Foundation Size needed for a part brick and timber porch

[markblue777]

Hi all,

Looking at building a porch and I was going to make it part brick and part timber. I was going to have the brick part 500mm (or should it be more or less, im dont mind if it is just dpc height and then the timber frame sits on that) in height and then the timber frame built on top of that (probably going to clad in softwood pine painted or may just go for plastic)

My question is how far would I typically need to take the foundations for something like this and I have a soil line that runs outside my door (i will be building about 100mm away from it, possibly a bit more not 100% on the sizing yet) Do I need to dig to under the pipe to run the foundations.

The timber frame will be secured to the brick course and the back of the timber frame to the house wall, not sure if that makes any difference with regards to footing depths.

(separate note, does a DPC need running between the wall and timber frame where it joins to the house, I was going todo it between brick course and timber frame for the base so assume it should be done for the back as well)

Cheers
Mark

 

[Fat ferret]

For foundations the rule of thumb is usually the thickness of the wall deep and 3 times the thickness wide. So in theory a single brick wall can sit on a 75 x 225 mm thick strip with the same of rubble under that.

How deep to dig? well I usually just dig till I hit something solid like compacted subsoil where no one has dug before. I prefer to do this by hand for a neater job as digger drivers can end up digging too much out and making more work and having "accidents". How deep the trench is, is a bit meaningless to me it depends on how likely the bottom is going to move once it's got all that weight on it.

This is just for garden walls, plinths and small stuff I have done in the past so not sure what difference it being part of a house makes.

My instinct would be to re-route the soil pipe if it's in the way. Don't know about your other questions.

Hopefully some one with more exacting knowledge will be along soon.

 

[RobinBHM]

If you are building a building regs exempt structure then you can decide on foundation depth.

Porches may be built under permitted development.

Depth of foundation is dependant on weight of structure, type of soil, trees within zone of influence, locality of foul drainage. A single storey structure is typically built on a foundation 450mm x 1000mm deep (down from ground level). Most foundations these days are trench fill, which is about 650mm depth of concrete, then a row of high density concrete blocks. Face brickwork will start a course or more below ground level. Ideally finished floor level should be 2 courses above ground level.

You can do a dwarf wall any height. It should work out to brick gauge (75mm), so 450mm, 525mm for eg.

A dpc on top of the wall before starting timber frame is probably wise. Where frame meets house wall a vertical slot needs cutting vertically and a dpc fitted. Where the roof of the porch abuts the house wall, you may require a cavity tray.

 

[markblue777]

Hi robin, yeah the building is under permitted dev and except from building regs.

What do you mean with regards to a vertical cut? There currently is a covering when the porch will be so the porch will meet up to that

 

[Lons]

Building regs used to stipulate 600mm depth below ground level and the vast majority of houses and extensions have been built on that until relatively recently unless there were tree roots, drainage pipes or unstable ground to take into account. I personally wouldn't build on anything less, exempt or not and my own porch has this depth of found. Width of found should be 200mm wider than the wall (100mm each side) with the wall built centre of found.e.g. a 300 cavity wall should have 500 wide found, most are 600 wide however.

Found has to cope not only with the weight but with frost penetration and heave hence the depth. You can trench fill which is easiest and most cost effective if paying for labour, otherwise just a 200mm concrete found in bottom, preferably reinforced, then engineering bricks, trench blocks or 7n concrete blocks to one course below ground then engineering bricks to dpc level.
BTW contrary to common belief, a clay base is not the best as clay shrinks when dry and swells when wet.

If a soil pipe is in the way across the trench (not in line with it) it is normally possible to lintel over that with cheap concrete lintels, much easier and cheaper than re-routing the pipe. Whatever you do, don't trench fill over a pipe without allowing for movement or you risk pipe fracture at some stage. I've seen many such problems!

Seems overkill for a porch but remember if the found isn't adequate and the wall / frames / glass moves or cracks because of that there's no easy fix and you've wasted your time and money.

cheers
Bob

 

[n0legs]

BTW contrary to common belief, a clay base is not the best as clay shrinks when dry and swells when wet.
cheers
Bob

Clay Heave, can cause really big problems and a lot of expense :evil:

Got to agree with all Bob has said =D>
I was told a long time ago "start right and you'll finish right". It's served me well over the years.

 

[markblue777]

Yeah I was going to go 600 minimum. Or even just dig to match existing house foundations. The soil pipe would run adjacent to the front of the porch. So o may even have to rake foundation lower to go under the pipe so the load is spread under it.

Cheers
Mark

 

[Lons]

Yeah I was going to go 600 minimum. Or even just dig to match existing house foundations. The soil pipe would run adjacent to the front of the porch. So o may even have to rake foundation lower to go under the pipe so the load is spread under it.

Cheers
Mark

You only need to deepen the found in the vicinity of the pipe and can step the found if you wish, just make sure it's overlapped and linked properly.
I would definitely consider reinforcement as belt and braces. A 3 x 2 Mtr sheet is less than £30 and can be cut up to suit.
Cheers
Bob

 

[No skills]

A timely thread, I have my own glazed timber porch to do soon.

Has anybody used the steel base and mini pile (sort off) system? Seen it on a few conservatory adverts over the years.

Any thoughts?