TIMBER GARAGE - BASE AND SUBBASE:

[RONCHAM]

Hi all - I'm proposing to build a timber car-garage (24' x 10') on clay soil. I'm advised to lay a 6" subbbase of hardcore or Type 1 (which are quite different size/grading) with 4-6" of concrete on top. Also, to lay a damp proof membrane, either on top or beneath the subbase. With this structure, is a subbase necessary and is Type 1 preferable? Is the subbase to disperse load or drain water (or both)? And is a DP membrane really necessary, is rising damp likely to be a problem? If so, is membrane best above or below subbase? I've been asked (by the company who will erect the garage) to lay a single course of engineering bricks around the perimeter, just a couple of inches inside the edge of the concrete - would several inches be better to prevent possible edge cracking? Do I need any reinforcement or mesh in the concrete base? Any advice greatly appreciated. Thanks.

 

[Deluks]

Sub can be a bit of both, smashed up brick, roof tile and stone, with a couple of bags of mot dumped on top and whacked.

Mesh (a142) is recommended, as is 6" deep concrete.

In heavy rain, water will likely run under any brickwork built straight up from the slab. (There are options but I haven't time to go into it now.)

Dpm not essential, but I would anyway, use 25mm building sand as blinding on top of the compacted mot.

 

[RONCHAM]

Thanks for your quick and very helpful reply. I take it sand layer is to sit DPM on? Re. water, I was thinking of ground water rather than rain. If no subbase, I'm not sure if there's much chance of water rising up thru 6" of concrete to give a damp floor??

 

[Deluks]

More for keeping the concrete from drying out by losing moisture to the hardcore. Yep, sand to protect dpm.

 

[RONCHAM]

Thanks again for your help.

 

[r896neo]

When he mentioned rain water he was refering to the course of bricks they asked you to lay around the perimeter.

Because concrete is relatively impervious to water but mortar is not, any rain which hits the outside edge of the slab will seep through the mortar and into the garage. A mortar fillet along the perimeter will help.

 

[RONCHAM]

Thanks for that - a good and simple idea! Garage supplier said set perimeter bricks 2" back from edge of concrete. Seems very close to me? - wouldn't weight be "off the edge of the concrete" (I assume it disperses at 45 degrees??).

 

[RONCHAM]

Hi again! -

I'm ordering some Type 1 soon to lay on clay subsoil for garage base - it comes in 850Kg jumbo bags and I assume they're 1 cu. metre? For estimating purposes, after compacting, what approx. volume will they fill? Calculated volume required is about 2 cu. metre so I'm hoping 3 bags will be adequate? There will be probably be a gap of a week or two before I lay concrete etc. - do I need to worry especially about keeping Type 1 dry or will it be dry enough from normal drainage/evaporation??
Thanks for any advice.

 

[theoldun]

MOT fill. Depending on moisture content and stone to fines ratio allow 2 tonne per cube metre.

Your base 7.31x3.00 = 21.93ms, less 0.225x0.225 perimeter toe = 4.63ms = 17.3ms x .0.100 thick = 1.73mc x2 = 3.46 tonne divide by 850 = 4 bags.

Do not worry how wet fill gets after laid and compacted.
oldun

 

[RONCHAM]

Thanks for that - I always respect advice from an oldun and very impressive maths! I'm guessing those jumbo bags are 1 cu. metre of material and although they seem already well compacted, must obviously compact considerably more at nearly two and half bags per compacted cube metre?

Your (4 bags) calc was for 100mm thickness Type 1 so for 150mm thick I assume another two bags? Re. perimeter toe, is that for thicker concrete around perimeter and lapping down over the Type 1??
Thanks.

 

[RONCHAM]

Hi again -

Can anyone advise approx. prices for ready-mixed concrete or better still those guys who mix it at the house? For an everyday material, so extensively used, it seems to be the world's best kept secret!! I'm only guessing but reckon about £120 cu. m. mixed at house (Horsham area).
Thanks.

 

[RONCHAM]

For the benefit of anyone undertaking a similar project to the above, I can advise how things have worked out (so far). The area concerned was less than envisaged as an existing concrete base is to be incorporated. Actual total area was 150 sq. ft (about 14 sq. m.) with Type 1 and hardcore to a depth of about 7" (175 mm.) ie a gross volume of about 90 cu. ft. (about 2.5 cu. m.). 4 bags of ballast were used (each 850/900 Kgs) but some hardcore to hand was also used. With ballast alone I am sure 5 bags would have been sufficient. The initial 3 bags were poorly graded with a high sand content and perhaps filled a smaller volume. Based on 5 bags, this equates to about 1.3T/cu. yd or 1.7T/cu. m. A bit less than advised by theoldun probably because the ballast was hand-tamped rather than machine-tamped. Thanks to all for most helpful advice.

 

[RONCHAM]

I can now answer my own (earlier) question about concrete prices which may be of interest to others. Those guys who mix at the door have a minimum Saturday charge of £440 for 3 cu. m. and on weekdays a minimum charge of £305 for 2 cu. m. - so about £150/ cu. m. (dearer than I thought!). Hope this info hasn't contravened the Official Secrets Act!!

 

[RedHerring2]

Sorry, didn't see your enquiry re concrete prices. I had 4 m ³ for £510 very recently. (about £127.50 per m ³ ) That was a full load.

I also had a pump at £150. Just 1 hour so no extras to pay for. That includes the two guys to work the pump and hose. Excellent value I think.

 

[RONCHAM]

Hi - thanks for that . I think concrete price varies by region - I'm in Horsham, Sussex (but I don't earn megabucks! ). I was offered a dumper for an extra £40 which is not bad and would save a lot of wheelbarrow trips, especially as I won't be mob-handed for this job.