The Brew Cabin

Posted: **Fri Jul 22, 2005 6:04 pm**

Hi,

I have recently had a new conservatory fitted and need to decide on an approach to the bedding and sub base for foot traffic only around it.

Area 1, is 0.5m x 5.5m and will pave over 20mm limestone chippings which contain a land drain below. The depth of the chippings is about 300mm or so.

Area 2, is approx 4m x 3m currently has 100mm of concrete over it in one part and just earth in another (ex flower bed). Ideally I would like to reduce the level by an inch or two.

The underlying ground is very solid and full of rocks/stone etc (I live near an old quarry), its quite hard to dig by hand and the location is not very suitable for a mini-digger ... so I would like to avoid too much hand digging if possible.

I have looked at this sites reccomendations and I am not sure if I need a sub base at all for foot traffic.

A builder recently suggested putting 100mm of compacted MoT aggregate down first which seems overkill and mean taking off a minimum of 150mm (probably more) off whats there now.

Thanks in advance for any advice,

Posted: **Tue Jul 26, 2005 2:11 pm**

Hi, I'm not an expert but can offer my opinion based on laying my own Indian Sandstone patio and from hanging around this forum for a while :;): .

A sub-base of MOT type 1 material is not mandatory for a light use foot traffic only patio. However, it may be required if you have soft ground or need to raise levels or fill voids.

I'm not a drainage person so don't understand the implications of paving over chippings covering a land drain. I know that land drains work by the water migrating from adjoining soil into the chippings, but not sure if covering them is a problem. However, if the chippings don't provide a sound base, then you may need a subbase material before your bedding material.

In terms of levels, you can work out how far you need to dig down using the following formula :

(average depth of flagstones + 35-50mm bed + any sub-base required)

You mention having hard ground, so this leads me to believe you probably don't need a sub-base if the levels are correct. However, you must not extend the depth of the bedding material to account for any variations in level. A maximum of about 50mm is recommended otherwise the bedding layer can become too weak.

Good Luck

.

Posted: **Tue Jul 26, 2005 5:42 pm**

Thanks for the reply.

The land drain is around the perimeter of the conservatory to keep the stone courses below the DPC dry-ish. The ground water for the area when flagged will discharge into a gully connected to the land drain which is then drained into the main drain.

Currently the chippings will take the water from above ... but this is just how the builders left it!

The upshot of flagging over the chipping covered drain is the same amount of water in the land drain but just entering at different point (closer to the main drain).

The chipping area to be flagged over the land drain is only a few m2 and I have tested that the gully and land drain work for prolonged periods (with a hose pipe, and inspected the coupling to the main drain

and observed it working).

Also the chippings when compacted should form a solid sub-base for the flags themselves I guess?

Hence, I am now thinking flagging Area 1 will be fine?

Re: Area 2 the existing 100 mm concrete has to come out and so:

(average depth of flagstones + 35-50mm bed + any sub-base required)

Gives:

30mm + 35..50mm = 65...80mm

So the level should be a bit lower I think. Although it will need to rise an inch or two to the gully at the boundary of area 1 ... I don't care too much at this point though.

The ground is quite solid so I am now thinking I'll omit the sub-base and just fill in for the levels with aggregate, as suggested and compact.

... unless anyone can forsee problems with this approach!

Cheers,

Alex.

Posted: **Fri Aug 19, 2005 3:55 pm**

Seems fine to me and I assume it's all done and dusted by now.

The Brew Cabin

Indian stone bedding/sub-base - Advice on indian stone bedding/sub-base