Conventional Raft Slab

A ‘Conventional’ Raft Slab is a concrete base laid directly on a compacted base.

It is strengthened by cutting trenches in the base and adding reinforcement.

These ‘thickened beams’ are formed at the edges and also under loadbearing walls.

This photo shows the base covered in poly , reinforcement placed and edge forms in position ready for placing of concrete.

Advantages

  • Higher thermal mass, as it incorporates the thermal mass of ground  so better suited to passive solar design.
  • Less susceptible to bad workmanship by concretors
  • More resistant to point loads, such as jacking a car.
  • A thicker overall slab makes it easier if you want Floor Drains (To allow for the dropped floor in those rooms)

Disadvantages

  • Generally uses more concrete, than ‘Waffle Pod Slabs‘, with more waste as volume is less predictable
  • Rain can cause construction delays as the trenches for the beams can fill with water
  • More complicated excavation can lead to additional cost

 

Also see Ground Conditions

 

Drop Edge Beams

If you want to build your new home on a slope you may need to have the additional cost of needing drop edge beams on the low side of the slabs.

These are a sort of retaining wall to hold the fill under the house slab as shown in the drawing below.

Here is a photo of a typical drop edge beam before the brickwork has been started.

Costs (2013) are in the order of $500-$700/sqm of beam, plus the cost of the fill under the slab.

Say. . . $12,000 to $18,000 for a 20m long house with a 1.5m drop beam so a significant extra!

The photo below shows the same wall after the brickwork has been completed.


If you are building on a slope you may also want to know about Retaining Wall Fairness

Thanks to Grumblebum54 for the photos

Slab Insulation

I have previously posted about the relatively small heat loss from a slab on ground
But what if you have got in slab heating, or just want to minimise heat loss/gain from your house?

Before Construction

This sketch shows the placement of the insulation, if you can arrange for the builder to install it before construction.

The way this is installed is the insulation foam is installed inside the slab formwork.

A 40mm foam board with an R value of 1.0 will typically reduce the heat loss from the slab by 50%.

If you have a small builder or are having a custom home built this should be possible……some project builders however will probably be unwilling to do this installation.

After Construction

If you want to insulate after construction this detail is as effective as the previous method.

It works by using the soil as insulation.

Although soil is not a great insulator by stopping the heat escaping upwards 1m of  soil will provide a R value around 1.

 

See Insulation for similar Posts

For Posts about Green Building see Sustainability

 

Insulation – Heat Loss Slab On Ground

Why is there less fuss about insulation under a concrete raft slab than ceilings and walls?…………well here are a few interesting facts:

  1. A thick layer of earth provides a reasonable amount of insulation.
  2. The soil contributes to the thermal mass of the structure which helps smooth out any temperature variations.
  3. The temperature of the ground below the surface varies much less than the air temperature. For Victoria a ground temperature range in the order of 13 degrees in winter to 22 degrees in summer is typical.

As a consequence the main heat loss from the slab is only from the edges of the slab rather than from the middle.

Heat Loss Calculation

When estimating the heat loss a key factor is the ‘ Perimeter to Area Ratio’ (PAR). Examples are:

For a 10m x 10m slab the PAR = 40/100  = 0.25

For a 20m x 5m slab the PAR   = 50/100  = 0.5

The  table below provides some values of ‘U’ for the total structure for various values of the ‘PAR’ .

PAR

.2

.3

.4

.5

.6

.7

.8

.9

‘U’

.37

.49

.6

.7

.78

.86

.93

.99

So for a typical single storey house of 20m x 10m

The PAR = 60 / 200 = 0.3

From the table ‘U’ is  0.49

The Heat loss from the slab  =   Area x ‘U’   =   200 x 0.49   =   98 watts/degree C

The ‘U’ value for this standard slab is similar to an Insulated  Brick Veneer  Wall.

A Waffle Pod Slab will have a slightly better insulation value but have a lower thermal mass.

If you want to install slab edge insulation see Insulating Your Slab.

 

See Insulation for similar Posts

For Posts about Green Building see Sustainability

 

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