Will a 400mm foundation take the load of a GRP roof ?

EachPenny

Bandia wrote: »

The current roof is nothing but plastic panels, so there is no structure on the top what so ever to place the Kingspan. I am intending to brick up the existing walls with some more brick work and add large sized windows. As the current walls are cavity walls, when I get rid of the current 20 year old windows I am hoping that I can insert quality insulation all around the walls. Idea is have a thick underlay and carpet on the current tiled flooring and to pray for the GRP roof to do its job with out any problems around the 400 mm foundation.

If you are substantially changing the walls and changing the roof then you may need to get planning consent as well. Have you looked into this?

Bandia wrote: »

I live in Ashford Middlesex area and I was told by a builder that the bungalows we live in here don't have foundations greater than 500mm when they were originally built in 1930 (really unsure how true that can be) but if he happens to be right, I wouldn't have to worry too much about GRP roof on 400mm foundations.

Are you on clay soil there then?

The problem is what you are proposing isn't just replacing the roof, you are also talking about raising the existing 400mm high walls up to roof level (unless I'm mistaken). Even with large windows, you will still be adding a considerable weight onto the foundations before you then add the new roof structure.

What you are talking about is going to cost you a lot of money, and if the end result is not safe then it puts you and your family in danger. This isn't something you should be making guesses and assumptions about. The builder might be right about the main foundations only being 500mm, but does he know how wide they are? The size of a foundation isn't just about the depth, the width is important too, as well as the thickness of concrete. If you don't know all of that information and have the foundations properly designed then you risk differential settlement, subsidence and potentially even the walls just collapsing.

Davesnave

Bandia wrote: »

The current roof is nothing but plastic panels, so there is no structure on the top what so ever to place the Kingspan. I am intending to brick up the existing walls with some more brick work and add large sized windows. As the current walls are cavity walls, when I get rid of the current 20 year old windows I am hoping that I can insert quality insulation all around the walls. Idea is have a thick underlay and carpet on the current tiled flooring and to pray for the GRP roof to do its job with out any problems around the 400 mm foundation. .

This is substantially different from the impression given by your first post. You are intending to do more than replace the roof, you have cavity walls etc....all new information

So ignore my first post, although it wouldn't be beyond the wit of most people to fix the Kingspan up there using some temporary supports made from s/h timber or cradling it with Visiqueen etc.etc, but it's now irrelevant.

Yes, houses from the 30s often don't have great foundations. My last one had 250mm. I wasn't exactly overjoyed about it when I found out. It explained some cracking of the rendering. The below DPC bricks were disintegrating too. Again, not something I would disclose to a buyer!

Bandia

Okay, I was under the idea that adding 1.5 x 1.5 windows on both sides and having a 2.3 metre patio doors with side upvc posts must be light enough, with the intent of increasing the brick work to upto 2.3 m in height.

I was given a quote of 6000£ for the GRP roof and another 2000£ for brickwork. Alternatively, the builder gave me a quote of 18000£ for underpinning all the 3 walls from 400mm to 1 metre foundation. But when I got the foundations dug out, we didn't check the width of the foundation. I may have to spend another 300-400£ for checking this information.

I am really in a fix. I need that space but can't afford to spend more than 7-8k on it as prices have almost dived by 10% in the 2 yes since I bought this place.

Bandia

EachPenny wrote: »

If you are substantially changing the walls and changing the roof then you may need to get planning consent as well. Have you looked into this?

Are you on clay soil there then?

I don't know exactly if the soil is clay soil but when we dug it up it looked like a thick mixture of black clay and coarse soil.

teneighty

Bandia wrote: »

EachPenny wrote: »

If you are substantially changing the walls and changing the roof then you may need to get planning consent as well. Have you looked into this?

Are you on clay soil there then?

I don't know exactly if the soil is clay soil but when we dug it up it looked like a thick mixture of black clay and coarse soil.

It would appear the ground conditions in your area are predominantly gravel over clay so you might be OK with a shallow foundation. However, there are pockets of silt which could be the "black clay" you described as clay isn't usually black, so not good and would require a much deeper foundation. Silt can be very soft and could need a very deep foundation maybe even piles.

You need someone to dig a trial hole and inspect the ground conditions, preferably someone who knows what they are looking at.

From your later posts I'm beginning to think you are flogging a dead horse. Probably better to demolish the conservatory and build a new extension from scratch.

EachPenny

Bandia wrote: »

I am really in a fix. I need that space but can't afford to spend more than 7-8k on it as prices have almost dived by 10% in the 2 yes since I bought this place.

If that is the case then you won't want to devalue your home further by building a dodgy extension without planning/building control consent, or which a future buyer's surveyor would take one look at and advise the buyer to pull out. Making your property unsaleable is not a good plan.

What do you need the space for?

Bandia

EachPenny wrote: »

What do you need the space for?

Sure, I already decided to not to go ahead as I obviously wouldn't want to do something that is not safe in the first case. I was following up on advise from people who were reassuring me that light weight air bricks and GRP roof don't add much weight on the current foundation but would increase the warmness of the place.

My intention was never to sell this place advertising it as an extension. I just wanted to keep it as a conservatory itself with double glazed doors separating the main house.

The current living room is very small and the conservatory is big size, so thought of converting the space so that my small kids can play and store their toys etc.

I will either have to leave the whole place as is or just spend the extra 18 grand for underpinning and full refurbishment.

Davesnave

Bandia wrote: »

I will either have to leave the whole place as is or just spend the extra 18 grand for underpinning and full refurbishment.

That's around what we paid, or what I would have paid if the conservatory company hadn't messed-up contractually to the extent that we could leave then £thousands short. I have a structure about 30m2, but it includes a porch.

I did quite a lot of the donkey work myself. Otherwise, it would have come in at well over £20k, which is too much for something that can't be regarded as totally permanent. The base will last 100 years, but the plastic bits, maybe 20-25yrs.

With a GRP roof you would have more of a garden room. Fine, if you can stomach/ameliorate the loss of light. We couldn't.