Looking for foundations detail and cost reduction advice - detached garage

[Pabbles]

Hi everyone.

 

My head is spinning. I've just spent an hour on Google trying to find a detailed cross section of a suitable foundation for my new garage build. I've seen loads but can't seem to find one that is a close match for our build.

 

The plan is attached.

 

• The only thing to add is that the floor slab will be quite thick at 200mm, reinforced (specific requirements for the vehicle lifts).
• Construction is 65mm Imperial Olde Shire facing brick with 7n dense blocks on the inner course with a 100mm cavity.
• SE has advised a depth of 1.3m based on findings from test holes. But frustratingly did not fully detail construction before we parted ways.

 

It is quite a long length of foundations at 39m so I'd like to be able to draw and then cost up options for trench fill, trench blocks and dense block as a few pounds a metre could quickly add up.

 

Does anyone have. nice, well proven cross section they could share? Ideally looking for one with detail on of block/brick types at each course and positioning of DPM/cavity tray.

 

Any advice on how to keep the costs down given current concrete prices would be very welcome.

 

I already know I'm going to feel silly when someone links to a blindingly obvious place, but I would really welcome the help as my Google and NHBC kung-fu is failing me.

 

 

 

 

 

Edited December 9, 2023 by Pabbles

[Chanmenie]

I’m sure more experienced members will be along, but for starters 600mm wide strip footings of 300mm deep concrete followed by trench blocks.

compare the cost of this against going for trench fill with concrete.

Concrete is expensive but probably less than the labour costs of laying the trench blocks unless you’re planning on doing it yourself.

[Pabbles]

Thank you.

 

Yes - my ideal is to be able to draw up all three options so that I can work out the material costs for each. I won't be doing it myself, I have a great builder - we work well together but the quotation phase is always tricky. We've found a good pattern where I plan everything out to Nth degree, cover materials costs and then he quotes a healthy but transparent labour price that keeps his margin safe but means I'm not paying over the odds.

Edited December 9, 2023 by Pabbles

[markc]

200mm seems way over the top for a workshop slab, at work our yard and workshop slabs are 150mm with 1 layer of mesh and we often have 20t point loads. Nothing wrong with thicker at strategic locations, like under lifts or presses etc.

Edited December 9, 2023 by markc

[ETC]

Why are you putting in a DPM/cavity tray?

[Pabbles]

@markc I'm going off the specs for the 2-post lift. Some of my vehicles weight close to 3 tonnes. The lift manufacturer specs 200-250mm of reinforced C25/C30 concrete as a base. A failing 2-post lift is a scary thing. They do offer a drawing showing how to lay a H-pattern of thicker concrete in strategic areas as you suggest, but I haven't yet been able to compare the costs of the additional shuttering/layout for a deeper H shape vs just doing the whole slab in 200mm.

 

@ETC I thought that was standard practice? Am I missing something obvious?

[Roger440]

1 minute ago, Pabbles said:

@markc I'm going off the specs for the 2-post lift. Some of my vehicles weight close to 3 tonnes. The lift manufacturer specs 200-250mm of reinforced C25/C30 concrete as a base. A failing 2-post lift is a scary thing. They do offer a drawing showing how to lay a H-pattern of thicker concrete in strategic areas as you suggest, but I haven't yet been able to compare the costs of the additional shuttering/layout for a deeper H shape vs just doing the whole slab in 200mm.

 

@ETC I thought that was standard practice? Am I missing something obvious?

 

Are you sure?

 

General rule of thumb for any installer ive come across is 150mm if its reinforced concrete. All the ramps ive had put up, none had, nor specified 200mm. None fell over either.

 

If it really has to be 200, then as suggested thicken in the appropiate area. Going to save a heap of concrete. 200mm is total over kill for something that will only see cars.

 

Ive also installed a 2 poster on 4 inch non re-inforced concrete. But it did have an H frame. Which substantially reduces the floor rerquirements.

 

Have you considered changing ramp supplier?

 

[ETC]

It’s a garage. 

[Roger440]

Given your intended use, i think you would be nuts not to fit a DPM.

 

But then, id be insulating under it too if it was mine and going to the lengths you are.

[Pabbles]

Thanks both - that's really helpful.

 

So, going off @ETC's point then from the above if the building was just used for periodic storage then DPM may not apply. However in this case, the building will be used extensively (starting at 1000 hours a year and moving up towards 2000 hours as I hit retirement and contains a lot of expensive machinery. It really is an obsession of mine. As such I'd really like to have a good level of protection against moisture ingress so would stick with DPM.

 

@Roger440. Mate, I owe you a pint. The lift manufacturer I had planned to use (TwinBusch) specified 200mm of C20/C25 concrete. However you are quite right, most alternative manufacturers specify 150mm (granted, as a minimum) even for higher capacity lifts. As such, you've just saved me a 25% of my concrete costs simply by picking another lift. Thank you!

Edited December 10, 2023 by Pabbles

[kandgmitchell]

80m2 x 50mm = 4m3 x £138 +vat = £662 - that's about 100 London pints