New suspended patio

[Edward]

Our current patio is ground bearing and is held in place by a retaining wall, which is failing as it has no footings. The structure is independent of the house foundations so there's no risk to the house. The patio is currently about 1.5m above the garden - we also need to raise it slightly so the step from the new doors is reduced (see picture)

 

My solution is to remove the retaining wall and the necessary material currently being retained and create a new suspended patio using steel tied into the house and a new retaining wall. The steel will then support block and beam, to which we can then build the patio back onto using the existing York flags. The new retaining wall will have (i think) a cavity of which the steel will rest onto the blockwork, whilst we can reuse the bricks from the failing wall to finish the new wall. I have a few questions:

 

a) How can i specify the stiffness of the new steel without the use of a structural engineer? 

 

b) Does the block and beam need a screed on top to accept mortar to then accept flags? Is there a spec for the mortar.

 

b) how does this new steel tie into the existing house - is there a standard detail knocking around somewhere? 

 

Airbricks, DPC and drainage im comfortable with.

 

I hope the above makes sense.

 

Thanks.

[Declan52]

That's a very very expensive method of doing this job. Remove the wall that's there and dig out the what's needed to allow you to put concrete foundations in. If you are looking to hold back 1.5m worth of height the concrete is easily going to be 1200mm wide at the bottom. So from the front of that wall measure 2200mm and that will give you enough distance for the foundations and a 1000mm gap to allow you to work safely without the fill under the patio causing the trench to cave in. It might just be easier to remove all of what's under the patio.

Pour the concrete and build a block wall 900mm wide for 2 courses then step it in to 675mm for 2 courses then 450mm for 2 courses then 225mm till your finished height. Don't forget to leave drainage holes in the blocks to allow water to escape. 

Once the wall is built and everything is rock hard put a drainage channel behind the bottom course and fill around it with pea gravel. Then backfill with clean stone in the area against the wall and using a Wacker plate run it over every 200mm of stone you put in so each layer is well compacted. Just keep doing this till your at whatever height suits to start the bedding of your flags. Put this down and then your flags. 

You can use membranes in-between layers of the stone and have this resting on the stone and the wall so it's all tied together. 

If you want the Face of wall to look like it is now then you will have to add on whatever that stones width is to the concrete foundations otherwise put some render on it.

[Edward]

Ok thanks. It would be my intention to remove some fill behind the wall to ensure the new wall would not be retaining anything. I was also considering removing much the fill to accept EPS blocks to make the levels. This would negate the need for a block and beam and possibly reduce the size of the footings.

[Declan52]

Why would you want to use block and beam on this job. They are very heavy to move around. A wheelbarrow with stone is a lot easier.

[Edward]

Because the current retaining wall is failing and needs replacing. I also need to raise the patio inline with the doors to reduce the drop. If i use stone and a wheelbarrow, the new wall will need significant footings to retain the stone. In my suggestion, the new wall would not be retaining anything. 

[joe90]

 Cost of retaining wall vs cost of beam and block ?   Amount of work building retaining wall, filling with stone vs amount  of work for beam and block? You pays your money and takes your choice.

[Edward]

11 minutes ago, joe90 said:

 Cost of retaining wall vs cost of beam and block ?   Amount of work building retaining wall, filling with stone vs amount  of work for beam and block? You pays your money and takes your choice.

 

My original post was concerting the specification around the steel. 

[Declan52]

39 minutes ago, Edward said:

Because the current retaining wall is failing and needs replacing. I also need to raise the patio inline with the doors to reduce the drop. If i use stone and a wheelbarrow, the new wall will need significant footings to retain the stone. In my suggestion, the new wall would not be retaining anything. 

You need to put concrete in no matter what width of wall your building. So you  make it slightly wider than you have planned and do it right. The cost of steel and block and beam flooring wont be cheap or easy to put in place. How are you planning on lifting these beams over 1.5m into the air. They are heavy. As well your going to end up with a void that will be the new 5 star rat hotel in your area.

[joe90]

41 minutes ago, Edward said:

 

My original post was concerting the specification around the steel. 

Ah sorry. Why steel? Do you mean along the house wall to rest the beams on?

[Edward]

27 minutes ago, joe90 said:

Ah sorry. Why steel? Do you mean along the house wall to rest the beams on?

Perpendicular to this, from the house to the retaining wall

[Mr Punter]

Draw a plan to scale so it is easier to work out quantities.  Use an engineer to spec the steel.  They should be cheap.  Get a quote first.

 

You can just brush in sand cement slurry to the beam and block to lock it all together.  Lay the slabs on them into mortar or a sharp sand / cement screed.

 

I would not want to tie the steel to the house but your engineer may have a suggestion.

 

You don't want the concrete beams more than 3.5m or you will not move them.  Even then you will need a helper and some Weetabix.

 

My preference would be a wall at the front and back to support the concrete beams.  You could have storage underneath.