Concrete outbuilding foundations

[nordita]

I'm building a 5x3m 2.5m high single skin concrete outbuilding. It will act as a gym.

I'm looking for advise with the foundations.

Based on Structure: Approved Document A the width of the footing is advised to a be a minimum width of 250mm and depth of the excavation 450mm based on my wall and ground type.

I initially planned to pour a concrete footing but discovered thermalite trench blocks have a width of 300mm. Access to pouring concrete is problematic and would prefer not to do if possible. I would

1. Given the trench blocks have a width greater than the advised minimum footing would it be okay to solely use the trench blocks 2 courses high with no concrete poured footing?
2. The walls will be 3.6n aerated concrete. Is this also permissible for the below ground structure? I think there are higher density blocks available in that size but unsure if they would be overkill?

Thanks

 

[George_engineer]

Interesting question. I don't see why not. It'd probably be sensible to pour a 50mm levelling/blinding layer, but you could achieve something similar with dry mix mortar.

Guessing you're on gravel/sand?

Other option if you're doing a concrete floor and need to pour that, is to change it to a raft slab. Walls would be built off the slab.

 

[nordita]

Thank you for the reply.


I've made the assumption I'm on chalk. I'm pretty sure I will need a pick to excavate given the local area. I will dig a few test holes.

 

[jks7492]

I’d just pour a 150mm thick x 300mm wide concrete strip foundation. It’s less than a cubic meter of concrete which in comparison to most foundations is practically nothing.

What are you doing for the floor?..

 

[Notch7]

I'm building a 5x3m 2.5m high single skin concrete outbuilding. It will act as a gym.

I'm looking for advise with the foundations.

Based on Structure: Approved Document A the width of the footing is advised to a be a minimum width of 250mm and depth of the excavation 450mm based on my wall and ground type.

I initially planned to pour a concrete footing but discovered thermalite trench blocks have a width of 300mm. Access to pouring concrete is problematic and would prefer not to do if possible. I would

1. Given the trench blocks have a width greater than the advised minimum footing would it be okay to solely use the trench blocks 2 courses high with no concrete poured footing?
2. The walls will be 3.6n aerated concrete. Is this also permissible for the below ground structure? I think there are higher density blocks available in that size but unsure if they would be overkill?

Thanks

Foundation depth is decided by the load bearing capacity of the ground and its stability. That’s influenced by ground type and whether there are any trees within the zone of influence.

You could hi density concrete blocks up to DPC and thermalites above.

Im not sure what wall construction you are intending but a single skin wall has both damp and thermal issues. If you want a modern external finish like smooth render, I would use tyvek housewrap, vertical battens then rendaboard. That creates a cavity and keeps the wall dry.

 

[^woody^]

That foundation recommendation seems too narrow and too shallow.

Two course of blocks direct onto the ground won't have sufficient load transfer capacity and risk cracking.