Mortar mixes and levels - Tips needed on patio

[Rolyan]

We are laying approximately 18 square metres of Marshall's Chancery flags. We already have a concrete patio, and they are going on top. We are using three sizes; 600 x 600, 600 x 300 and 300 x 300 (all are 40mm thick). Advice on the following would be most appreciated (complete novice).

1) I've read Tony's comments on mixes, re wet, moist and semi-dry. Are all suitable for laying onto concrete?

2) If I use semi-dry, am I correct in saying that I just mix the sand and cement in the mixer, with no water added, then spread it out. Do I trowel ridges/lines into it, or should it be flat?

3) Is semi-dry suitable for the smaller flags, 300 x 300?

4) The builder's merchant recommended 3:1, but this site seems to suggest 4:1. Which is best, or would both be okay.

5) If I lay semi-dry, do I try and get the correct level and fall on it before laying the flag, and if so how? I can only really do individual bedding because of the shape of the patio. I assume that there will only be a very small amount of 'tapping down' I can do. Does it have to be the correct level before the flag is laid, with no adjustment after.

6) Finally, there is a slight slope on the end of the exisiing patio. It's not really enough to start cementing up or digging out and re-doing (and I've got time limits). How thick can I lay the mortar in this area, to make the flags level. Could I go up to 75mm if necessary.

Your help and comments would be most appreciated.

[Tony McC]

1 - yes

2 - yes, with ridges

3 - yes

4 - 3:1 for what? The bedding? Too strong: The jointing? It's about right. The BM has a vested interest in persuading you to be overly generous with the cement as they make more profit on a bag of cement than they do on a bag of sand.

5 - The bedding needs to be 10mm or so high and then the flag is tapped down to the correct level. This ensures any deviation in the base of the flag is properly accommodated.

6 - 75mm is best done by changing over to a semi-dry concrete. Use a 10:1 or 8:1 mix of grit sand and cement for those areas where the bed will be 50mm or less, and then use a mix 6parts 10mm gravel to 3 parts grit sand t 1 part cement for the deeper spots.

[Rolyan]

Tony - many thanks for your helpful comments. I think i know what I'm doing now, but if you could help with the following i would be most grateful (the job starts in a couple of days!).

1) The BM recommended 3:1 fmix or the bedding (the materials have now been ordered, paid for, and due on Thursday). Am I correct in thinking that you suggest and 8:1 or 10:1 mix for the bedding?

2) When I do the semi-dry concrete, is this done the same as the semi-dry mortar, without adding water i.e. mix the 6 parts gravel, 3 parts sand and 1 part cement in the mixer, then spread out as required.

3) The BM has recommended and ordered a bulk bag of builders sand. Is this okay for doing the above, or does it need to be another type (grit, river, etc.). I don't mind asking them to change it if absolutely necessary. However, if it is now on its way, can I use it without too many problems.

Thanks again for an excellent site, which I have recommended to those planning their Spring projects!!!

[Tony McC]

1 - yes. I recommend a 8:1 or 10: mix for bedding as this minimises problem with efflorescence. 3:1 is way too strong for a patio.

2 - yes, add no water.

3 NO!! Pillock of a BM - you need grit sand, sharp sand or concreting sand, but definitely NOT building sand for all the reasons explained on the website. When it comes to recommendations, I recommend you tell this BM to stick to selling sticks and stones coz they know bugrall about paving!

[Rolyan]

Tony - phew, that was close.

I've just rung the BM, and changed the order to grit sand. Hopefully I can now crack on. The plan is:

Brush the concrete patio clean. Mix up semi-dry mix, 8:1, one barrowful at a time. Lay an individual bed (50mm thick) of this mix directly onto the concrete base, 100mm wider all round than the first flag, with trowelled ridges. Lay the first flag to the string line, to the correct fall. This will consolidate the mixture (which won't have been compacted), reducing it by about 10mm. Tap down with rubber mallet, level off with the spirit level and string and check for rocking. Lay another individual bed of the 8:1 mix to suit the next flag, thick enough to allow for it compressing as above. Repeat the tapping and the levelling, measuring across several flags at a time, working the pattern as we go.

If this is correct, could you just add a quick "Yes" so I know I'm doing it right. Otherweise please point out my errors.

Thanks again for all your help and advice.

[Tony McC]

Yes.....


...but what about the jointing?

[Rolyan]

Yep...good point! I was planning to point them once I had laid them all, including the final cutting and fitting around the edges (very irregular shape).

I think it will take me about a week to lay the flags, prior to pointing. I was then going to use a 4:1 mix soft sand and cement and follow the website details to point and polish off. Is this wait okay? Can I wait up to 2 weeks to do the catch up pointing if I absolutely need to?

Once again, thanks for your prompt help (as the day approaches, the size of the job starts to become apparent!).

[Tony McC]

Sounds ok - as long as you get them pointed asap and try to do it in one operation so all the joints end up being (more or less) the same colour.

Let us know how it goes.