DIY concrete block outbuilding (15m2)

Nordita

I want to construct a non-habitable outbuilding in my back garden to serve as a gym. When I decapitate myself bench pressing I want to be in a comfortable, warm, dry environment and with okay sound insulation to muffle my screams. I also wouldn't mind some electricity for spotlights, speakers, tv etc.

I no building experience or tools. I aim to keep it as cheap as possible.

• The dimensions are 5 (L) x 3 (D) x 2.5 (H)m and will be within 2m of my boundaries.

• I think I have chalk 0.5-1m below me and living on the coast means things are cold and wet.

I don't really know what I am doing but here is my first draft:

• Walls: Render > concrete blocks > wooden studs / insulation > breathable membrane > plasterboard
  

• Roof: Slightly sloped flat roof: Waterproof felt > osb sheets > wooden studs / insulation > breathable membrane > plasterboard
  
  
• Any particular considerations about doors and windows?

• Foundations / floors I am not sure.
  
  For the foundations I researched and some suggest pouring 3 cubic meters of concrete given the dimensions. This is out of my budget. Instead would is be structurally safe to opt for concrete blocks as underground footings instead? If so how deep and thick should these be? I was then thinking of pouring a thinner layer of concrete in the area above the footings and between the main block walls. Above this I could integrate a weightlifting platform and the rest I can figure out later.

Finally the gym equipment (power cage) may require bolting to the floor or wall. Probably just the floor so I would need to consider alterations. Any idea what sort of weight such a simple block wall could tolerate? Would be sort of funny if the entire thing collapses and kills me. 

Regarding planning permission and building regs:

• It's within 50% of area of the original property
• The height limitations are adhered to considering the nearby property boundaries*
  
  * I need an extra foot above 2.5 to do certain exercises. Am I permitted to lower the standing space within the structure as long as I remain within the 2.5m height limit?
  
  
• Do I need to apply because I would be digging a deep foundation?
• Do I need to apply because I am pouring concrete / and or installing concrete block foundations? (non porous material)
• If I get an electrician to fit the wires and stuff do they apply the certificate and that's it for building regs?
  
  

THANK YOU!

Section62

Nordita said:

I want to construct a non-habitable outbuilding in my back garden to serve as a gym. When I decapitate myself bench pressing I want to be in a comfortable, warm, dry environment and with okay sound insulation to muffle my screams. I also wouldn't mind some electricity for spotlights, speakers, tv etc.

I no building experience or tools. I aim to keep it as cheap as possible.

• The dimensions are 5 (L) x 3 (D) x 2.5 (H)m and will be within 2m of my boundaries.

• I think I have chalk 0.5-1m below me and living on the coast means things are cold and wet.

I don't really know what I am doing but here is my first draft:

• Walls: Render > concrete blocks > wooden studs / insulation > breathable membrane > plasterboard
  

• Roof: Slightly sloped flat roof: Waterproof felt > osb sheets > wooden studs / insulation > breathable membrane > plasterboard
  
  
• Any particular considerations about doors and windows?

• Foundations / floors I am not sure.
  
  For the foundations I researched and some suggest pouring 3 cubic meters of concrete given the dimensions. This is out of my budget. Instead would is be structurally safe to opt for concrete blocks as underground footings instead? If so how deep and thick should these be? I was then thinking of pouring a thinner layer of concrete in the area above the footings and between the main block walls. Above this I could integrate a weightlifting platform and the rest I can figure out later.

Finally the gym equipment (power cage) may require bolting to the floor or wall. Probably just the floor so I would need to consider alterations. Any idea what sort of weight such a simple block wall could tolerate? Would be sort of funny if the entire thing collapses and kills me. 

Regarding planning permission and building regs:

• It's within 50% of area of the original property
• The height limitations are adhered to considering the nearby property boundaries*
  
  * I need an extra foot above 2.5 to do certain exercises. Am I permitted to lower the standing space within the structure as long as I remain within the 2.5m height limit?
  
  
• Do I need to apply because I would be digging a deep foundation?
• Do I need to apply because I am pouring concrete / and or installing concrete block foundations? (non porous material)
• If I get an electrician to fit the wires and stuff do they apply the certificate and that's it for building regs?
  
  

THANK YOU!

Render on concrete block is not very forgiving of settlement or other movement.  You need to make sure you have adequate foundations. Alternatively, with no previous DIY experience, consider a timber frame construction.

Digging a hole to lower the floor level to keep within the PD rules is not a very good idea - it creates additional risks with damp and flooding, and when you come to sell the property it will look like a bodge.  If you need the additional headroom then apply for planning consent for the building you want... almost always better than trying to scheme a way around the PD rules.

Nordita

Reply is much appreciate thanks. Points considered. Would you know what would qualify as adequate foundations for a block construction as described? A pit fille with concrete is out of my price point.

FreeBear

If you are going to the expense of building a solid structure in blockwork, it needs a decent foundation. If that requires 3m³ of concrete, then you'll need to stump up the money for it. Whilst the Victorians often built footings directly on to compacted soil, that style of construction is just not good enough today.

Do it properly.

stuart45

It's possible to use less concrete if you use just 225mm at the bottom of the trenches, or the minimum for regs is 150mm. However you need more blockwork then. It always used to be done like this, but nowadays the trench fill method is the norm.
In chalk you will probably find that once down about 2ft the ground is quite solid. Back in the 70's houses on the Sussex coast were usually built on 9 inches of concrete in a 2ft 6in deep trench. You have to remember to make the trench wide enough to be able to work in.

Section62

stuart45 said:

It's possible to use less concrete if you use just 225mm at the bottom of the trenches, or the minimum for regs is 150mm. However you need more blockwork then. It always used to be done like this, but nowadays the trench fill method is the norm.
In chalk you will probably find that once down about 2ft the ground is quite solid. Back in the 70's houses on the Sussex coast were usually built on 9 inches of concrete in a 2ft 6in deep trench. You have to remember to make the trench wide enough to be able to work in.

Good point about the trench width - which also means care needs to be taken to make sure the backfill is well compacted.  People I know stopped using strip foundations because of the hassle of dealing with the excavated material.  If there's no use for it as fill on site it gets expensive to have it carted away.

The other issue is needing to use suitable blocks below DPC... which don't always work out cheaper than mass concrete in a narrower trench.

stuart45

Trench fill does work out cheaper, especially when the labour for brickwork is added to the cost. Whichever way it's done A decent foundation is a must for an OPC built shed. It wasn't so bad when lime mortar was used and the footings were spread.

Nordita

@stuart45 @Section62

Thank you for your answers I think I understand more.

• How would I go about working out the depth of the trench from which the concrete pour begins?
• And similarly the width of the concrete layer?
• Is it permissible to return the disturbed soil into the trench?

 

stuart45

Once the concrete is in the trench and gone off the blockwork can be taken up to DPC height. When that's gone off you can backfill the trench with some of the earth that was dug out. The depth of the trench depends on you getting down to solid ground. You need to peg out the trench to get the concrete level.
The width will depend on whether you need room to work in the trench.