250mm footings or raft

[ari2019]

Ive been given the option of a raft or footings for foundations for a garage however i havent priced them up yet. Been getting conflicting advice concerning pricing from others that have had work like this previous. Its for a basic garage, ground conditions are good. House was built in 1997 and theres been hardcore stones on the area where where the garage is going. I park here, theres a slight slope and never no damp issues.

Given that ive 2 options, im thinking of going with the strip but just need some advice to refresh my memory. Ive worked on footings before but its been a some years, i forget the thickness of them. I will be getting my engineer to draw up the correct footings however for now, im trying to get a rough estimate of what ill need. With the ground being good to build on( or so i think for now before any trail holes dug) Here we dig 800mm to 1m deep by 600mm wide if the conditions are good. This is below the frost line and 250mm-350mm thick concrete x 600mm wide then build up in block of the founds to dpc level. 250mm being the minimum thickness if i remember correctly. This is for single story garages and bungalows. For double story houses the thickness and width could be more.

Ive been on jobs where there was no need for rebar or mesh however if i was to do this, throwing in some 18mm mesh into the footings for all the cost of it, would help greatly. Rebar even more.

The raft is a raft. Lots of hardcore, steel and concrete. Been getting conflicting advice some sating they work out the same amount of money and work, others say the raft is dearer and had 2 say the footings can work out dearer if they are deep plus one has to take into consideration all the extra work needed for the slab floor along with the hardcore wacking, dpc and mesh.

 

[tony1851]

A strip only needs to be 450-500 wide by 150 thick if it's just for a garage.

 

[ari2019]

A strip only needs to be 450-500 wide by 150 thick if it's just for a garage.

Sounds good however any ive worked on the clients/builders probably went over kill for the sake of a few extra pound.

 

[Ian H]

Fill them closer to ground level to save the labour of laying more blocks.

 

[ari2019]

Fill them closer to ground level to save the labour of laying more blocks.

But isnt that above the frost line?

 

[stuart45]

It's the base of the concrete that counts, not the base of the blockwork. Trench fill is common nowadays.

 

[^woody^]

And yet if you have strip foundations, in addition to the concrete strip and the foundation walls and extra excavation and back fill, you still need hardcore and a big, errr, slab of concrete just like a raft.

 

[John D v2.0]

I thought the big slab of concrete for the floor didn't have to be as strong as a raft that has the actual walls loading right on the edge of it? Or is a single story single skin building light enough to just go straight on the slab?

 

[^woody^]

The edges would be thickened to accommodate the wall loading.

 

[noseall]

The edges would be thickened

A bit like a strip foundation.

Just build a strip footing - its simpler. Rafts require reinforcement and it's time consuming, costly and
unnecessary.

 

[^woody^]

A bit like a strip foundation.

Just build a strip footing - its simpler. Rafts require reinforcement and it's time consuming, costly and
unnecessary.

You should find that all in, in terms of material amount and costs, labour costs, excavation, spoil removal, skips, time and ease, a raft will win every time for a garage or similar simple structure. And even in terms of risk of future potential ground movent or maintenance, a raft is less susceptible thus a safer risk.

Reinforcement is a sheet of a252 dropped in. It's not the Hoover dam.

All those pre and post war garages sheds and outhouses in gardens up and down the country are still going strong.

 

[Diver Fred]

Go for a raft. 500/600 deep by 300/350 wide at edges, 150/200 deep across the full width. Compact the ground before adding A252 reinforcing mesh with sides turned down into edges. Can even finish 50mm above ground level.
One pour of concrete and level off.

Neat, simple and the 'pad' and garage will move as one. Mesh will help stop cracks or the pad from 'spreading'.

Don't forget the weight of a car will be similar or greater than the weight of the garage. If you go for footings and separate 'pad' for the vehicle to stand on it will sink over time.

 

[Notch7]

A friend recently did a small house extension. It was originally planned to be conventional trench fill, but due to a tree just under 5 metres away, the BI insisted on something more involved.

The job ended up being a raft with reinforcing. After adding up the cost the raft came out slightly cheaper.

The problem with a raft is dealing with the DPM arrangement. Drawings often show the skirt stepped so the brickwork starts below the top of the slab -possible to do, but needs a fair bit of formwork to achieve.

As Noseall says, for a builder the quickest, simplest option is generally dig a trench and mass fill -its what all builders do these days, more concrete but less labour. I suspect the introduction of ready mix concrete made the move from strip foundations with blockwork out of the ground to be phased out.

The annoying thing with trench fill is the need to cast foundations and slab at separate times. Expensive on jobs where a concrete pump is used as it means hiring twice. Almost all the jobs I get involved in now are pumped -Its £350 for the pump, but only needs 1 guy on site, is done in 1 hour and less messy if done well. Helps with difficult access too. The disadvantage is you need a space about 25 metres x 4m for the volumetric concrete mixer plus the pump truck together. It wont be long before trucks that mix and pump become standard -mixermate have one.

 

[noseall]

Yeah go for a raft and then join the queue of people on DIYnot wanting to know why they have water entering into their garage, via the slab, every time it rains.
Honestly, strip footing is easier - every time.

 

[^woody^]

No need for a DPM or DPC in a garage.

Condensation is going to occur regardless, and the walls are typically not cavity then there will be lots of moisture and humidity, so the main criteria is ventilation.

Lots of people build garages like houses and is a waste of time.

But, if need be a water proofing additive can be used in the raft.