Digging up "raft" foundation under houseb for block paving

[Toilet Duck]

Hello all,

For the resident PH experts out there, if a house sits on a "raft" that borders the entire property (semi detached) by about two feet, if you wanted block paving on the driveway that runs along the side of the property (against the house wall), would the block paving sit on top of the "raft," or would the "raft" be dug up?

Thanks in advance

[Busamav]

do not interfere with, or undermine the raft

[Gav147]

It has to sit on top, don't start breaking up your foundations.

If you do lay it on top make sure you have a minimum of 150mm between the finished surface of the block paving and your houses dpc.

[Globs]

Busamav said:

do not interfere with, or undermine the raft

+1

Trust us on this, do what you like with the blocks, but leave the raft intact.

[Toilet Duck]

Thanks for the replies.

I've had two firms round for quotes so far. One said they would lay the block paving on top of the raft, the other said they would dig it up. I got the details for both firms off the Marshalls website/register thing. Jesus Christ it makes you wonder doesn't it.

[m3jappa]

Canvey Island?


I would not touch the raft, that said i do know of people who have and have stood there for 3 days or so taking it back which imo is both stupid and dangerous potentially. From a contractors point of view its just not worth the risk either way.

The only problem with paving over the raft (and i have done it a couple of times) is that the sharp sand bed which is over the raft will compact more than the sharp over the type 1 (if they have said crushed concrete do a runner).

What this means is when the blocks are compacted they will barely move. The trick is to just tread in the sand over the raft so its still quite soft, then when you compact the paving it will wack down better. You will still get a few broken blocks though.(which can be changed), and it still wont go down quite as much as the rest of the paving. It requires a little thought to do right thats all.

Overlaying concrete is not easy or nice to do either way but in this case you dont have a lot of choice.

hth

[davidjpowell]

obvious alternative is to leave yourself a small border where the raft is and just pave upto.

[Toilet Duck]

m3jappa said:

useful info

Cheers for that. Looked at your profile/website as well

davidjpowell said:

obvious alternative is to leave yourself a small border where the raft is and just pave upto.

The trouble with doing that is that as the raft is (guesstimate) two foot plus wide, bordering up to it would mean a driveway reduced in width by two foot plus along the side of the house. Might not sound much, but this would make a big difference and, in my opinion, look a bit cack (nice block driveway bordered by a band of crappy looking concrete).

Thanks again for all the replies, PH truely is the font of all knowledge

[Toilet Duck]

Gav147 said:

If you do lay it on top make sure you have a minimum of 150mm between the finished surface of the block paving and your houses dpc.

I was just thinking about this. How do I tell at what level the house dpc starts? If the block paving/base etc is, say, 150mm deep in total, then if the dpc needs to be another 150mm higher, what happens if its not? Other than digging up the raft to get the block paving lower (which PH has taught me is a definate NO NO), how do you modify the existing dpc to make it higher if its currently too low (again I don't know if this is the case, I'm just thinking ahead for possible problems).

Cheers all

[Henry-F]

m3jappa said:

Some handy advice

Great website and really enjoyed looking at your gallery of completed jobs. Some excellent work in there, not just from a quality point of view but also some interesting features and solutions.

Your passion and attention to detail shines through.

Keep smiling, and get back to laying them bricks

Henry

[Stegel]

Toilet Duck said:

I was just thinking about this. How do I tell at what level the house dpc starts? If the block paving/base etc is, say, 150mm deep in total, then if the dpc needs to be another 150mm higher, what happens if its not? Other than digging up the raft to get the block paving lower (which PH has taught me is a definate NO NO), how do you modify the existing dpc to make it higher if its currently too low (again I don't know if this is the case, I'm just thinking ahead for possible problems).

Cheers all

The reasoning for the 150mm between DPC and paving is to prevent the former being bridged and to limit splashback bouncing on the paving and wetting the wall above the DPC. Your DPC will usualy be identifiable by a thicker than usual horizontal mortar joint, possibly with the DPC material (bituminous felt, polythene etc.) visible; there may also be a different brick used above and below DPC. The height of the DPC is fixed, and cannot be altered, so if there is insufficient height between the DPC and the exposed raft, paving up to the edge of the raft will be your only option.

[Munter]

Just to visualise rafts gone wrong as background rather than advise...

http://maps.google.co.uk/maps?hl=en&q=oswestry...

[m3jappa]

Thanks for the comment! It means a lot when people appreciate the effort that goes into it all.

With regard to the d.p.c

Sometimes you have no choice at all but to go higher than the 150mm, some houses will dictate that it has to go higher, for example they may have a funny level gargage or just be built at bizaree levels compared to neighbours houses.

I am not 100% on this but the 150mm below rule will only properly be enforced on new builds, i am fairly certain on that?

I have had to do it numerous times and am totally happy to go to 1 course below, obviously i explain everything to the customer and often it is suggested as a way to me (as sometimes they see it as a way to save money, especially on patios). Above one course and i'm starting to tell them to seriously consider what they are doing. I have a couple of times had to paint on dpc, its their houses and its up to them at the end of the day but i dont like it at all.

If the water runs AWAY from the house it WILL be fine, i have even seen ground levels at dpc level before and they had no problems, even my parents house was actually like that and i lived there for too long There was just lots of fall away from it so water never sat there.

If it falls towards the house then regardless of dpc then there should be an aco (line) drain fitted either way as water sitting against a house will cause problems no matter what the level.

You then need to keep it as low as you can with the sand layer (but not too thin as it won't wack down).

The first thing any decent contractor should be looking at is what am i going to do with the water, after that its plain sailing (usually).

[Toilet Duck]

m3jappa said:

If it falls towards the house then regardless of dpc then there should be an aco (line) drain fitted either way as water sitting against a house will cause problems no matter what the level.

I'd forgotten about this; the firm that wanted to dig up the raft said that they would fit aco's.

[Gav147]

Toilet Duck said:

I'd forgotten about this; the firm that wanted to dig up the raft said that they would fit aco's.

The thing is you can't just dig up a raft, it doesn't finish at the brickwork, it incorporates the foundation and sub floor as one reinforced foundation. here's a pic that shows the detail of a raft a lot easier than i can explain it on here and also shows why you shouldn't let them dig it up



eta - Just read jappa's post, the 150mm I pointed out is a requirement on new builds but also should be stuck too unless absolutely forced to do otherwise as jappa posted you will probably be fine with a bit less and you should be aware of the fact if the contractor is going to do so as splash back from rain onto brickwork can cause it to deteriorate over time.

also nice looking site Jappa and some very nice looking patios/drives.

Edited by Gav147 on Friday 1st April 20:50

[Toilet Duck]

Gav147 said:

useful diagram

Cheers for posting that diagram, its enabled me to see what a raft actually is. I wasn't 100% sure but thats made it a lot clearer. Thank you!