Concrete block foundation - how many should I use?

[Danv]

I am in the process of final bits of design on my self build garden office. However I have hit a conundrum and can’t seem to find any advice online as to how many concrete blocks plinths I need for my foundations. The building is to be made from timber as you will see below. 

 

How many concrete block plinths to support the base? and how far should each be spaced from each other?

 

My building is: 4211mm x 3188mm

 

These are the two options I was playing with. One with more blocks is what I had before watching a few videos and the one on the right is what I'm now thinking... if it'll hold up! It's just for an office / play room so doesn't need to house extremely heavy equipment. Any advice is greatly appreciated!!

 

 

Edited February 3, 2022 by Danv
Adding pictures

[Thorfun]

I've no idea but, for the cost of a few extra blocks, why even consider the second option? a garden room may now only be an office but it's the sort of space that could easily be turned into a gym which would have heavy equipment and so might as well plan for something like that now. very hard to retrofit later!

[scottishjohn]

the real question is how soft is your garden - will it sink  and how much will the floor flex if you skimp on supports

 nd how big a concrete pad under each block- or no pads just set on earth ?

Edited February 3, 2022 by scottishjohn

[SuperPav]

Use paving slabs instead of concrete blocks - usually can pick these up for free locally, and the larger footprint means they're unlikely to sink unless your ground is REALLY soft.

I'd go for the 32 you have in Option 1.

[Danv]

Thank you for all your input!

 

@Thorfun The benefit of having less is to lower the amount of manual effort and time it will take for the additional amount - which i was trying to reduce. I do like the thoght for the future though, especially if we sell the house etc.

 

@scottishjohn The plan for each block is to dig down 30-40cm, fill with rock/stone and then top with sand and then add the block on top, probably in stacks of 2 blocks (cemented), depending on leveling.

 

I should also add that the option with 32 did have joists at 600mm, which on the lesser blocked version i reduced to 400mm centres.

 

Looks like I will need to continue looking at the 32, however if anyone things I can get away with less without risking the foundation - please do shout!

Edited February 3, 2022 by Danv

[Conor]

Make sure you hire a whacker and make sure your base is well compacted. Do in 100mm layers and make sure there are plenty of fines in the mix.

[markc]

I’m guessing the sand topping is for ease of levelling? Don’t! The sand will wash/leach away leaving voids. Under the blocks.

as Conor said, well compacted base, add the blocks and then level the top with mortar or shims under the timbers.

[Chanmenie]

Have you considered gravel grids base, like Ibran ?
easy and cost effective to lay and will keep the timber base dryer than concrete blocks or slab 

[Temp]

What size are the floor joists going to be?  If you know that you can use span tables to work out the max allowed unsupported span.  This ignores the issue of them sinking into the ground though.  

 

If you decide to use fewer blocks than option 1 then don't do option 2!  Put the rows of blocks the other way so that every joist has three blocks under it. As currently drawn some joists have 4 blocks and some have none at all.

[saveasteading]

1 hour ago, Chanmenie said:

Have you considered gravel grids base

But still use blocks on top, to get air flow and also allow adjustment for precise height.

 

I am not one to overdesign, but then I could design the timber spans.

 

In your case it is much simpler to use extra blocks, laid flat , which are going to cost you £4 each or so x 20. 

option 1 then.

Option 2 is just wrong I think as you show the blocks under every 4th joist, and the others allowed to deflect.

[scottishjohn]

18 hours ago, Danv said:

hjohn The plan for each block is to dig down 30-40cm, fill with rock/stone and then top with sand and then add the block on top, probably in stacks of 2 blocks (cemented), depending on leveling.

If you are going to dig a hole and fill with hardcore --just  add some cement and make them concrete +level them up at that stage when concrete is wet -then blocks -no need for a wacker ? then 

Edited February 4, 2022 by scottishjohn

[PeterW]

I would work the spans out - tbh 6x2 would be fine here - and go the long way across so the 4m span at 600mm centres and you will need to just drop in a row of pillars at each end and one in the middle if you turn it into a ring beam. I would use 2 blocks on flat and then one on the top of them to lift the base but unless you’re on very odd ground then you won’t sink as the ground pressure from 9 pads will be negligible. 

[Danv]

Thanks @PeterW, I’m using 6x2’s and like the sound of this, any less holes to dig and fill the better! Are you able to inform me what you mean by a ‘ring beam’?

 

thanks @Temp aiming for joists of 6x2. Do you have a link to a span table I could reference?

 

thanks in advance!

[saveasteading]

1 hour ago, Danv said:

joists of 6x2

pressure treated.

[Temp]

1 hour ago, Danv said:

thanks @Temp aiming for joists of 6x2. Do you have a link to a span table I could reference?

 

I only have an old set but if you PM me an email address I'll send you them

 

6x2 C16 joists are OK for at least 3m spans. Your shed is just over 4x3. If you decide to run them in the 3m direction I'd still support them mid span just to make it all feel more solid. 

[PeterW]

13 hours ago, Danv said:

Thanks @PeterW, I’m using 6x2’s and like the sound of this, any less holes to dig and fill the better! Are you able to inform me what you mean by a ‘ring beam’?

 

thanks @Temp aiming for joists of 6x2. Do you have a link to a span table I could reference?

 

thanks in advance!

Ring beam - essentially put a 6x2 across both ends of the floor so your floor joists are screwed to this (or use joist hangers for even more strength) and it creates a “ring” of  timber. 
 

Latest span tables here but a C16/C24 44x145 will be fine at 600 centres and supported mid distance. If you’re thinking of putting anything heavy in the centre of the front span (French doors etc) then just double up the front joist or even just put an extra pier of blocks so there are 4 on the front not 3.