Foundations for a small out building next to a plum tree

[Adsibob]

We are pouring a 160cm by 90cm reinforced concrete foundation for a brick barbecue we will be building. The construction of the barbecue will be single skin brick side walls and a single skin brick back wall up to about 95cm high (a U shape). Then there will be a 6cm thick reinforced cement platform and on top of that we will continue the three single skin walls with the same facing brick, but in addition add an inner skin made of refractory brick. That will go up to about 1.95m in height total, then we will buy a lightweight galvanised steel cooking hood and flue on it.

The concrete platform will also be covered with a layer of refractory bricks.

 

i estimate a total of 420 regular facing bricks plus about 170 refractory bricks.

 

Builder initially advised 30cm reinforced foundation would be enough. But now he has asked about a nearby plum tree which I planted a month ago. I could move the plum tree, but I’m wondering whether I really need to. It is currently 1.2m tall and will grow to a maximum mature height of 2.5m (it’s a VVA1 semi-dwarf variety, known as a Plum (Prunus) Jubilee Bush). 
 

We are on London clay.

 

The base of the tree is 1m from the planned location of the nearest side of the foundation. The root ball was only 30cm in diameter when I planted it last month.

[IanR]

51 minutes ago, Adsibob said:

The base of the tree is 1m from the planned location of the nearest side of the foundation. The root ball was only 30cm in diameter when I planted it last month.

 

I feel you need to separate them, more for not having the canopy overhanging the barbecue. I've found Plumb trees to be vigorous growers, even dwarfs and columnars. For instance I had to really cut back a columnar plumb last year that was only 3 years old and had already spread 1.5m either side.

[Adsibob]

44 minutes ago, IanR said:

 

I feel you need to separate them, more for not having the canopy overhanging the barbecue.
 

assume that’s not an issue, because we can train the plum to grow away from the barbecue and onto the trellis on the nearby fence.

44 minutes ago, IanR said:

I've found Plumb trees to be vigorous growers, even dwarfs and columnars. For instance I had to really cut back a columnar plumb last year that was only 3 years old and had already spread 1.5m either side.

Supposedly the one I have is not off the vigorous type. But maybe you are right and it’s safer to move it now whilst we still can.

[Carrerahill]

1 hour ago, Adsibob said:

We are pouring a 160cm by 90cm reinforced concrete foundation for a brick barbecue we will be building. The construction of the barbecue will be single skin brick side walls and a single skin brick back wall up to about 95cm high (a U shape). Then there will be a 6cm thick reinforced cement platform and on top of that we will continue the three single skin walls with the same facing brick, but in addition add an inner skin made of refractory brick. That will go up to about 1.95m in height total, then we will buy a lightweight galvanised steel cooking hood and flue on it.

The concrete platform will also be covered with a layer of refractory bricks.

 

i estimate a total of 420 regular facing bricks plus about 170 refractory bricks.

 

Builder initially advised 30cm reinforced foundation would be enough. But now he has asked about a nearby plum tree which I planted a month ago. I could move the plum tree, but I’m wondering whether I really need to. It is currently 1.2m tall and will grow to a maximum mature height of 2.5m (it’s a VVA1 semi-dwarf variety, known as a Plum (Prunus) Jubilee Bush). 
 

We are on London clay.

 

The base of the tree is 1m from the planned location of the nearest side of the foundation. The root ball was only 30cm in diameter when I planted it last month.

300mm reinforced concrete slab... for a BBQ? Seriously?

 

First, the slab is too thick, not needed. 

 

Secondly, if you build the slab properly, the slab will be a raft found, the BBQ will move with the slab, therefore the tree/BBQ should be OK, as the tree does its thing it may move the slab a bit, but it will move the slab and BBQ construction as one. 

 

I think for this I would be happy with a 100mm slab, A252 or A393 mesh set centrally with 50mm cover top and bottom obviously, and 50mm cover at all edges or you risk it rusting and popping and a C20 concrete. You could stick in another 25-50mm if you want, but 300mm is madness. 

