Marking out/planning foundations

[an203888]

Hey all,

I have hired a digger next week to dig my extension (39m2) foundations so I need to mark out - I will give my thoughts below but if anyone has any advice it would be appreciated.

1 - Going to secure short battons to existing wall where footings will be required and then add nails in the center and outside of where footings are needed.
2 - Will do the same the other end of wll but using some profile boards (two stakes with a cross member)
3 - I will ensure everything is square using the 3-4-5 method as well as running a line down the side of the house and measuring diagonals..
4 - I will then run a line across the nails and use a level to mark footings position and mark with spray.

5 - To get the right height for the bottom of the foundations I was going to create a datum inside the new extension at same height as current floor. From this I will work backwards with bricks/blocks/mortar/foundation depth so that I shouldnt have to cut any bricks/blocks.

Does this all sound about right? I will plan on using the same method to set out for the blockwork.

 

[noseall]

Does this all sound about right?

Mostly, yes.

Squareness can be thrown by and out-of-square existing building, so don't be fooled into adjusting your square extension to suit. Rather use a builders square (a foldaway 1200mm x 1200mm fella) to counter-check your own corners before adjusting the lines. Not all existing elevations are dead straight either.

We use tile batten rather than a tape. We make 'spray battens' that we use instead of a tape because they are more reliable over distances and on windy days. Especially useful when butting the batten against the exsiting house then using the marks on the batten to spray the lines for the parallel trench. They are useful across the site too. We will join battens for longer distances. You can mark the building sizes as well as the trench sizes on the batten. We deliberately spray white blobs on the batten to determine the trench outline.

Only spray short distances because the digger will churn up the line. Your spray batten should be kept handy for this purpose. Much better than setting up the line every time. Don't mix up your battens!

The trench bottom will be determined by soil conditions and a reasonable depth below ground. Your finished concrete level will be determined by the existing DPC and the distance to that DPC divided by the masonry units. I.e. 225mm or 75mm increments unless you are using reclaimed old imperial bricks above the soil line.

If the existing DPC has an unreasonably thick bed and/or the existing screed or FFL has sneaked up above the DPC, or if the distance from DPC to window height happens to gauge with ball breaking beds, then it doesn't hurt to finish your DPC slightly higher than existing.

Every little helps.

 

[an203888]

Great feedback thanks. I have a bundle of tile spline so will make use of what you suggest. Might also use some to make my own tempory square up. Thanks

 

[noseall]

Might also use some to make my own tempory square up.

We set up the lines in all directions the place the square on the ground parallel to one of the lines in the corner, then 'eyeball' it against the other line. Observing takes a bit of getting used to and a fair bit of squinting, but it is reliable....as long as it isn't windy! You can go a bit nuts (if you are OCD like me) adjusting and faffing, but it is worth the effort.