Garden Room Foundation Near Trees

[Antony11]

Hello,

I am looking to build a garden room ideally 8 x 3.5m. However behind my fence there are a lot of tree's and bushes, keeping these trees healthly is important since they act as a screen between my house and a college building behind them. There is quite a big Oak tree that sits right behind my fence and a Chesnut tree with an 60cm meter wide trunk a metre away from the back of my fence.

I know the soil in my garden is going to be pretty tangled up with roots, so I was thinking to build my garden room on concrete blocks, on a 10cm layer of MOT Type 1. Then placing a weed membrane in between the blocks. Then followed by a garden frame etc

I thought this method would still allow air flow underneath the building so the soil could still stay slightly oxygentated which would benefit the roots, although I am not sure how the weed membrane would effect that.

Any advice or feedback about the best foundation option for this situation, or tree knowledge is much appreciated.

I roughly laid out the building dimensions with stakes, please see attached pictures

Thanks,

Antony

 

[^woody^]

The issue with roots is not the root damaging foundations or the root not getting oxygen, but the moisture that the root extracts from the ground which could cause the ground to shrink, and the foundations to subside or move. Or if trees and shrubs are removed the subsequent swelling of the ground due to excess moisture.

Weed membranes are of no use in this context, unless you want to stop weeds.

Large established tress will not tend to cause much of a problem, but otherwise a raft foundation may be more appropriate, or deeper foundations than normal.

 

[Antony11]

Hi Woody,

Thanks for your reply.

My main concern was covering the area which the trees have established roots and in effect starving them of water and nutrients, but generally speaking you don't think this is an issue for established trees? I was thinking that by having the garden room elevated on blocks it would still allow water to soak into the area covered by the garden room from the surrounding ground that would still get rained on. So I was hoping subsidence wouldn't be such an issue. In an ideal world I would like a raft/ slab but I think there are too many major roots in the area to have one.

And yes, the weed membrane is only to stop weeds growing beneath the building.

So if I can't do a raft, do you know if concrete blocks would be the next best thing? I have seen something called easy pads that can be adjusted should subsidence occurs but they literally work out 10 x more expensive than blocks.

Thanks,

Antony

 

[frutbunn]

Large established tress will not tend to cause much of a problem

Yes they will!

 

[^woody^]

Hi Woody,

Thanks for your reply.

My main concern was covering the area which the trees have established roots and in effect starving them of water and nutrients, but generally speaking you don't think this is an issue for established trees? I was thinking that by having the garden room elevated on blocks it would still allow water to soak into the area covered by the garden room from the surrounding ground that would still get rained on. So I was hoping subsidence wouldn't be such an issue. In an ideal world I would like a raft/ slab but I think there are too many major roots in the area to have one.

And yes, the weed membrane is only to stop weeds growing beneath the building.

So if I can't do a raft, do you know if concrete blocks would be the next best thing? I have seen something called easy pads that can be adjusted should subsidence occurs but they literally work out 10 x more expensive than blocks.

Thanks,

Antony

The way a tree or most plants tend to work is that they have other roots to get the water, and the roots are far reaching sideways and downwards. Water and air and whatnot still travel under buildings anyway.

Pads are less adapt at dealing with movement and will move independently and unevenly

 

[mrrusty]

I don't believe that the area covered is going to make any difference whatsoever to the growth of the trees - if there's water in the ground, the trees will find it - think about city trees with only a little square of earth exposed at the trunk and the rest under the road and pavement - they still manage to grow.

So the issue is potential ground movement and foundations. If it's a garden room it isn't super-critical - yes, you don't want it falling down, but neither is it going to impact on your house value etc. I'm looking at easypads for a gardenroom I am planning too. The good thing about these is they are height adjustable, so although access might be tricky for the inner pads, you at least retain the option of adjusting them in the unlikely event the ground moves enough to warrant adjustment. A reinforced concrete raft that size will cost you ~£2K in concrete, reinforcing ballast etc - I've just done one almost exactly the same size, so easy pads will be less than that.

 

[Antony11]

The way a tree or most plants tend to work is that they have other roots to get the water, and the roots are far reaching sideways and downwards. Water and air and whatnot still travel under buildings anyway.

Pads are less adapt at dealing with movement and will move independently and unevenly

Thanks for this information, with this taken into consideration it seems like a raft would be best, however I don't see how one could be constructed as there are plenty of thick roots in this area. Especiallly with the correct sub base depth

 

[Antony11]

I don't believe that the area covered is going to make any difference whatsoever to the growth of the trees - if there's water in the ground, the trees will find it - think about city trees with only a little square of earth exposed at the trunk and the rest under the road and pavement - they still manage to grow.

So the issue is potential ground movement and foundations. If it's a garden room it isn't super-critical - yes, you don't want it falling down, but neither is it going to impact on your house value etc. I'm looking at easypads for a gardenroom I am planning too. The good thing about these is they are height adjustable, so although access might be tricky for the inner pads, you at least retain the option of adjusting them in the unlikely event the ground moves enough to warrant adjustment. A reinforced concrete raft that size will cost you ~£2K in concrete, reinforcing ballast etc - I've just done one almost exactly the same size, so easy pads will be less than that.

Hi Rusty, thanks for your reply, that is a good point about trees in the city. Ideally I would like a slab but like I just said to Woody in this thread I don't see how one could be built as there are plenty of thick tree roots.

You are right easy pads would definitely be cheaper and much easier than a slab however since like you say the inner ones couldn't easily be adjusted it kind of makes them lose one of their main benefits, I felt more inclined to go for some thick 7n blocks from builders depot but still undecided at this point.