Planning the build
Why build an extension?
Initial design sketches
On-line Building Regs
Meeting planners
 Meeting building control
 Planning the foundations
 Planning the floor slab
 Marking out the footings
 Damp proof course issues
 Possible drainage problems?

 Construction of the walls
 Proposed roof structure
Windows diary
 Window challenges
 Initial window plans
 Making the casements
 Making the frame
 Designing leaded lights
 Making leaded lights
 Assembling the window
 The little window

 The door frame
Demolition diary
 The demolition starts!
 The demolition continues
 Still more demolition

Foundations diary
 Foundation issues
 Drain issues
 Digging the foundations
 Rethinking foundations!
 Pouring the foundations

Floor slab diary
 Rethinking the slab!
 Floor vent extensions
 Preparing the slab #1
 Preparing the slab #2
 Pouring the slab

Walls diary
 Preparing to build walls
 Build up to the DPC
 Build the walls #1
 Build the walls #2
 The gas men cometh
 Build the walls #3
 Finishing the walls
Roof diary
 Roof structure build
 Tiling the roof
 Finishing the roof
 Finishing the gables
 Roof insulation

 Floor screed
 Shower room
 Shower room floors
 Shower room walls
 Utility room
 Utility room walls
 Utility room floor
 Finishing the windows

Project finished!
 Final inspection
 The certificate
 Final thoughts

 Twelve months on!
 Material costs
Sister sites
 House re-roof project
 Home gas usage data

 Stop paint flaking
 DIY secondary glazing


Planning the floor slab

I imagine that the building the floor is one of the most complicated and one of the hardest jobs to be undertaken in the construction of the extension as there are so many layers to the job!

The drawing below shows what has to be laid down and in what order.

All the layers that form the floor

From the bottom up...

  • The hardcore base that will need to be consolidated with a vibrator and be at least 100mm thick.

  • A thin layer of sand to protect the damp proof membrane from being damaged

  • The damp proof membrane usually 1200g polythene (shown thicker here so that it can be seen)

  • The 80mm thick phenolic or polyurethane insulation layer

  • The 100mm concrete slab that might need to be reinforced if the floor is large

  • The final screed to provide a flat finish

  • The final floor - tiles for me.

The damp proof membrane shown in red

The above diagram shows the damp proof membrane in red (and thicker than it is in real life) and shows how it folds over the DPC material in the wall to form a complete watertight seal.

The thicknesses of the individual layers

The above diagram shows the thicknesses of the individual layers which might vary around the country so it's worth checking with your local planning control officer.

Note: The above theory is all wrong following a talk with the Building Control Surveyor - see here

You will see that during the build I found out that the slab needs to be keyed into the existing house and the slab rests on top of the inside block wall.

Source: Government. Planning Portal

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