Foundation strength question?

jagrace

I have my foundations up to radon barrier level with the first row of the inner cavity built (9 inch) high. I was going for ordinary rads but now want to put in under floor . I have poured a 3 inch subfloor and plan to put 100mm quinn insulation with a 65mm screed containing the under floor pipes to finished floor level. will this floor be strong enough? I know most floor are poured as at least a 6 inch solid slab. all replies gratefully appreciated.

Mellor

In my opinion the subfloor is not enough. The more insulation you have the stronger the each individual elements needs to be.
75mm subfloor is too small imo. I like to use 150mm.
It is possible to increase the thickness of the slab? By pouring more, or by dropping the starting level?
You will be able to make some extra room by decreasing the screed, there are calcium sulphate screeds that are suitable for as little as 30mm over insulation.

sinnerboy

min 150

250882

If you already have the 75mm floor slab poured then pouring a further 75mm on top is just compounding the problem, you now have 2 sperate layers that can move on top of each other. I would reccomend that you remove the 75mm and put in a proper 150mm reinforced conc slab.
I know you are probably thinking thats a large cost but if your slab does fail it will probably start showing as a crack on your tiled floor, a alrge gap forming between the floor and your skirting board, or leaks appearing in your underfloor heating pipes. What I am trying to say is that you could end up gutting your finished house to fix a problem that could have been solved at this stage

jagrace

Thanks for the replies. I know it should be 150 min but that would leave almost 8 and a half inches of concrete in the floor. That sounds crazy to me? the hard core is well compacted and the ground was hard enough to start off with. I know if there is problems down the road it will be a big hassle to sort them. But these floors are not carrying much weight the foundations are carrying the weight and they have reinforceing steel. My only other concern is i plan to put concrete first floor (dormor bungalow). if i did pour another 75mm would the fllor not be good enough then. If it moves ir shouls all move together? if i come up more i will have to put a soap bar on which will cost more money. How are fllors with under floor normally finished? mellor that screed you mentioned sounds good but the IFH crowd want the finished floor 65mm above the pipes?

sinnerboy

pour 150 over the 75 . add mesh reinforcement . your building raises by 75mm . carry on as before then

Is reducing your ceiling height an option? -but If I'm reading this correctly you're still at rising wall level??

Else an extra course of blocks would keep everything relative assuming no serious height restrictions.

jagrace

Sinnerboy that would leave me with 9 inches of a sub floor and re-inforced? thats along way off my 3 inches which a few people say is enough . There are no real height restrictions but i was trying to avoid going up a second block for the floors. how would i screed them I was hoping to screed off the top of the fisrt block. What if i put heavy chicken wire in the top half of the floor (UFH level) to tie it together? there has to be an easier way than pouring mass amounts of concrete or digging up what there. it aint cheap stuff!!!!!!!!!!!!!!!

Mellor

If you want to keep the floor to the minimum size, break out the existing 75 slab (this is poured already, right),
pour 150 slab, 100mm insulation, 35mm CS screed. total size 285mm, 320mm if you use a 65mm concrete screed.

If you want to keep the floor to the minimum cost, leave the existing 75 slab,
pour 150 slab over this, 100mm insulation, 35mm CS screed. total size 360mm, 395mm if you use a 65mm concrete screed.

250882

Mellor, clear and precise.
OP those are really your only options, concrete is a good material for compression but unless it is reinforced then it really is very brittle, you'll be putting quite a load on those floors another screed tiles, furniture, kitchen units etc.
I have discovered 2 failed floors last year. One had been caught on time but the other wasn't. Client had a marble floor in the hall and now its all cracked with rises and falls and because its so shiny its showing all the dips and hollows plus its presented loads of sharp edges where the slabs have cracked.
Kitchen tiles are the same and the french doors dont close right either. They have chosen to live with the problem but it takes the shine of a new house

ircoha

jagrace wrote: »

Sinnerboy that would leave me with 9 inches of a sub floor and re-inforced? thats along way off my 3 inches which a few people say is enough . There are no real height restrictions but i was trying to avoid going up a second block for the floors. how would i screed them I was hoping to screed off the top of the fisrt block. What if i put heavy chicken wire in the top half of the floor (UFH level) to tie it together? there has to be an easier way than pouring mass amounts of concrete or digging up what there. it aint cheap stuff!!!!!!!!!!!!!!!

