I have just finished a small natural stone patio ( 8 x 9 feet ), gravel sub-base, sharp sand / mortar bedding layer and have a few follow-up questions ( horse, stable .. bolted ). The patio is immediately behind our house ( mid-terrace victorian ) and as our neighbour has an extension the patio effectively abutts a wall on 2 sides ( L shape ). I removed a tiny crap cracked concrete patio ( 5 x 8 ) that was originally there.
So here goes.
We aimed for 6:1 bedding mix ( supplied bags of portland cement were 25Kg, sand was 40Kg so couldn't do 4:1 !). I suspect we actually started with 8:1 before strengthening. Do you think this change will cause a problem ?
The flags were laid on a baking day. Because we didn't want to walk over the flags we dry-mortared as we went with the first few rows done with 8:1 mix. These joins seem a bit crumbly. Do you think
it is worth redoing the dodgy bits straight away or only if movement results ?
Also we aimed to leave around a 10mm gap but a couple of flags were touching due to us not getting the rows absolutely straight. As these flags are wider at the top than bottom and taking into account the heat I'm not confident that we have the cavity nicely filled. I'm wondering whether we could then get problems with water entering the join and frost damage as well as unwittingly having created a home for ants ( we have many ). Again do you think this is worth redo-ing straightaway ? With the blocks almost touching ( although the irregular join means a small gap ) how would I add new mix ?
Next. The rear wall on our house has been re-rendered down to the level of the old patio. There is now a recess to the original brick where I have removed the old patio and the new flags start about an inch out and below where this render finishes. I plan to fill this gap with 3:1 sand - cement mix plus waterproofing agent. As I have several bags of sharp sand left over could I use this or is it a big no no to use sharp sand instead of builders sand. Is the builders sand better for waterproofing ?Does the above ratio sound ok ?
Finally when I had removed the original patio I found that it was above our neighbours dampcourse. There is even a flower bed with the soil also at a higher level ! I'm assuming this was possible as the wall had a smooth render ( like ours ) going down and joining the old conrete patio ( 3 inches above DC in 1 place ) and also going down below the top level of the soil ( and inturn DC ) where the flower bed was located. Have you come across this before ?
Apart from this the patio looks great ! Sorry about the wordy-ness of all this ..
General Patio Questions
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84-1093879891
OK - taking your questions one at a time....
An 8:1 mix for bedding is fine. I normally recommend a 10:1 mix for patios, as I found that was always adequate.
I'm not sure what you mean by 'dry-mortared' but I'm assuming you buttered the joints with a mortar and then pointed them immediately, but because of the heat, they mortar was 'parched', ie, it had the water sucked out of it by the thirsty flags and natural evaporation before it had a chance to ensure full hydration of the cement content. This is why the resultant mortar is weak and crumbly, and the only remedy is to cut out all the weak mortar and replace it with fresh, prefeably on a cloudy or overcast day, when it's out of direct sunlight, and late in the afternoon.
The joint width is a problem. When laying these stone flags, or even the imitation stone flags that are popular, it's best to leave the joints unpointed until an area has been laid and the joint widths adjusted by levering with a bar or a spade to even out any differences. Ideally, no joints should be less than 6mm and none wider than 18mm, (12m +/- 6mm), but there is always some variation.
If you have flags in direct contact with their neighbours, this is less than ideal, but, if it's only one or two, then it's not worth losing sleep over. However, if there is a lot of very tight joints, it may be better to re-lay a section to even out the joints. Without seeing it, I can't say which is best.
When it comes to filling those tight joints, you could mix a slurry mortar and pour that into the joints to find its own level and then top them up later, once the slurry mortar has settled - it always 'sinks' by 6mm or so as the water content drains out and is used up by the cement. For a slurry mix, use a 4:1 mix with a plasticiser and plenty of water to make it a soup-like consistency. Keep the slurry agitated by stirring and pour it into the joints from an old watering can or similar. Wipe up and sponge down any spillage immediately. Take your time - you will probably get some slight overspill and that may stain the stone slightly, but it can be washed down with an acid wash in 7-10 days and no-one will be any the wiser. :)
An alternative, and in my opinion less successful method, is to use a dry mortar, ie a mix of very dry sand and cement with no added water. Brush that into the joints using a hand brush, and then pour water into the joint to wash down the dry mortar into every nook and cranny of the joint and initiate hydration. I find that this method always results in a lot of settlement in the jointing material as the water washes through the dry mix, and you end up having to repeat the process two or three times to get properly filled joints with no great gaping voids.
