Building a block shed on a concrete slab, is this okay?

happyhero

Hi I have a 10 x 8 feet concrete slab built on very tough to dig soil. It has 4.5" of compacted Mot1 base and 4.5" of concrete with a rebar grid in it. I want to build a block wall shed (not sure what blocks I would need to use) laid around the edge of the slab up to a height of about 7 feet and then a timber felt roof on top.

I want to clad the walls with timber to match adjacent wood shed so that it looks like a timber shed on the outside but is block built inside.

Can I use the existing base as it is? 

What blocks would/could I use outside for lightness etc?

grumbler

Not sure about usual blocks, but thermalite ones are really very light and have better insulating properties (more expensive though).

jonnydeppiwish!

For a shed I’d say it should be okay - use engineering brinks for 2 courses, followed by 2 concrete block courses then thermalite or equivalent. 

Assuming a flat roof (no tiles, then that should be perfectly adequate.

GDB2222

Can I ask what the reason is for the blocks, if you are cladding it with wood? 

Mutton_Geoff

I'd also use EDPM not felt on the roof.

35har1old

happyhero said:

Hi I have a 10 x 8 feet concrete slab built on very tough to dig soil. It has 4.5" of compacted Mot1 base and 4.5" of concrete with a rebar grid in it. I want to build a block wall shed (not sure what blocks I would need to use) laid around the edge of the slab up to a height of about 7 feet and then a timber felt roof on top.

I want to clad the walls with timber to match adjacent wood shed so that it looks like a timber shed on the outside but is block built inside.

Can I use the existing base as it is? 

What blocks would/could I use outside for lightness etc?

The roof would require a minimum fall of 1:80
To fix cladding you would need to masonry nail laths to the blocks to attach cladding to.
Two of the walls would need timber wall plates bedded then nailed once cement had set for a number of days to fix the timber joist to.
If you are going to have a window you will need a head and then fully trim around window with timber so you have a edge for cladding to finish into.
The door frame would also require to be fixed with masonry nails. You would need to fix a weather bar to prevent damp from entering.

Joist 47 x 150mm x 2.4m required ?

Concrete block around 0.95p 
Thermalite from £1.92 3.6N to £3.30 7N depending on strength

I know my choice would be to construct a timber shed

Section62

happyhero said:

Hi I have a 10 x 8 feet concrete slab built on very tough to dig soil. It has 4.5" of compacted Mot1 base and 4.5" of concrete with a rebar grid in it. I want to build a block wall shed (not sure what blocks I would need to use) laid around the edge of the slab up to a height of about 7 feet and then a timber felt roof on top.

I think 4.5" is a little on the thin side to act as a foundation for a wall of that height + heavy roof.  Where depth-wise was the steel mesh laid within the concrete?

chrisw

35har1old said:

happyhero said:

Hi I have a 10 x 8 feet concrete slab built on very tough to dig soil. It has 4.5" of compacted Mot1 base and 4.5" of concrete with a rebar grid in it. I want to build a block wall shed (not sure what blocks I would need to use) laid around the edge of the slab up to a height of about 7 feet and then a timber felt roof on top.

I want to clad the walls with timber to match adjacent wood shed so that it looks like a timber shed on the outside but is block built inside.

Can I use the existing base as it is? 

What blocks would/could I use outside for lightness etc?

I know my choice would be to construct a timber shed

I built a shed out of concrete blocks at our last house. The condensation in it was terrible. I put a load of air vents around the roof but it still dripped off the ceiling making things wet and rusty.

Ive got a timber shed now and there's no sign of condensation.

stuart45

Section62 said:

happyhero said:

Hi I have a 10 x 8 feet concrete slab built on very tough to dig soil. It has 4.5" of compacted Mot1 base and 4.5" of concrete with a rebar grid in it. I want to build a block wall shed (not sure what blocks I would need to use) laid around the edge of the slab up to a height of about 7 feet and then a timber felt roof on top.

I think 4.5" is a little on the thin side to act as a foundation for a wall of that height + heavy roof.  Where depth-wise was the steel mesh laid within the concrete?

A raft foundation for a garage normally had the edge thickened to around 12 inches, so technically you are correct that 4.5 inches is a bit thin. 

Section62

chrisw said:

I built a shed out of concrete blocks at our last house. The condensation in it was terrible. I put a load of air vents around the roof but it still dripped off the ceiling making things wet and rusty.

Ive got a timber shed now and there's no sign of condensation.

The problem would primarily have been with the roof construction rather than the walls though. Although if the walls has no exterior weather protection then that would have been an additional source of damp (air) within the shed.

With outbuildings condensation tends to form on the least thermally-efficient interior surface, i.e. the coldest surface.  This tends to be the underside of the roof because the roof covering typically has low thermal resistance, the roof is more exposed to cooling effects, and warmer moister air will tend to be in contact with the underside.

Concrete blocks also provide a decent thermal mass, so heat absorbed during the day will help keep the walls warmer compared to the (uninsulated) roof during the night.

It may seem counterintuitive to insulate an unheated outbuilding, but the most effective solution to condensation dripping from a ceiling is often to add insulation (preferably during construction).