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

Bandia

Hi Guys,

I have a conservatory at my home which is 7 x 3.2m (7 m wide, 3.2 length).

It has a 400mm tall wall on all 3 sides and very old double glazing windows sitting on the wall. The existing roof is nothing but plastic panels. The foundation beneath the walls is 400mm.

The places becomes a freezer during winter and my heating bills just don't look good trying to keep that place warm.

If I want to have the roof replaced with a GRP roof with insulation, I am hoping that I can use that space more often.

Is it feasible for the 400 mm foundation to take the load of a fibreglass roof ? I am being told by builder that it must be okay, but does anyone here have experience with this. I was initially planning on underpinning the walls but got put off with the cost involved for that.

Thank you in advance for your suggestions and responses.

Davesnave

I would buy the Kingspan (other brands are available) now and rig it temporarily to the existing roof, if you can, in order to judge whether the expense of fitting GRP is going to be worth it.


My conservatory is usable all year round, but the only thing that makes it different from a house is the roof, which is not building regs compliant. If your walls, floor, and these old windows are going to leak heat like a sieve, having an insulated GRP roof may not be enough and you may also lose a lot of light to the room behind.

Doozergirl

Your builder knows little about foundations.

400mm is deemed inadequate as you don't have solid ground - all ground moves and the closer to surface it is, the more susceptible to movement it is. The idea of any good founsation is to find solid ground, firstly, and then consider the weight, which in most houses isn't a huge load overall and so the required depth will only really vary according to the ground the house is built on. The minimum depth is 1 metre under building regulations but foundations can be calculated properly for the conditions by a structural engineer.

These roofs are expensive and in my opinion, false economy when they are fitted retrospectively to a building that was never intended to be permanent, was cheaper than an extension for good reason, is susceptible to movement and is already some way into its feasible life.

teneighty

Doozergirl wrote: »

Your builder knows little about foundations.
The minimum depth is 1 metre under building regulations but foundations can be calculated properly for the conditions by a structural engineer.

That simply isn't true. The Building Regulations does not stipulate depth of foundations, only width and thickness. Each building project is determined by the local ground conditions, the 1 metre depth mentioned by Doozer only applies to clay sub soils.

It is the type of sub-soil under the conservatory foundation that is important. It could be gravel or chalk or solid rock. I went on a job many years ago where a DIY builder had hired a breaker and spent 2 weeks cutting through solid limestone to get his house extension foundation down to the mythical 1 metre depth.

In reality a GRP roof will add very little additional load to the overall structure so will have no meaningful affect on the foundation. By far the largest load for any roof design is the live load, not the weight of the roof itself.

It might well be that the 400mm deep foundation is inadequate for a "proper" extension but is perfectly acceptable for a lightweight prefabricated conservatory as the framing can take the seasonal movement of the foundations rising up and down with the changes in moisture content of the soil and freeze/thaw action without causing damage to the structure.

Doozergirl

My bad re: depths. I've only ever worked on clay sub soils in London and the Midlands and the minimum depth has always been 1 metre and is accepted in these parts as the rule. We've certainly never hit anything like solid rock and I'd still stand by my comment of a builder not knowing much if there answer to whether a conservatory footing is adequate with an answer like "it must be".

I don't believe that the average built conservatory would cope well with seasonal movement . Our structural engineer spends an inordinate amount of time looking at failing conservatories with inadequate foundations. People's expectations of them stretch further than the quality of the initial build and now people are expecting them to perform just like a properly built extension and are opening themselves to potential problems when making alterations.

stator

You shouldn't be heating a conservatory. Whoever fitted a radiator into a conservatory was breaking building regulations

phill99

stator wrote: »

You shouldn't be heating a conservatory. Whoever fitted a radiator into a conservatory was breaking building regulations

Which building reg exactly?

Typhoon2000

My house came with a light weight tiled roof and insulated and plastered ceiling in the conservatory which is 5m x 4m. It already had radiators in it and is perfectly useable all year round. We widened the opening from the kitchen to the conservatory and took off the new doors once building control signed off the work.

Doozergirl

Typhoon2000 wrote: »

My house came with a light weight tiled roof and insulated and plastered ceiling in the conservatory which is 5m x 4m. It already had radiators in it and is perfectly useable all year round. We widened the opening from the kitchen to the conservatory and took off the new doors once building control signed off the work.

Yay! It's a wonder Building Regulations even exist.

People complain about cowboy builders but they're quite happy to ignore the rules when it suits.

EachPenny

teneighty wrote: »

That simply isn't true. The Building Regulations does not stipulate depth of foundations, only width and thickness. Each building project is determined by the local ground conditions, the 1 metre depth mentioned by Doozer only applies to clay sub soils.

Approved Document A Section 2E states:-

Minimum depth of strip foundations
2E4
Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth, however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground.

2E4 then goes on to explain the issues with clay soils.

So unless you have (competent) rock within 450mm of the surface then the absolute minimum depth in all cases would be 450mm... deeper if soil conditions dictate necessary.

Typhoon2000

Doozergirl wrote: »

Yay! It's a wonder Building Regulations even exist.

People complain about cowboy builders but they're quite happy to ignore the rules when it suits.

Yeah right, my house must fail every single building regulation going being built before such things existed.
I have the doors in the garage to back on if every looking like the place is going to fall down from the cowboy builders.