All about civil construction knowledge- PARAM VISIONS

3 common mistakes in the roofing sheet calculations.

 I have mentioned below, the 3 most common mistakes done while calculating the number of roofing sheets required for the building. 

1. Slope calculation:

If the length of the building is 10ft. and suppose if the slope gave is 2ft then the length of the sheet to cover the building will be the hypotenuse of the triangle, i.e. 10.20 shown in the fig.

Instead of calculating the slope length, some contractors simply take the outer dimension of the plinth i.e. 10ft. which gives a shortage of 0.2 the sheet length.  

 2. Overlapping calculation :

Suppose if the length of the roofing sheet is say ' L ' and if the length of the overlapping is ' d '  as shown in the fig.

then the length covered by the 3 sheets is equal to ( 3L - 2d )

If you simply deduct the overlapping ' d ' for the no. of sheets taken i.e. ( 3L -3d ) then it is a calculation mistake.

Note: This calculation should be done rightly in both the way, i.e. length as well as width side of the roofing sheet.

3. Dimensional unit calculation :

Normally when we purchase the sheets from the market, the dimensions referred by the trader is say 10 ft. then the actual size of the sheet maybe 3.0 meters. When we convert the 3mtr. in to feet, it comes to  (3.0*3.281) 9.84 ft. instead of 10 ft. 

This difference of 0.16 ft. cumulatively for all the sheets gives a big calculation mistake.




 Now, let us observe the 5 main differences between the one-way slab & two-way slab



One-way slab

Two-way slab


(Lx / Ly) > or = 2

Where Lx  = Longer span


            Ly =  Shorter span


(Lx / Ly) < 2


Lx =  Longer span

 Ly =  Shorter span



The main reinforcement is provided in one direction i.e. Ly.

Distribution bar is provided

in longer span i.e. Lx.

Main reinforcement is provided in two directions i.e. Lx & Ly



Loads are carried by the structure in one direction.

Loads are carried by the structure in both the direction.



Supported by the beams in opposite directions.

Supported by the beams on all four sides.



Bends like a cut portion of the cylinder

Bends like a cut portion of the sphere.


7 common mistakes in the plinth beam

While providing a plinth beam for the building, some common mistakes which I have observed are given below. 

1.  Development length :

Provide the rebar with needed development length and 10 times bar dia. which is given as Ld + 10Db or say L -length/ hook. Curtailing the bar at the column end is technically wrong.

 2.            Bar diameter

The diameter of the bottom reinforcement should not be less than 12mm (IS 456) and top rebar should not be less than 10mm. diam. as shown in fig. Providing the rebar of 10mm. and 8mm one of the mistakes which should be avoided.

 3.     Nominal & clear cover :
Providing the cover for the reinforcement bar on all the 4 sides are essential, as shown in the fig., but when the beam is placed over the brickwork, avoiding cover for the bottom rebar is a wrong practice.

4.   The spacing of stirrup :
Providing the spacing of 150 -175 mm at the center and 125 - 150 mm near column end is the good practice to avoid the settlement of masonry work. One of the mistakes done here is keeping the spacing at 300mm and above to save the stirrup rings is a mistake which should be avoided.

5.      Ld at the T- junction :

 As explained earlier, the development length Ld should not be curtailed at the junction of the 2 beams or at the T formed at the column and another plinth beam.

 6.    Cleaning the beam:
After completing the plinth beam work for the building, usually, the next procedure followed will be muroom filling in the plinth. After completing the filling work, the contractor on the site, neglect to clean the plinth beam surface. Brick masonry work builds on such an uncleaned beam top will result in the weak bonding and ultimately ends in the cracks at such joints. Please take care to clean them properly.

7.           Formwork :

Using building blocks or bricks as the side formwork for the plinth beam is also a mistake. Do you know why? The reason is, they absorb the water and cement slurry, making the weak concrete of different grades.

If you want to understand them in a better way with explanation, click on the video given below



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