Raft foundations for a garage - am I doing it right?

[impreza280]

I'm building a garage down the side of my house. It will be a freestanding timber framed barn style single garage, with a wooden frame on a dwarf wall. It will measure 6m x 2.5m (exempt from building regs) and will have the house one side and a 6 inch gap until the neighbour's fence on the other side.

So far I have dug a 300mm deep hole just a fraction bigger than I need. My current plan is as follows:
1. 100mm of scalpings, levelled, followed by wacker plate, followed by 50mm sand blinding, followed by wacker plate.
2. Formwork made from 150x25mm with posts every 1m
3. Damp proof membrane inside formwork - not sure what guage yet.
4. Steel mesh on spacers - not sure what spec of steel or how high spacers should be.
5. Self-levelling concrete (LaFarge Agilia) - 150mm deep.
6. Remove formwork after 4 days.

I'd really appreciate comments on this plan.

Thanks
Dave

 

[Shytalkz]

Why are you making a raft?

 

[noseall]

Why are you making a raft?

My thoughts entirely. Strip foundations are tried and tested and relatively simple.

 

[impreza280]

Fair question - though I'm relatively close to neighbour's trees, one of which has a TPO on it. I'm also about a foot lower than the neighbour's garden. A stipulation of the planning permission was that I wasn't allowed to do strip foundations. 2 builders who came to quote both thought that a raft was the best option.

 

[Shytalkz]

Soil type? Beam and base alternative?

 

[^woody^]

AFAIK, the planners can/should not put such a condition on any permission.

Planning conditions have to meet certain criteria, and foundations are a technical issue and not a planning one - in which case it will be unreasonable and unenforceable.

 

[Shytalkz]

You sure about that? They get very uppity when footings are close to trees, especially ones with TPOs on them.

 

[^woody^]

Well there is a bit more to it.

The planners can consider the effect of the development on the protected trees, but it is not a simple matter of them saying you must use a particular foundation design. There has to be clear evidence of the proposal being likely to cause damage to the tree, and it is something that the applicant can disprove if necessary.

In this case, if a raft is specifically made a condition of granting the permission, then if a raft was technically unsuitable for the location or disproportionately expensive, then it will be unreasonable.

So yes, the planners must consider the effects of the proposal in deciding if permission should be granted or not, but any conditions imposed have to be carefully considered. In this case (and others) a blanket statement requiring a raft could be considered unreasonable unless it is the only way of preventing definite damage if permission was granted and the work gone ahead. And I don't think the planners can make that assertion.

What would happen if these trees cause the raft to bob up and down and the walls crack?

 

[Shytalkz]

Unless what they actually said was the OP can't use strip/trenchfill founds and would have to think of sommat else, and the raft is what's come out of that; but not a stipulation of planners to use a raft, per se?

 

[impreza280]

The tree with the TPO is 6+ metres away (3 metre canopy) and the other trees are relatively small or further away. The soil type is a mix of clay and flint/chalk.

If I go for an alternative to a raft then I'm likely to need to get someone in to do it, whereas a raft I feel confident of doing myself with the right advice, which I'm sure will then be the cheaper option. And having asked builders for quotes they pointed to a raft as being the best solution in this particular case.

If I were to still go ahead with a raft, am I along the right lines in terms of what order to do things in?

 

[Shytalkz]

Can't see why you're not just using strip/trenchfill at those distances, then - you won't be causing any irreperable damage to the root system. That'll be much cheaper than a raft.