 

 

Edited June 2, 2023 by Carrerahill

[Adsibob]

Refractory bricks are very heavy. At least 25% heavier than a standard brick. But yes, there is probably a degree of over engineering here. 

[saveasteading]

100mm concrete base on decent hardcore, extended 100mm past the wall position.

Warehouses holding 20t/m2 only need 20cm.

Reinforcement optional, but perhaps wise with the tree there. Myself, I would compromise and use fibres.

You don't need all those refractory bricks. I would build in normal bricks then line the fire base with refractory lining bricks/ tiles and also 1 x high to back and sides. 

 

For the concrete shelf I think i would build a base shutter of thin ply or estate agents sign, propped, and leave it in. Now that does need some mesh as it is suspended, but any will do it.

I don't think commercial bbqs have any reinforcement except maybe chicken wire.

Actually the concrete doesn't need fire proofing as the heat goes upwards and there should be ash protection.

The charcoal burns best if on a mesh with air beneath it.

 

It is worth building an extra shelf below, for hot pans.

 

Are you plugging in a kit, or making your own cooking shelves?

 

It will work better and save materials to bring the flue size down to about 30 x 30 internally. 

 

 

[Adsibob]

7 minutes ago, saveasteading said:

Are you plugging in a kit, or making your own cooking shelves?

I’m plugging in a kit. 

 

7 minutes ago, saveasteading said:

Warehouses holding 20t/m2 only need 20cm.

So why did my BCO insist on footings deeper than 2m for a part single storey part double storey rear extension? SE had specified 1.7 and BC increased these to 2.2m from what I recall.

[saveasteading]

37 minutes ago, Adsibob said:

footings deeper than 2m for a part single storey part double

I can't answer your specific point but it is an easy assessment from a table.

2.4m would be for heavy clay with a potentially big tree, eg oak or ash, very close.

 

Footings are for structure. Slab is just the floor.

The warehouse example will have foundations for columns and for structural walls, down to 0.9m in low risk  and 2.4m if clay near trees.

 

The big supermarkets use thick slabs due to caution/ have money to spare.

 

Commercial warehouses typically carry 20t /m2, sometimes on racking, and have forklifts trundling around, all on 200mm, sometimes less. The floor isn't going anywhere unless the ground is very soft indeed.

 

It is also related to risk. If your barbecue settled 10mm it wouldn't matter, and you wouldn't know. If it fell down some winter night nobody would die.

 

I really can't see why 300mm is even suggested. I suppose it is only costing you an extra £100 or so, so it is your choice.

 

[TonyT]

300mm madness

[joe90]

I am going half that for a car garage (timber structure tho). And it’s been assessed by an S.E.

Edited June 2, 2023 by joe90

[Adsibob]

It’s because of our neighbours’ trees and the deep clay, that the SE specified such deep foundations and that the BCO then increased them to something over 2m. I’m in very bad clay, the bbq is 1.1m away from a boundary line. I have no control of what is grown on other side of that boundary, so I based my ballpark assessment on the basis that the builder said 200mm and I thought, let’s add a bit to err on the side of caution, in the same way the BCO did. On reflection, based on the comments here I’ve clearly added a bit too much, but its future proofs the thing against any eventually, whether that be sustained drout causing shrinkage of the soil, or the neighbour’s trees growing even bigger. The extra 0.1m depth is an extra 0.144m3 of concrete, so extra cost but not extortionate. And given I will only have myself to blame if anything goes wrong, I just rather pay for the extra concrete.

[joe90]

I get that @Adsibob but a slab that is strong enough (100mm?) will not crack and as said above it will all move as one, I am no S.E. but an additional 0.1m depth of concrete won’t stop that movement unless like your house extension you go much deeper (then 100mm will still be enough?). It’s the depth of excavation, not the thickness of the concrete, Someone prove me wrong,

[saveasteading]

52 minutes ago, joe90 said:

Someone prove me wrong,

No you are right. Warning: Short essay follows.

The concrete slab is made to be flat and hard. It sits on stone which is rougher and not bonded, but strong enough for the load. This sits on ground that also has to be strong enough...which it will be unless someone has put soft fill in. 

