The Brew Cabin

Hi there,

I am a bit stuck and would really appreciate some help

I have three questions I would like your advice on:

1) The old patio, crazy paving, is 165mm below the DPc, but I have built a pergola and there is no water coming down onto the patio area. So I still need to worry about the DPC? the new slabs are 32mm thick and I am intending to lay them onto a 50mm bedding layer, ie the new patio will be 83mm below the DPC is this ok under the pergola?

2) which is the better method of construction, rigid or flexible? I seem to prefer rigid but want to do the best job possible, which would you say is the better method and why?

3) If you tell me to remove the old patio, should I then lay a 75mm layer of hardcore, ie 6(Ballast):1 (cement). Will this be enough. Or as the old patio has hardcore beneath it, should I remove this as well or just the crazy paving and therefore use the old sub gbase layer?? Also how do i prevent the new hardcore from spilling out around the edges?


Please help, I am unsure as to which way I should proceed.

Kind Regards

Ajay

1 - yes, even though you think there is no water penetrating beneath the pergola, you should still aim to comply with the 150mm below dpc rule. 83mm below dpc is less than ideal and, as I've said a thousand times, it's just the sort of thing a surveyor notices when valuing a property and it can lead to a reduced valuation, or an insistance for remedial work. It's always best to do the job properly.

2 - Laying method for a patio? Depends on the materials being used, but for flags/slabs, I'd nearly always go with a rigid construction because it offers better stability for larger paving units that are bed-dependent for their load-bearing capability.

3 - if the old patio comes out, then you can make up the levels using a cement-bopund material (your 6:1 ballast mix) or you could just use granular sub-base material. No need to remove existing sub-base material, if it's in good condition.

I'm not sure how the 'hardcore' (as you call it) can 'spill out' unless this is an elevated pavement, in which case you need some form of retainer at the edges. Under normal circumstances, the sub-base is buried, so there's nowhere for the sub-base material to spill.

Thanks a lot for your reply to my question.
I didnt think that the sub base would be below ground level because I was thinking of laying over the existing patio, but from the sound of it, you are suggesting that I remove the old patio before starting work on the new one. The crazy paving sems to be bedded directly onto the sub base, there appears to be no bedding layer, could it just be really thin, ie 10-15 mm? Hence it is not immediatley visible.
In your experience how easy would it be to remove the crazy paving, all types of shapes and sizes, and leave the sub grade in tact? The sub grade is definitley sound as it was very hard to penetrate, although I am worried as to why the low spots, where puddles form, occur? Is this because the sub grade is not good enough and has sunk?
PS Hardcore is aterm i have picked up from people in the trade who seem to think of hardcore as a mixture of 6:1 ballast:cement.
For the bedding layer do I use sharp sand (3): cement(1) with small quantites of water?
Or do I use Ballast(mix of sharp sand and gravel)3: cement 1.
Thank you very much.




Is 50mm layer for bedding ok?

Taking up crazy paving is a doddle, usually, especially if, as you believe, it is on a very minimal bed. A pick will break it up in next to no time and then it's just a matter of dumping it into a skip.

Once it's out, the sub-grade can be re-graded in afew minutes, by raking over it with the back of a spade or a rake, and then re-compacting the lot with the vibrating plate.

The low spots are probably of long standing and occurred when there was some minor settlement of the sub-base or inaccurate laying of the crazy paving.

Hardcore is a term I prefer to avoid because it means different things to different folk. Quite a few one-handed surfers end up on this site, looking for hardcore, and are somewhat deflated when offered pictures of crushed rock or broken bricks. Strange men! ;)

Hardcore can be anything from half-bricks, to slate-waste, to gravel, or, as you believe, a lean-mix ballast-based concrete. On this site, I try to use accurate terminology that refers to specific materials, so that there is less chance of confusion amongst the site-users. Obviously, I can't (and don't) expect everyone else to be as specific as I try to be, but if I slip into generic or loose terms, then the accuracy of my information is reduced.

For your bedding, I would suggest that you use my standard recipe for patio paving, which is a 10:1 mix of grit sand and cement, with no added water, as is described in some detail on the Laying Flags page, which I heartily recommend to you as essential reading. :)

Hi,

Thanks for all your help. I am a bit confused however, you suggested that you would opt for a rigid construction when answering my first post but in your last post you say that I should use the

Hi,

Thanks for all your help. I am a bit confused however, you suggested that you would opt for a rigid construction when answering my first post but in your last post you say that I should use the dry 10:1 mix?? Which shuld I use?

The 10:1 mix is a rigid bedding system. The cement content, although minimal, is sufficient to bind the sand, making it hard and therefore it qualifies as a 'rigid bedding medium'.

Hope that's cleared it up for you. :)

Thanks.
Just one more thing, since my patio is 8 metres by 2.8metres do I need to have an expansion joint?? Or can I leave this out? The probklem is that the concrete sub base has already been layed with only construction joints at 3m widths. The Expansion joint would just sit in the mortar bedding layer. The mortar layer is approx 40mm. So my question is whether it is worth placing it now in the bedding layer.

If there's no expansion joint in the existing base, then no need to incorporate one in the patio. The usual rule is that expansion joints are continued upwards, if overlain, so, if there was one present in the existing slab, you'd be best making a joint in the patio directly over it.

Normally, any slab longer than 6m would have a movement joint of some description, even if it were only to act as a crack control joint. If you have concerns abouit the existing slab, then you could saw a control joint into the surface before laying the paving. This would act as the slab's weak point and then, if it was going to crack, it would crack along that line. However, even if you did this, I wouldn't worry about incorporating a complementary joint in the patio paving - if the slab cracks, the crack will be reflected through the bedding but it would then find a route along the jointing between the flags, which can easily be re-pointed.

I think you're worrying unduly about things that might never happen, and, even if they did, are very easily rectified. :)

Hi Tony,

Many thanks for all your help on this site. I have now finished the patio and it looks brilliant, I cant believe how satidsfying it is to look at it. I had to use an angle grinder to make cuts in awkward shapes, ie around the drain and stuff. I prob cut around ten slabs this way, they are standard wet cast concrete slabs. I have a question about dust from grinding. I wore a decent filter mask through all the cutting and no dust could be seen in my nose afterwards but I still felt a bit clogged up and irritated, breathing wise, is this normal as I have not done this before. Have you ever suffered anything like this??
Many thanks
Aj

I suspect that, even though you had the dust mask on, you still breathe in some of the dust through your mouth, and there's still dust in the air for several minutes after cutting has finished (assuming there's no breeze).

I think we are going to see a lot of silicosis-like problems with construction workers in the next few years. We had many years from when power saws first became popular when wearing a dust mask was seen as being 'girly' or 'soft'. Even now, I can go onto a site and see pavings or roof tiles or kerbs being cut by an operative with no dust mask, because they seem to think a few gobfulls of dust won't do them any harm, and the mask is buried under a pile of wet-weather gear at the back of the van, and by the time they've trudged across the site, they could have had the cut done....but they don't seem to realise that it's not just a few gobfulls: it's a few gobfulls on a regular, daily basis, and it's nasty, abrasive dust that will irritate their lungs and trachea.

In these litigious times, when employers are being sued for all sorts of minor transgressions, such as shouting at numpties, making the tea too hot or not providing moisturiser, I find it surprising that many employers and site supervisors do not insist on full protective gear when operatives are using a cut-off saw. Even for occasional use DIYers, I would compel them to wear a mask and goggles when using a saw - it's just not worth the risk!