And then the gap in the rendering - using sharp sand will give you a coarser-textured mortar. Structurally, it's fione to use it to make up the 25mm fillet between the old render and the paved surface, and, if you're happy enough with the texture, then there's nowt wrong with it.
Building sand is normally used for redering because it is finer and has a higher clay content, which means it makes a 'stickier' mortar and adheres more immediately to the substrate (it's all to do with grain size, the size of the voids between the grains, and capillary action), as well as giving a smoother finish when trowelled. But for a 25mm gap, at the base of a wall, that no-one will ever notice, using sharp sand will not be a problem. :)
The problem with the levels and your neighbour's DPC is more of a problem, though. It may be that some previous occupant laid the old patio and didn't understand the way that DPCs work, and so, in blissful ignorance, laid paving and constructed flower beds that breached the DPC.
Your idea about the render protecting the DPC is not right - a render would, normally, stop at or above the DPC, not bridge it, as seems to be the case here.
Does your neighbour have any problems with damp in their property? If they do, of if damp problems were top develop, and it could be shown that your patio was the prime cause, you could be left facing a large-ish bill for remedial work and re-decorating.
When you bought the property, did your surveyor not identify this problem of a bridged DPC?
An 8:1 mix for bedding is fine. I normally recommend a 10:1 mix for patios, as I found that was always adequate.
I'm not sure what you mean by 'dry-mortared' but I'm assuming you buttered the joints with a mortar and then pointed them immediately, but because of the heat, they mortar was 'parched', ie, it had the water sucked out of it by the thirsty flags and natural evaporation before it had a chance to ensure full hydration of the cement content. This is why the resultant mortar is weak and crumbly, and the only remedy is to cut out all the weak mortar and replace it with fresh, prefeably on a cloudy or overcast day, when it's out of direct sunlight, and late in the afternoon.
The joint width is a problem. When laying these stone flags, or even the imitation stone flags that are popular, it's best to leave the joints unpointed until an area has been laid and the joint widths adjusted by levering with a bar or a spade to even out any differences. Ideally, no joints should be less than 6mm and none wider than 18mm, (12m +/- 6mm), but there is always some variation.
If you have flags in direct contact with their neighbours, this is less than ideal, but, if it's only one or two, then it's not worth losing sleep over. However, if there is a lot of very tight joints, it may be better to re-lay a section to even out the joints. Without seeing it, I can't say which is best.
When it comes to filling those tight joints, you could mix a slurry mortar and pour that into the joints to find its own level and then top them up later, once the slurry mortar has settled - it always 'sinks' by 6mm or so as the water content drains out and is used up by the cement. For a slurry mix, use a 4:1 mix with a plasticiser and plenty of water to make it a soup-like consistency. Keep the slurry agitated by stirring and pour it into the joints from an old watering can or similar. Wipe up and sponge down any spillage immediately. Take your time - you will probably get some slight overspill and that may stain the stone slightly, but it can be washed down with an acid wash in 7-10 days and no-one will be any the wiser. :)
An alternative, and in my opinion less successful method, is to use a dry mortar, ie a mix of very dry sand and cement with no added water. Brush that into the joints using a hand brush, and then pour water into the joint to wash down the dry mortar into every nook and cranny of the joint and initiate hydration. I find that this method always results in a lot of settlement in the jointing material as the water washes through the dry mix, and you end up having to repeat the process two or three times to get properly filled joints with no great gaping voids.
And then the gap in the rendering - using sharp sand will give you a coarser-textured mortar. Structurally, it's fione to use it to make up the 25mm fillet between the old render and the paved surface, and, if you're happy enough with the texture, then there's nowt wrong with it.
Building sand is normally used for redering because it is finer and has a higher clay content, which means it makes a 'stickier' mortar and adheres more immediately to the substrate (it's all to do with grain size, the size of the voids between the grains, and capillary action), as well as giving a smoother finish when trowelled. But for a 25mm gap, at the base of a wall, that no-one will ever notice, using sharp sand will not be a problem. :)
The problem with the levels and your neighbour's DPC is more of a problem, though. It may be that some previous occupant laid the old patio and didn't understand the way that DPCs work, and so, in blissful ignorance, laid paving and constructed flower beds that breached the DPC.
Your idea about the render protecting the DPC is not right - a render would, normally, stop at or above the DPC, not bridge it, as seems to be the case here.
Does your neighbour have any problems with damp in their property? If they do, of if damp problems were top develop, and it could be shown that your patio was the prime cause, you could be left facing a large-ish bill for remedial work and re-decorating.
