Pour Concrete Slab after timber frame garage built?

Hey there,

Thank you for the detail highlighted in your question!

Please have a look through the below and get back to us; let us know if this helps and if you have any further questions then give us a ring and we’ll walk you through what potential avenues you have :)

It is possible to pour a concrete slab after a timber frame garage has been built, though it may require careful planning and some workarounds to avoid damaging the existing structure. Here's a general outline of how you might approach it:

1) Removing the Floorboards
Since you mentioned the floorboards can be removed easily, this is a good start. Once they’re out, you will have access to the joists and the underlying ground, which is essential for preparing the base for the concrete slab.

2) Reinforcing the Ground
If the ground beneath the paving slabs is stable and compact, you may still need to excavate a little to create a proper base for the concrete. This could involve:
- Removing the paving slabs
- Compacting the soil beneath
- Adding a layer of hardcore or gravel to improve drainage and prevent the slab from cracking over time.

3) Adding a Damp-Proof Membrane (DPM)
Before pouring the concrete, you’ll need to lay a damp-proof membrane (DPM) to prevent moisture from rising through the slab. This is crucial to prevent damage to both the slab and the timber frame in the long run.

4) Pouring the Concrete
You could pour the concrete into the exposed joist areas, but the key will be ensuring the depth and thickness meet local Building Control requirements. Typically, a concrete slab should be at least 100mm thick (though local regulations might vary), and you might also need to reinforce the concrete with steel mesh.

Since the timber frame garage is already in place, you'll want to be careful to prevent the wet concrete from damaging the timber. You could use temporary formwork or barriers to protect the garage's walls during the pour.

5)Potential Height Adjustment
Depending on how high the concrete slab needs to be, you might end up with the new slab sitting higher than the original floor. You may need to adjust the joists or the floor's overall design if you want to ensure the final floor is at the correct height.

6) Consulting with Building Control
Since Building Control is already involved, it would be wise to consult them before proceeding. They may have specific requirements or suggestions on how to remedy the situation without having to take down the entire structure.

Alternative Options:

Screeding or Thin Concrete Over Existing Paving Slabs:

In some cases, you might be able to pour a thinner concrete screed or topping over the existing slabs without removing them, though this might not satisfy Building Control if a full slab is required.

Structural Beams with a Concrete Topping:

Another option could involve adding structural beams (e.g., steel or stronger joists) and pouring a reinforced concrete topping over them. This would allow for a stronger foundation without full excavation.

Conclusion:
Pouring a concrete slab is feasible, but you will need to ensure the ground is properly prepared, the slab is of adequate thickness, and that it doesn’t interfere with the timber structure. Consulting Building Control early in the process will ensure that whatever approach you take will be acceptable.

Yes, it is possible to pour a concrete slab under an existing timber frame garage, but it will require careful planning and execution. Here’s a detailed breakdown of your options and the process:

Option 1: Pouring Concrete Under the Existing Garage
This can be done without dismantling the garage but requires some preparation:

Remove Floorboards and Assess

Take out the floorboards to expose the joists.
Evaluate the ground below to ensure there is no major movement or instability.
Temporary Support

The timber frame should be temporarily braced to ensure structural stability during the work. This might involve installing diagonal supports or external braces.
Prepare the Base

Clear out loose debris, dirt, and stones from the ground.
Compact the remaining ground to create a solid base.
Add a layer of hardcore (e.g., crushed stone) and compact it to about 100mm.
Add a Damp-Proof Membrane (DPM)

Lay a DPM to prevent moisture ingress. Ensure it laps up the sides of the timber frame walls and is taped securely.
Install Perimeter Formwork

Use timber boards to create a frame around the interior edges of the garage to contain the concrete.
Pour the Concrete

Pour the concrete in sections if needed (depending on access). A thickness of 100–150mm is typical for a slab.
Level the concrete and let it cure for at least 28 days before reinstalling the floorboards.
Option 2: Retrofit a Floating Concrete Slab
If access for pouring is limited, a floating slab (not fully connected to the timber walls) might suffice for Building Control:

Same as Option 1, but the concrete slab would not be directly bonded to the garage.
Add expansion joints around the edges to allow slight movement without affecting the structure.
Alternative Solution: Screw Piles with Flooring Upgrade
If pouring a slab is challenging or not feasible:

Use screw piles or ground anchors driven into the ground to provide a stable foundation.
Attach joists or beams to the screw piles and add insulated or reinforced flooring panels.
This could satisfy Building Control if stability and load requirements are met.
Considerations
Building Control Approval: Ensure they agree with the proposed remedy before starting. They may have specific requirements.
Access for Concrete Delivery: If the garage has limited access, smaller batches of concrete or a pump may be required.
Cost: Pouring a slab under the existing structure will likely cost more than doing it beforehand due to labour and complexity. Expect to pay £2,000–£5,000 depending on garage size and site conditions.

Next Steps
Consult with a structural engineer to confirm the feasibility of your plan.
Discuss your proposed solution with Building Control for approval.
Hire experienced contractors for the work to ensure it’s done safely and effectively.
This approach should help you meet Building Control requirements without dismantling the entire garage

I might be wrong as it depends on your council but my initial thought was that the frame will still be sat on the old stuff. Putting concrete on the inside doesn’t alleviate the main issue that is that the weight of the main structure is sat on a base that doesn’t conform to building regs. One way would be to shutter off each side individually and concrete under the frame in sections. Then pour the main part