How and what to do when preparing for your decking/suspended floorslab

For a building that must have more than one floor, you need to factor in the technicalities involved when putting in place the concrete slab. For most of the poorly done projects, decking or suspended slab contributes majorly to the cause. Knowing what to do is more important than been present 24 hours on site to achieve nothing.
These are the basic steps to follow…Get your architect or engineers to calculate the number of planks and other wooden materials you need for the decking, the number of reinforcements or iron rods, the volume of sharpsand, granite and numbers of bags of cement required.After getting these calculations right,call your electrician and plumber to site  for the estimates so that they can place their pipes before the concrete is formed. They may need to break some blockwalls even cut iron rods in extreme cases for the pipes to fit in.After getting the materials to site,invite the carpenter to start placing planks for beams that will transfer loads from the concrete decking to the foundation through the columns/pillars in the building.But before you give the carpenter go ahead, make sure you call a builder to site to give the right level for the decking if not, you will have very uneven decking especially when the soil is weak and buildings settles very fast on it.

HOW TO TAKE LEVEL

The builder is expected to carry his levelling instrument to site with an assistance along with
him to do the job. A reference point is taken in the building and is marked out, then measure
the height you wish to achieve from that point up. Now transfer the level or point to the four
corners of the building, when this is established, the carpenter will only need to throw one
line from one place to the other and he can begin to place the planks along the line. Some artesans will tell you that they can use plumb to get this level, this is not true, the best method is to use the levelling instrument. Measuring same distance directly from the ground floor to the top and doing same all over the building especially when its not a precast
structure, shows technical deficiency on the part of your builder, because most floor slabs are
not on same plane out of errors due to settling,lack of technical knowhow and carelessness from
the casters of the ground floor slab.

After the level is determined by the builder, next is for the carpenter to get ‘water level’ he will use this to transfer the level round the building. Water levelling instrument is an ordinary hollow transparent cylindrical rubber pipe that is filled with water and is easy to turn around the walls of the building. Remember water finds its level, the level of water at the end of the pipe in one place will be the same level at the other end of the pipe, so is easy for you
to transfer and determine the level round the building.  After placing the planks supported by bamboo or iron rods under with other wooden materials like 2″ x 3″ wood, the iron bender is called to start fixing the iron rods on top.He must have prepared ‘concrete biscuit’ which is used to raise the iron rods to avoid contact with the wood, if not, by the time the plank is removed some parts of the iron rods will be exposed to air or moisture which will cause rusting and ultimately decay.The iron rods for the floor slab is run along, and the electrician is also expected to fix his pipes and sockets, likewise the plumber will also do some work if need be before you call the casters in to cast the concrete for you.

WHEN AND HOW TO CAST CONCRETE

Be prepared to rent a concrete mixer and labour for the casting especially when the area of
concrete is much. Mostly the concrete is in ratio 1:2:3 or 1:2:4 i.e one bag of cement to four headpans of sand,
and eight headpans of granite.Except for peculiar sites this ratio is ideal.

Inform the caster to bring a vibrating machine along, this is used when casting so that the concrete will settle properly and  there will be less holes in the concrete.If you can get granites of smaller sizes like half inch to do the beams, it will  be good for compactibility. The concrete will form well with finer smaller stones aggregates.As the concrete is been poured, follow the casters behind closely with vibrating machine and work on the concrete. make sure the beams are well vibrated at the joints especially because of the clusters of iron rods if not the concrete will not cover the beam properly.

Get a good mason that will spread the concrete round evenly with range and plumb for best result.Next day pour water on the concrete and blockwork can start on top that same day. By 21 days or three weeks the bamboo or cylindrical iron pipes support under can be removed, it must have fully set. Remove any cantilevered side after 4 weeks or a month.

An architect with decades of experience

13 Comments

  1. This advice is too general, following it may be costly, it is saver to employ the service of a professional Builder, this are technical issues that are better handled by professionals.

  2. Gud article but we have professionals that can do these job. Civil engineers are trained to do these jobs likewise builder. The concrete cover placed on the form work b4 the reinforcement work is based on design calculation and is not just by the iron benders. The iron benders also need the detailed structural design drawing and bending schedules to placed the reinforcement under the watchful eyes of a structural engineer. Settlement of building to a greater extends is un desirable for building bcos it can leads to failure of the building, the engineer try to put the settlement to a safe limit by choosing appropriate foundation type. Finally, it is more advisable to built all building of 2 story and above as a framed structure

  3. Would you suggest gravels or granite for decking? By the way, I do not see article as portrayed by Tope. I like the simplicity of the article. Ofcourse building engineer will oversea what is being done and make corrections as needed.

  4. i will like to know the difference in using various sizes of granite in casting. will my concrete have more strength when i use bigger size granite than the smaller size?

    1. No the smaller sizes are better but will cost more. Smaller granules give room for better bonding.
      Holes won’t be as common as what you’ll see in bigger granite casting. Above all you are most likely to have a slab that will resist water percolating through easily.
      Thanks

  5. When i project manage a contruction project, i always insist that the contractor should give me a Grade C20 concrete. This can be confirmed only by a lab test of the cubes. I dont ask for 1:2:4 concrete. It is left for him to use any mix that can give my the grade of concrete.

    By doing this, you are sure of getting the prescirbed strength of concrete from your contractors.

    Sometimes, the 1:2:4 nominal mix might not give you the specified strength because of the way the labourers load the headpans indiscriminately. Hence you could have a strength less than the specified strength.

    Do a trial mix, check the strength in the lab and stick to the nominal mix ratio that gave that specified strength of C20.

  6. It will be good the author of these articles responds to the comments or apply some of them in future articles. The write-ups are well intended but if not handled properly might lead innocent clients to error. I’ll also advice that he puts a disclaimer on these articles since most of them are not DIY. In other circles he may be persecuted by ignorant people who take on such projects without seeking professional help.

    1. Thanks for your observations. This site is not a substitute for a professional service in a construction site , but is just to encourage people and allay fears and some doubts when you want to embark on a building project. In otherwords work done on site will not be totally strange to a layman.

  7. I want to start a storey building of 4flats but i dont know how much it will cost neighter do i ve the money to finish it immediately.I intend to start and do it gradualy not minding when it finishes.i am sure i can put in N1M every year.kindly advise.hw much will the foundation work cost? then wat next?

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