When you bought the property, did your surveyor not identify this problem of a bridged DPC?
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steveb
- Posts: 6
- Joined: Wed Jun 18, 2003 2:54 pm
- Location: london
Thanks for the full reply.
Yes the pointing is as you described and I will gouge out and redo the bad sections. In many places it has actually hardened well. For some reason I have it in my mind that I should have pushed the mortar down having brushed it in - which I didn't.
Does this matter if it's hardened well on top ?
One additional point on the pointing ( yup bad pun ). I've noticed that some of it looks wet and there is a damp patch a few cms either side on the flags ( parallel to the join ) even though it's been dry and warm for a while. I remember reading about this in another post but can't find it. Does this indicate a cavity underneath ?
The tight join indeed only affects a couple of edges so I will leave them down I reckon.
As far as the DPC goes we've been in the house for 12 months and are neighbour hasn't mentioned damp problems. Having said that it's not in the weather / trains .. hmm / weather list so I don't think it would come up and I don't think the surveyor
looked at the ajoining property as it wasn't mentioned.
As a complete building novice I'm pretty ignorant of DPCs. When I removed the old concrete it showed as a thin black layer of what looks to be slate. It is very low compared to the level of the garden. Yet if the wall is rendered and this forms a neat join with a concrete patio how would you know where it was ? This is the case with our section of wall. I just made sure that my
flags weren't higher that what I replaced. We have no damp problem. Around the corner ( path to kitchen door ) on our house
the unrendered wall has been mortared over the bottom 8 or so inches so I presume the DPC is under there but I can't visibly see it unlike the newly exposed section of our neighbours wall ...
Yes the pointing is as you described and I will gouge out and redo the bad sections. In many places it has actually hardened well. For some reason I have it in my mind that I should have pushed the mortar down having brushed it in - which I didn't.
Does this matter if it's hardened well on top ?
One additional point on the pointing ( yup bad pun ). I've noticed that some of it looks wet and there is a damp patch a few cms either side on the flags ( parallel to the join ) even though it's been dry and warm for a while. I remember reading about this in another post but can't find it. Does this indicate a cavity underneath ?
The tight join indeed only affects a couple of edges so I will leave them down I reckon.
As far as the DPC goes we've been in the house for 12 months and are neighbour hasn't mentioned damp problems. Having said that it's not in the weather / trains .. hmm / weather list so I don't think it would come up and I don't think the surveyor
looked at the ajoining property as it wasn't mentioned.
As a complete building novice I'm pretty ignorant of DPCs. When I removed the old concrete it showed as a thin black layer of what looks to be slate. It is very low compared to the level of the garden. Yet if the wall is rendered and this forms a neat join with a concrete patio how would you know where it was ? This is the case with our section of wall. I just made sure that my
flags weren't higher that what I replaced. We have no damp problem. Around the corner ( path to kitchen door ) on our house
the unrendered wall has been mortared over the bottom 8 or so inches so I presume the DPC is under there but I can't visibly see it unlike the newly exposed section of our neighbours wall ...
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84-1093879891
Steve B scribbled.....
For some reason I have it in my mind that I should have pushed the mortar down having brushed it in - which I didn't.
Does this matter if it's hardened well on top ?
It's essential that the mortar is pressed firmly into the joints and then tooled on the surface to create a full joint that bonds properly to the edges of the flags and forms an effective seal at the top. Simply brushing-in the mortar mix will, of course, fill the joint and help keep out litter and fag-ends, but it doesn't actually help reinforce the construction.
With regard to the mystery DPC, these older properties can often be very confusing as to whether they have a proper dpc at all, and, if they do, where it is and why. I'd suggest that, as long as you're no higher than what was there before, and there's no problem with damp walls for either yourselves or the neighbour, then forget about it and just enjoy your patio. :)
For some reason I have it in my mind that I should have pushed the mortar down having brushed it in - which I didn't.
Does this matter if it's hardened well on top ?
It's essential that the mortar is pressed firmly into the joints and then tooled on the surface to create a full joint that bonds properly to the edges of the flags and forms an effective seal at the top. Simply brushing-in the mortar mix will, of course, fill the joint and help keep out litter and fag-ends, but it doesn't actually help reinforce the construction.
With regard to the mystery DPC, these older properties can often be very confusing as to whether they have a proper dpc at all, and, if they do, where it is and why. I'd suggest that, as long as you're no higher than what was there before, and there's no problem with damp walls for either yourselves or the neighbour, then forget about it and just enjoy your patio. :)