see http://www.roadstone.ie/Products/Blocks/Standard.htm
for a 140 block on flat plus mortar may allow you to screed the 150 new slab.

jagrace

lads thanks for all the replies on this. Talked with engineer(which i should have done on the first occasion) expensive mistake !!!!!! i am going to pour the 150mm on top of the original slab.Then i am going to put a 6 inch soap bar around the top and screed the finished floor off this. The bed of motor should give me the desired height for the finished floor. so basicaly 75 mm down already, then 150 on top of that which brings me flush with the top of the 9 inch block alrady there,this also allows me to tamper off the top of this block to make sure the base is level for the 100mm insulation to sit on. Then get builder back to put on 6 inch saop bar all round. With the correct motor gap this should finish perfectly for me to screed the second floor containing the underfloor heating. from top of insulation to finished floor =65mm. thanks again for the replies and if anyone see's a big problem with this please let me know before it costs me more money

Unfortunately, that's one of the pitfalls of building the first (& only house) is that it is a learning experience and you'll look back and know where you could have done some bits of it better &/or cheaper.

If I was doing it again there would be a couple of things done differently

jagrace

tell me what tell me what????????? i have a long way to go and i am sure there will be a few more mistakes. While were on the subject i have a few more questions.
1. wall insulation is it worth going more that 60mm in a 100mm cavity?
2. Is a precast slab worth the money for the first floor?
3 what would you run under floor heating with (geotherm,oil gas)?
any suggections on avoiding problems gratfully appreciated. you cannot beat experiance!!!!!!!!!!!!!!!!!!!!!

sydthebeat

jagrace wrote: »

tell me what tell me what????????? i have a long way to go and i am sure there will be a few more mistakes. While were on the subject i have a few more questions.
1. wall insulation is it worth going more that 60mm in a 100mm cavity?
2. Is a precast slab worth the money for the first floor?
3 what would you run under floor heating with (geotherm,oil gas)?
any suggections on avoiding problems gratfully appreciated. you cannot beat experiance!!!!!!!!!!!!!!!!!!!!!

1, if you are going with 'partial fill cavity insulation' you need to retain at least 40mm cavity... there are plenty of threads already about wall insulation
2. yes, definitely.. but remember there are knock-on effects... again check existing threas
3.UFH is best suited to geothermal heating as it runs about 55 deg and not 70 deg like rads.

Mellor

sydthebeat wrote: »

3.UFH is best suited to geothermal heating as it runs about 55 deg and not 70 deg like rads.

i would reverse that yo be honest. Geothermal is suited to UFH, as geothermal isn't capable to running rads.
But There is no reason why a good gas boiler can't run UFH just as well.

I personnally don't like geothermal. As it is portrayed as this great free clean source of heat. Which is isn't. In terms of energy, carbon emissions etc it is no better that a gas or oil system. Until electricity is sorted out in this country, it is a bad option imo.

jagrace

jagrace wrote: »

tell me what tell me what????????? i have a long way to go and i am sure there will be a few more mistakes.
any suggections on avoiding problems gratfully appreciated. you cannot beat experiance!!!!!!!!!!!!!!!!!!!!!

Post frequently here before doing anything major (after searching for the answer as it's likely to have already covered), if still in doubt seek professional help.

Try to look as far ahead as you can, like where are you going to put the duct for the cooker hood for example, these things can sometimes come back and bite you. Thinh about where your services are going to run, it's easier for a block layer to leave opes rather than having holes bashed in the walls later for pipes etc (not always successful as I had to "move" some holes).

Try to co-ordinate tasks, you have a digger on site for a day, get him to do as many digging jobs as practably possible in one go (much cheaper that three visits)

Don't skimp on bracing/scaffolding etc, makes the job much harder & increases risk.

Do as much research as possible, but be resigned to the fact that you are certain to see something that you wished you'd found earlier!!

sydthebeat

sydthebeat wrote: »

3.UFH is best suited to geothermal heating as it runs about 55 deg and not 70 deg like rads.

I have my system running at 35c (end stopping the mixer valve), but that's with an enhanced insulation specification.
With a thermal store set to 60c you can use any heat source you like.
UFH & Geothermal do go well together!
I currently use oil, temporary solution, untill the wood burner & solar systems are installed in the garage (that is still being built)