Building a Sun Room extension - expert opinion required

frogga

Good Morning All,

Five years ago we had the footings built for our Sun Room. It is just a square box on the back of the house, like a conservatory but with brick walls large windows and a proper tiled roof. We only had enough money to do it in stages and didnt want to get into debt, so we only got up to 'oversite' (hope that's the correct spelling) hoping to save up for the next bit.

We have proper plans and we have planning permission. The builder we used was a grounds work and drainage expert, we researched him and got checkable references. I was happy but Mr Frog had reservations. He had 5* reviews on Trust a Trader with photos. He dug the footings, did the brick up to ground level, put in hardcore layer, sand, the big black plastic sheet and then the concrete. Mr Frog said there was no way he dug deep enough. The plans said we needed 100mm hardcore, 150mm concrete and reinforcing mesh. We did not see the mesh go in at all, I am confident that's not there. At each stage he took photos and the Building Regs company approved that he moved onto the next stage so I trusted that building regs company knew what they were doing. The project was then put on hold. Building regs company said we could just contact them again when we were ready to start again.

Five years later we want to build the walls and the roof. We will wait for the electrics and plaster but for now we hope we have enough to get the shell built. We will be using a different builder and I am researching local people again. We asked the building regs compnay for the paper work report so we could show the new builder how far we had got. We have got the plans out to look at again and NOW I am worried! I think it is too high. Mr Frog said this at the time but I trusted building regs company as they approved it. The top of the concrete reaches just 70mm below the DPC but the plans say it should be 150mm. The plans say inside the room should now be another 80mm insulation and 65mm screed. That makes 145mm more hight inside, but the french door leading to the sunroom is only 75mm higher than the concrete. The building Regts company have stated on their report 'discussed dpc requirements as the ground levels appeared high at the rear, but the builder is confident that 150mm between ground level and dpc will be achieved'. This really worries me. The building regs company appear to have just looked a photos and taken the builders word for it. This is the first time we've seen the report. I know I should have asked for it at the time.

Is it a MUST that we need to have 145mm more height inside the room, or can we do less than the plans state? If it is any higher than 75mm we will not be able to open the french doors into the room! I recall the builder kept saying 'oh you dont need that much, the plans are over the top, we can get away with less, the building regs would not approve it if it was not ok' etc. I'm worried that we've got to dig the whole thing up and start again?

Thanks for reading if you got this far. I'm very interested to hear the thoughts of proper builders/experts and not arm chair critics as I have heard enough from them in my own family . I am preparing for an onslaught of people telling me what an idiot I am.

Frogga 

Doozergirl

150mm is the required measurement from the DPC to the ground level outside, not inside, but it does sound like there might be an issue.  Outside won't be an issue as there are different ways to skin that cat.  

There are different ways of building the slab, a lot of builders will agree the route with the building inspector on site.  

Are you sure that there is no insulation in the floor?  My H doesn't like to do the two layers of concrete, he tends to put the insulation over the MOT and blinding sand, then pours one big slab of concrete and gets it neat, so you don't need a separate screed, in which case you would have loads of leeway for your finished floor.   

If the insulation isn't already in then you will need a screed to keep a good solid floor over the insulation which isn't stable.  In theory, if your DPM is still exposed then you can extend it upwards with the slab, but doesn't help with your FFL inside the existing house.  

I'm not an expert on concrete, but was it a structural engineer that specified the mesh or just architect?  It isn't always needed and reinforcing fibres are also added into concrete.  I wouldn't be too concerned over that element in a modest sized room. 

frogga

Hello Doozergirl THANK YOU for taking the time to reply.

Yes that's what the builder said - he would sort it out with building control.

I'm SURE there is no insulation under the concrete. I watched. And the photos show it went - hardcore - sand - concrete. 

I remember him saying as it was a small room we didnt need the mesh so glad to hear you agree. It was 'just' an architect who did the plans. 

We have only one brick thick gap (70mm) between the top on the concrete and the bottom of the door. The builder said we can get the insulation and screed in that gap. But we also need a tile for the floor?? That's never going to fit is it?

What would you do? What are the options apart from taking it up and starting again?  Cross with myself for not being on top of this five years ago before I paid both the builder and the Building Control. 

stuart45

You need to discuss a solution with the BCO and new builder.
Kingspan do a VIP insulation board about an inch thick for floors with height issues and polymer added thin screeds are availabe.
Reinforced slabs are normally used in poor ground or deep fill etc and usually sit on the inner skin. They do the same job as a block and beam system. 

frogga

Hello Stuart, 

Thank you for replying. You are right, I will need to speak to a new builder and the BCO. I have arranged for two different builders to come round next week so will know more then. I guess I'm just panicking - but it sounds like you feel all is not lost with your suggestions of the VIP insulation for height issues and thin screeds. Thank you, I fell reassured, hopefully not prematurely.

Frogga

Section62

stuart45 said:

Reinforced slabs are normally used in poor ground or deep fill etc and usually sit on the inner skin. They do the same job as a block and beam system. 

^This.

The significance of mesh being omitted will depend on the type of soil the structure is being built on, and the care taken in preparing the formation and sub-base.

The way most domestic work is done, the function of the mesh has more to do with crack control rather than providing massive structural strength.

stuart45

Looking on the bright side, if the rebar is left out it makes the slab a lot easier to break up.

Section62

stuart45 said:

Looking on the bright side, if the rebar is left out it makes the slab a lot easier to break up.

Reminds me of my dad... "Don't build it too strong, it will be a b...r to demolish". 

frogga

he he he he thanks for that! - always a bright side

FreeBear

stuart45 said: Looking on the bright side, if the rebar is left out it makes the slab a lot easier to break up.

In time, rebar will rust and cause the concrete to crack. It may take 50 years or longer to do so.

stuart45

FreeBear said:

stuart45 said: Looking on the bright side, if the rebar is left out it makes the slab a lot easier to break up.

In time, rebar will rust and cause the concrete to crack. It may take 50 years or longer to do so.

The alkaline content in concrete protects the rebar. As long as there is enough covering over the rebar it will be OK, otherwise concrete bridges and multi storey car park floors and columns would be in serious trouble.
An inside floor is less likely to suffer.