relaying concrete floor

[alex6192]

Hi guys,

My downstairs concrete floors are uneven and have cracks in them so I'm going to relay them. I have detailed instruction on what I need to do (depth of insulation, DPM, etc) but i have a few other questions which I;m hoping you can help me out on.

- Do I need to notify building control?
- What mix should I use for the concrete? Type of sand?

More info:

The first room i'm going to do is 5 * 8 meters.

Many thanks,
Alex

 

[JerryM]

are you sure you can't just cover them with a 3 to 5 mm thk self leveller containing latex. it does depend on what the condition of the existing is but often a few larger cracks can be filled with mortar and the leveller applied over the top.

assuming you've already discounted the above then:

1) i would notify the building control (i like to get a completion cert for selling on and peace of mind that it's done to the right spec)
2) the concrete spec is called c20 (mix 1:2.5:3.5, cement : conc sand : aggregate) Nb max water : cement of 0.5 : 1 by weight). Water content is v.important in terms of achieving the final strength
3) The sand must be what's called grit or concreting sand unless you use all in one ballast which already contains the sand

 

[alex6192]

thanks for the info jerry, thats a great help.

I think relaying the floor is my best option as the state of the current floors are pretty bad and as it's a relatively old house im sure there wont be any insulation.

Given your comment I will notify building control. Do you know what the normal procedure is for this type of DIY work? I'm assuming they will need to come out and have a look at my work?

 

[alex6192]

One more question:

Once we have dug the existing floor out, what is the best way to get a level line around the room. I have been looking at rotating laser levels (not sure if i'm allowed to post the link?) but I was wondering if there is a cheap option?

 

[JerryM]

i don't use any instruments other than spirit level.

for floors i just go off the course of brick that i want the concrete level to finish at. if the brick course is out for whatever reason i use a straight edge (timber something like 3x2) and chalk to transfer from my datum wall to the rest (mark full length of datum wall, put 3x2 against datum & adjacent wall, use bricks and wood packers to support, transfer level from datum moving around the room in a circle).

at the concreting stage i don't make a big fuss on the level as i leave the concrete 25mm below the finished surface. i then use screed (sand/cement) to take it up 20mm. leaving 5 mm for self leveller.

the accuracy getting more the closure you move to the finished floor level - concrete is very difficult to get line & level. the screed is much easier to get line & level but even so you ideally need a final self leveller to get a top notch job.

nb if tiles are going on top then i may or may not use leveller depending on how level the screed turns out. buy the leveller from a builders merchant (around £13 per 25kg for the latex version which is best to avoid fine hair line cracking).

on the building control you need a form for what's called a "building notice". you can fill it out whilst at your BC office. they have a table of rates based on the cost of the work. i would go for the lowest which is about £50 +vat. the inspector will want to visit before starting the work (to make sure you know where your going and they are very helpful), before the concrete is poured and finally at completion.

don't forget to get some gaffer tape to seal the dpm at the corners (there's a technique which you will soon work out to cut the dpm but still give a 150mm overlap - try out on a small sheet 1st to get the idea). don't forget to take the dpm up the walls above the dpc (above the level of the finished concrete - ideally lap behind the wall plaster/render/skirting).

 

[alex6192]

thanks again jerry.

I contacted building control the other day and they have given me all the info I need. About to order the insulation board + DPM, so i'm getting there.

I'll let you know how it turns out.

Thanks.

Alex

 

[JerryM]

No probs.

i've found that an old carving knife is good for cutting the insulation board. also i use an old piece of timber typ 7"x4" by 1.3m lg as a tamper (for hardcore and sand) but anything relatively light but with a reasonably large flat surface will do.

 

[alex6192]

I bought myself a rammer from screwfix the other day. Wasn't a lot so thought it would be worth the expense.

Went into several DIY stores to get the quotes for the concrete mix today. Being generous with the quantities turns out at about £430.

Jewsons came out on top. I take it their all-in one sand/gravel is ok for the c20 mix i.e. a 6 - 1 (ballast - cement)? I got quotes for building sand as the bonding below the DPM, thats correct isn't it? or should I have gone with sharp? Sorry for so many questions.

thanks again.

 

[JerryM]

screwfix is spot on and i use it a lot.

i think the DIY's will be too expensive for the materials. builders merchants are what you need delivered in tonne bags. i've found the individual materials come out cheaper than the ballast and both are equally as good ie go for ballast if that's the best price. i've calculated at 44 bags cement (135£), 3 x 1te sharp sand (90£), 4 x 1te aggregate (120£).

the strength of the mix is given by the proportion cement to sand/aggregate. so the ballast at 1:6 is ok for the C20 mix (just make sure the ballast is for concreting - i am sure it will be).

building sand is best below the dpm.

 

[alex6192]

The £430 I mentioned previously includes 2tne bags of builders sand (jewsons only offer bagged aggregate) but it looks like your prices are a fair bit cheaper.

Just going on the cement, your calculations work out at 3.09 a bag where as Jewsons are nearer 3.50. What figures/store are your prices based on? I went to ridgeons (not sure if they have stores where you are based but they are merchants) and they offer very similar prices to Jewsons.

 

[JerryM]

we have several merchants in our area who are overpriced so it's worth shopping round to find the best. the best in the north west is Beesley & Fildes. Have checked their current price which is 3.35 inc vat (3.09 was my last purchase which was before a 2nd Jan price rise).

i had not included the 2te builders sand which brings costs within 30£ so i think your quote is probably as good as can be got.

sometimes worth trying B and Q on cement price as they can be competitive at odd times but any saving may not outweigh the hassle of splitting the order given you ideally need delivered.

 

[Lukasas85]

For example I have tried to Concrete my house floor, and now I am really happy about it