self build waterproof basement formwork membrane concrete rebar

Understanding steel reinforcement.

Generally speaking we ask that your engineer specifies 2 layers of A393 welded mesh in the slab and 2 in the walls as well.

If the slab needs to be larger to bear the weight of the structure above then it will have a heel and that might have trombone shape bars as well.
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The slab and the wall are connected, and the wall is prevented from being pushed over, by two rows of starter bars, 16mm or 12mm or one row of each depending on what he specifies.

The rebar should be clean before the concrete is poured. Here you see a compressed air line in use.

I recommend that the two faces of mesh in the wall are controlled by these U bars on 1.2m centres throughout. You should start fixing them and all the steel upright in the middle of a wall and work toward corners. Corners have L bars to join 2 walls together.
waterproof basement construction     waterproof basement construction

  • There should be safety caps on all uprights to prevent anyone falling on them being punctured.
  • A smooth blinding of semi dry or C20 very very wet concrete to virtually self level. Minimum 75mm thick.
  • steel neat, clean and securely tied.
  • scaffold board used for edge formwork where it equals the slab depth required.
  • This formwork is supported by steel road form pins banged through the concrete blinding and brought up to level with clods of earth underneath. Bits of hardcore underneath would be better or include some wooden pegs and nail the board at the right height. If you can put your pins or pegs in correctly before you blind, the blinding will make them stronger.
  • Mesh supported on concrete spacers and chairs. (Links to some concreting accessory suppliers are at the bottom of the page).
Your engineer is unlikely to specify your accessories. You need to order
  1. Concrete spacer bar.

    These are 1m long and you break them up into pieces. I allow 0.4m per square metre.

  2. Continuous wire chairs.

    These go between the layers of mesh. Tie each chair down both sides. At least 3 ties per m. I allow for continuous lines 600mm apart.

  3. Tying wire. At only £15 a roll get extra.
waterproof basement construction

waterproof basement construction I have sketched a corner to show how we actually get the bent starter bars in place. You need to imagine a string line between profile boards for the actual steel location. This is the idea ....

You might use two people for the first bits. By fixing two bars to the mesh back and front and using a bar as a diagonal, the 3 bars '1' will stand up. Ditto for 3 bars '2'. Then one person can fix the upright bar '3' and fix a corner bar '3' to it and to a bar '1' and to a bar '2'. Your corner is now strong. You will need two people again to fix a long length (of wall steel '4') from a corner to a bar at its far end. Having got this far, it's easy for one person to fill in the gaps with bars. Sometimes the bar '4' is moved along and pushed into the bank.

Try to keep your diagonals out of the first pour. Everything you've used above this pour is temporary and you will reclaim all that steel once the slab concrete has set.

If you can, tie the top mesh so that tying the starter bars to a straight bar in the mesh sets them in exactly the right place. Or else you can tie a straight bar to the mesh and fix the starters to that.

Or, I prefer, buying the mesh with flying ends and extra straight 10mm bar. In this way I fix extra bar to the mesh where bars are missing exactly where the starter bars need to be and I have maximum room to get my starter bars in.

waterproof basement construction
  • The rotating laser.
  • One man using the laser detector properly, that is, upright.
  • The vibrating poker.
  • Goggles, gloves and steel toe-capped wellies in use.

In case you want to order your steel yourself, (under Concrete Reinforcement in Yellow Pages) you will need to know the language. We found the following information on the Hy-Ten Steel Home Page. We've copied it because some readers may have trouble finding it on the Hy-Ten site.

British Standard preferred meshes in stock size sheets 4.8m long 2.4m wide.

British Longitudinal Wires Cross Wires Mass
Standard Reference Size mm Pitch mm Area mm²/m Size mm Pitch mm Area mm²/m kg/m² kg/sheet
Square Mesh Fabric
A393 10 200 393 10 200 393 6.16 70.96
A252 8 200 252 8 200 252 3.95 45.5
A193 7 200 193 7 200 193 3.02 34.79
A142 6 200 142 6 200 142 2.22 24.57
A98 5 200 98 5 200 98 1.54 17.74
Structural Fabric
B1131 12 100 1131 8 200 252 10.9 125.57
B785 10 100 785 8 200 252 8.14 93.77
B503 8 100 503 8 200 252 5.93 68.31
B385 7 100 385 7 200 193 4.53 52.19
B283 6 100 283 7 200 193 3.73 42.97
B196 5 100 196 7 200 193 3.05 35.14
Long Mesh Fabric
C785 10 100 785 6 400 70.8 6.72 77.41
C636 9 100 636 6 400 70.8 5.55 63.94
C503 8 100 503 5 400 49 4.34 50.00
C385 7 100 385 5 400 49 3.41 39.28
C283 6 100 283 5 400 49 2.61 30.07
Wrapping Fabric (basementexpert note: very thin)
D49 2.5 100 49 2.5 100 49 0.77 8.87

Size 6mm 8mm 10mm 12mm 16mm 20mm 25mm 32mm 40mm
Area (mm²) 28.3 50.3 78.5 113.1 201.1 314.2 490.9 804.2 1256.6
Weight kg per m 0.222 0.395 0.616 0.888 1.579 2.466 3.854 6.313 9.864
Approx no. of 12m bars per tonne 375 211 135 94 53 34 22 13 8


Major reinforcing suppliers are
Hy-Ten   Express Reinforcements   BRC   Rainham Steel  

Accessories suppliers are "Buildspan - the concreters warehouse"   Speedcrete   Max Frank   Lemon Groundworks   Formwork Direct

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