mm for timber and 5mm for a mortar bed. It wont happen.

Put the joists on hangers on the existing house, and this will remove the issue of a timber blocking the vents. Or bang out some bricks to form new vents below any fitted wall bearer plate

There is no point in building up the oversite below a timber floor. Just dig out what you need to, lay 100mm H/c and then 100mm concrete. Don't build it up to the level in your drawing.

A 150mm thick strip is deep enough for a centrally placed 300mm cavity wall. If your wall is 275mm, then the concrete will need to be thicker to deal with the 45 degree shear angle. In practice, the trench would probably be mass filled to 225 below external ground level

With the existing footing, generally its easier to leave then in and just dig your new trench under the overhanging bit and this is infilled with your new footing. There is no need to break the concrete off

 

[climber101]

Many thanks woody! much appreciated – you and everyone here that’s helped me are stars, that’s a lot answered .

I had been thinking about mass filling to get out of the ground quicker so it is reassuring to hear you say in practice this might be done. I’m thinking to mass fill it would be a found depth of 225mm at least (or more if I find my existing founds are deeper) Plus probably only 2 block heights = 450+225 = 675mm (to keep to 225mm below ground at one edge of the extension where the ground level is lower)

[Total found volume would be approx 13.4m length x 0.6m width x 0.675m depth = 5.427m^3 = 13 and a bit tonnes – two lorries at least? not sure would need to check]

I’m rambling on – two questions I meant to ask please is a) when you said in practice mass filling might be done is this common for extensions to reduce the blockwork and speed the build up a bit? and b) any thoughts for the floor joists how often timber wall plates are used versus hangers – I fancy going for hangers but am trying to work out pros and cons

(PS see what you mean about my lines being squint thanks - have double checked they are ok but my mistake was I should have used the orbit tool for a square on view before taking a snapshot.. )

 

[^woody^]

Mass fill concrete

Quicker, less labour, easier. Poured right in or barrowed.

Traditional footings

Slower to build, labour intensive, awkward to build, harder to set out for the novice, still require a concrete strip at bottom

There may be a slight cost saving for traditional footings per metre run if you are DIYing and time and trouble are not an issue.

If you are set on traditional footings, and DIYing, then I'd recommend using a Celcon trench-block as opposed to cavity walls.

A typical new extension footing will be 1000mm deep. Of which, at least 775mm of concrete, then 225mm of wall - 3 course of engineers or 1 course of dense concrete block. If you have shallower footings adjust the concrete. If the ground level slopes then the brick/block work caters for this. If the ground is level, then you could increase the concrete as it wont be seen.

But in all cases, work down from DPC level so that the wall works courses of brick, or block and brick to the top of the concrete.

ie if on your plans, you spec a 150mm strip, then in practice you may lay more than 150mm so that it works courses when you come to build, and then you dont have to mess about with the guage.

If you bolt a bearer timber to the existing wall, then you will need to insert a DPC behind it as it will be below the house DPC level. If you use hangers then (depending on their design) you may not have to.

A bearer plate will be easier for you to get the joists level

Hangers may not work courses, and so may be more work to chop them in.

So it depends on what works easiest. Either method will do, but consider how much work is involved with each method.

Concrete is ordered in m3 and a big wagon will carry 6m3. You will get a full load cheaper per m3 than a part load, or two part loads

 

[climber101]

Thanks again woody - much indebted !! Thinking about your comments I have made up a sketch showing cross section through an end wall of extension and I’m thinking I could either go either 0.9m deep mass fill concrete or 1.125m depending on whether wanted 2 or 1 block course before brick courses. ( If I went for 1.125m that keeps me 1 block plus approx 50mm below lowest ground level in line with your comment go to 225mm)

(The line between the 675mm and 225mm dimension in attached doesn’t exist in real life and for simplicity I am only showing the outer wall and not the inner timber frame wall or flooring )

(I reckon 1.125m will equate to about 9m^3 of concrete). What I now wonder about in terms of trench filling is whether 1.125 is too deep and needs reinforcing or some other consideration or whether that is perfectly within a normal range for mass filling? and whether to go for 0.9m deep or 1.125..

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[^woody^]

There would be no problem with mass filling, and at those depths, I would suspect that whoever does this will want to fill with as much concrete as possible