Also as these layers progress, the loading spreads out further. So a 1m2 slab is spreading its load over about 2m2.

The barbecue could sit on stone as far as strength is concerned, but the concrete keeps it controllable, tidy and clean, plus it spreads the load more efficiently.

 

If the clay shrinks in summer  the barbecue will drop a few mm and nobody will notice as all the ground will have dropped. Then in winter it absorbs water and rises again.

For a bigger structure like a house, the movement could be very different at various areas, due to the distance from the tree that absorbs the water, so could cause damage. So deep foundations take the structure below the drying and shrinking  clay. But only the foundations go deeper to where the clay stays unaffected. Slabs are usually sat on stone just below the original  ground level.

 

Building inspectors are notorious for instructing another 30cm be taken out. Sometimes there is good reason, and sometimes it is their insurance against not having researched the rules.

I say no, as can another SE....but usually a contractor will do as the bco says....esp as the client pays.

 

It is perhaps an anomaly that we never allow for trees that might be grown in the future. In reality, it us unusual to plant big species in proximity to houses.

[George]

Think I'd just get a portable BBQ! 

 

Agree with saveasteading mostly (mesh > fibres everyday). Foundations aren't magic, they just transfer load from surface level to competent sub-soil below ground. Buried, compacted stone is as good at doing this as concrete.

[Adsibob]

31 minutes ago, George said:

Think I'd just get a portable BBQ! 

It’sa cultural thing. My last barbecue was 14’ high and the subject of a planning permission dispute (which I won).

[SteamyTea]

4 hours ago, Adsibob said:

My last barbecue was 14’ high

Fourteen foot?

Have you seen Spinal Tap's Stonehenge.

 

 

[Adsibob]

3 hours ago, SteamyTea said:

Fourteen foot?

Okay, maybe not fourteen feet, but certainly more than 12’, including the flue pipe and cowl. The fire box starts at 90cm from the ground (like a kitchen worktop, you want it to be comfortable to cook), the fire box is about 75cm high. That takes you to about 165cm. Then there is the top support for the hood, the hood, the flue and the cowl. It was a thing of beauty.

[SteamyTea]

5 hours ago, Adsibob said:

It was a thing of beauty

Some say the same thing about Trevithick's engines.

[Adsibob]

Bloody good job I insisted on over engineered foundations. Look what they found:

 

A rather large tree root (diameter is about 9cm), almost certainly from an apple tree we cut down almost 5 years ago. Had I gone less deep, we might have missed this root, it would have eventually rotted and caused moree movement that my barbecuing skills can cope with!

[Big Jimbo]

Ỳou defo don't want the sausages rolling off.

[George]

You do need to go deep enough to get to clean sub-soil but you don't need to build back in concrete. 150mm layers of compacted stone is a lot cheaper.

[saveasteading]

Saw the trunk off and wrap some polythene up the face to keep out small roots.  your choice stone or concrete base.

[Lofty718]

Is that tree from a neighbours property?

 

I wanted a brick built bbq for this summer too but will have to wait till the next. If you don't mind me asking how much is your builder charging to build it?

[Adsibob]

6 hours ago, Lofty718 said:

Is that tree from a neighbours property?

 

I wanted a brick built bbq for this summer too but will have to wait till the next. If you don't mind me asking how much is your builder charging to build it?

We couldn’t agree a price. What I want is fairly involved as on top of the foundations, there are 3 walls to bring it to 90cm high, then a concrete platform, then the brick walls and firebox to bring it to around 1.9m. Then he just had to stick on the hood and flue which I will order separately. But I’ve had one of these built before and it took two semi competent guys a whole week to do it and even then it wasn’t finished, and I had to finish it myself. So he’s just doing the foundation, and then I will see whether I progress it this year, or next. 

[saveasteading]

I have one made of kit form concrete planks and slabs. Very happy with it  and easy to build.  It came with a cooking grille that conveniently slides into the mock brick cooking space at various levels.

I added some brickwork to the flue to take it higher. (About 4m?).