Porch Foundation Query

[berty3000]

Hi all,

I'm getting a upvc porch built on the side of my house. It would look something like the one below but more glass.

I've had a decent quote but the company in question is propsing to build the foundation for the porch onto the existing concrete driveway. It is a printed concrete drive that has been in place for about 25 years. It has the odd crack in it, but only a hairline one in the section that the porch would be on - probably from early settlement. The guy has said he is confident it will be fine and has to be as the job will be guaranteed, and they are in some insurance scheme.

He said he will do one layer of bricks up to the DPC of the house, use visquine, and fill in with concrete including reinforcing bars pinned down to the drive below. It won't ever move, particularly as the porch won't be placing much weight on it anyway. The only bit that will need to be dug out is some indian stone on the other side of the gate (gate and wall to be removed) - there he will do a 15" footing and tie in to the rest.

Does this seem like a sensible plan? My preference would be to cut the driveway out using a whizzer and build a new foundation for the whole thing but this would be more expensive and if it isn't necessary I don't want to waste money on it.

Thoughts?

 

[Notch7]

When my parents moved into their house in 1970 it had an entrance with 2 posts and a flat roof above - quite big, probably 2.5m x 1.0m. The ground was just paving slabs on a bit of a base. The porch lasted 40 years.

So if your drive is pretty stable with no movement it should be fine.

However bear in mind, it will be difficult to get a robust damp arrangement for your porch. Yes he can lay bricks on the driveway with a DPC, but driving rain and rain splash will soak the bricks above the DPC. Maybe he could use engineering bricks which aren’t very porous?

I don’t think reinforcing bars into the drive are a good idea, that’s just creating holes through the Damp proof membrane. Maybe he could do some just at the corners through the brick work.

I wonder if you risk condensation on the bricks or concrete floor inside in some weather conditions

 

[berty3000]

Thanks for the reply, Notch.

So there would only be one course of bricks forming the foundation. The DPC of the house is only one course above the driveway that the previous owners had installed. The visquine will be lapped up against the house. There would be the sill of the porch above the bricks.

I see what what you mean... if the visquine is laid on the drive, to prevent "rising" damp, the bricks and concrete within would be vulnerable to penetrative damp through the sides of the bricks from splashing... unless it is possible to lap the visquine up the insides of the bricks and over the tops of them?

 

[^woody^]

Do not allow a porch to be built on top of imprinted concrete. That drive is not a foundation.

The imprinted concrete should be cut smartly along the new porch outline and a proper strip or slab foundation laid, the walls built up and the floor added. Then the porch goes on that.

 

[berty3000]

Do not allow a porch to be built on top of imprinted concrete. That drive is not a foundation.

The imprinted concrete should be cut smartly along the new porch outline and a proper strip or slab foundation laid, the walls built up and the floor added. Then the porch goes on that.

The drive is 4" thick and has been stable for a long time. You definitely wouldn't compromise for a light weight structure?

 

[berty3000]

Do not allow a porch to be built on top of imprinted concrete. That drive is not a foundation.

The imprinted concrete should be cut smartly along the new porch outline and a proper strip or slab foundation laid, the walls built up and the floor added. Then the porch goes on that.

Also, what do you think is likely to be sound in terms of depth if we do cut the drive out? As in, from bottom of footing to floor surface of the porch?

 

[Notch7]

The drive is 4" thick and has been stable for a long time. You definitely wouldn't compromise for a light weight structure?

It’s true to say a concrete driveway is not a foundation, so it’s not the correct construction, as you know.

Modern construction is mostly: bricks, blocks, cement mortar, plaster finishes. They aren’t flexible and crack with the slightest movement.

Your porch is upvc construction, so small amounts of movement won’t be visible at all.


If you just want an unheated porch to give you a place for coats, shoes etc and improve warmth in the house, then it would probably be fine.

 

[Ivor Windybottom]

There's no fixed depth, you need to dig past the topsoil to undisturbed clay, sand or rock.

Anyone saying otherwise is a cowboy. They know it will last until your payment clears, beyond that they don't care. Don't even argue with them, get someone else to do it. If this is their approach then you can guarantee that every other corner will be cut too.

Porches aren't heavy but are particularly unbalanced, as they have 3 sides. I've seen a couple of houses for sale where a botched brick porch has tipped away from the building, due to all its weight being on the side away from it.

 

[Ivor Windybottom]

There's a world of difference between the first and last example photos you've given. The first is all brick, the last is all plastic panels. Which is it?

 

[berty3000]

There's a world of difference between the first and last example photos you've given. The first is all brick, the last is all plastic panels. Which is it?

The first photo is my house as is, where the porch will go. If the concrete driveway is cut out and there is compacted aggregate under it, couldn't the new foundation for the upvc porch be built up from that? Probably three brick courses or so? Assuming bricks used.

 

[Notch7]

I've seen a couple of houses for sale where a botched brick porch has tipped away from the building, due to all its weight being on the side away from it.

Foundation movement probably rather than weight.

 

[Ivor Windybottom]

Three courses of bricks is probably less pressure than someone standing on the driveway.

It's not without risk, but may be OK. It's not the proper way of doing things though, you may want to look into what the warranty covers and who provides it.

 

[^woody^]

You don't know the thickness of the concrete, it's grade, the sub-base or the ground condition. So don't know the suitability of the concrete to bear any load.

And it's not just about "pressure" or the load.

You will be placing a fixed load on a drive. Point loading from the frame, and the floor base. The frame is fixed to the house, and buildings and drives move seasonally, albeit slightly. So there is the risk of shear cracking at the perimeter.

So who manages that risk, the contractor or the homeowner? And what is the consequences of repair and cost of repairing any movement?
If its going to crack, that normally happens after several seasons or years. Is there an insurance backed warranty to cover the installation over the next say 10 years?

 

[berty3000]

Fair enough. I've rejected that approach. Thanks Woody and others for input.