# How much weight can ceiling joists take?

Discussion in 'Building' started by Simplez, 8 Sep 2020.

1. ### Simplez

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The ceiling joists are 4.2m in length and 4 x 8.5cm in dimension- the house is circa 1910. The house is 5m wide and 8.4m in length.

The joists meet on a supporting wall in the middle of the house.

I have removed the existing lathe and plaster ceiling so all that's left are the joists.

Will these joists be able to support plasterboard that is being skimmed as well as the loft being boarded out with p5 chipboard ontop?

So thats roughly 40sqm of plasterboard and skim. So that is 14 sheets x 25kg = 350kg

Plaster is about 6 bags to cover at 3mm so thats another 6 x 25 = 150kg

Total of 500kg

I am only boarding out the central half of the loft - so 20sqm.

That will take 14 loft boards at 17kg each for a total of 238kg plus 120 loft legs - (1kg each?) so 120kg for the legs plus 238kg for the boards is 358kg.

So plasterboard is 500kg plus loft boards and legs is 358kg for a total of 858kg

There are 15 ceiling joists.

So 858/15 = 60kg per joist?

3. ### jacko555

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How heavy do you think that was? Same or more than plasterboard & skim? How thick was it?

4. ### Simplez

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It was wooden battens and then the plaster was applied to it - battens were about 5-7mm and the plaster at least 1-2cm.

Do not sure how much that weight was.

I have walked on the joists when the original ceiling was in place and I weigh nearly 15st - 95kg.

5. ### tony1851

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You forgot to add the weight of the stuff you will be putting up there on the boarding, which can easily add up to as much as the weight of plasterboard, skim and chipboard.

Regardless of that, the ceiling won't collapse. What kills timber floor and loft joists is excessive deflection, which cracks the plaster ceiling below. Long before the joists fail by shearing off or splitting, they will have deflected to an alarming extent and rendered your new, plasterboard ceiling a mess.

So you have to check what maximum deflection will result from your loads, which involves some maths. Either that, or Google 'approved document A 1991 Superceded', which has load/span tables at the back, and which will give you an approximate indication of what size/span/spacing would be acceptable for your joists (the current approved documents don't have these tables).

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6. ### bennymultifinish

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run a 5x2 above centrally and perpendicular to your joists supported or resting independently and secure your 4x3ishs to it with joist hangers.
bazzinga . done it loadsa times.

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7. ### Simplez

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I can't make head nor tail of this as my joists are 30cm apart in the (but 34cm from centre of joist to centre of joist).

9. ### SFK

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Simplex - how to read the table:

Your Beam thickness and width= 40mm x 85mm
Your Beam Pitch (separation) = 340mm (note that you have to use 400mm as this is smallest they show).

So the table shows that
a 38mm x 97mm beam with a 400mm separation (units all similar to yours), will have a maximum length of 1.83m with minimum loading, and a maximum length of 1.42m with maximum loading.

As your pitch is closer, then you can presume a slightly longer beam length than these.

OR

As you need a beam length of 4.2m and have a pitch of say 400mm, then you can determine your beam dimensions by looking for any lengths greater than 4.0m under the 400mm columns.

Last edited: 9 Sep 2020
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10. ### tony1851

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@op - you are looking at the wrong table - you want the one for ceiling joists - not floor joists.

The difference is that floor joists are designed for an imposed load (furniture, people, stuff etc) of 30 lbs/sq ft, whereas ceiling joists are only expected to carry lighter loads (around 5 lbs/sq ft for light storage).

Looking at the ceiling joist table (left-hand column) the nearest joist size to yours is 38 x 97, giving max. span of 1.74m, so with your closer spacing of 340mm, you'd be looking at 2m max. reasonable span.

In practice, if you put most of the load over the central wall, you could go longer than this with your size of joists.

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11. ### Simplez

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Reading that again it seems that I wont be able to board the loft then?

As 0.25KN/m2 is 25kg per m2 - so for 40sqm looking at 800kg max weight inc the weight of items.

The loft legs weigh much less than 1kg each - more like 250g each so they go down from 120kg to about 30-40kg

So the weight of the ceiling and boards will be around 780kg - without anything stored on it!

12. ### tony1851

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0.25 kN/m² is approximately the weight of 18mm chipboard and 9.5mm plasterboard and skim.

It's not as bad as it might appear, if you keep most of the load near an internal wall. For the live load (ie the stuff you store) it's unlikely you will load it to the maximum because you can't get much stuff down near the eaves.

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13. ### Simplez

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Here is a diagram I made. I am only going to board out the central 4.2m leaving 2m on either side where the eaves are unboarded.

To clarify the joists from the back wall of the house run 4.2m to the internal brick wall. Then another 4.2m joist is connected to that which runs from the internal wall to the front of the house.

So as long as I keep the load over the internal wall then I should be ok?

In the diagram I have used loft legs every other joist but will be using loft legs on every joist to spread the load more evenly.

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