Raft Foundations ?

yop

Lads

There is a possibility that we have to go down the route of putting in a raft foundation.
Anyone know as a percentage home much more costly are they than a strip foundation?

Or is there even some way to calculate how much it would cost?

cheers

AidoX

Hey Yop,
I dont know how much the cost of a raft foundation but if it means more contrete then you can expect the price to jump up.

Ive started my self build in Dublin, we're in the middle of doing the foundations and what the architect put down could hold up a hotel... my builder was more than surprised... alone my founds are costing 25K thats including the garage founds also.

Looking for good brickies at the moment... wont be too long before the TF kit goes up ::touch wood::

Victor

Unless there are really special reasons, choice should be down to cost, based on the respective engineering designs.

The raft will be thicker and use more reinforcement but less excavation and blockwork.

Ive started my self build in Dublin, we're in the middle of doing the foundations and what the architect put down could hold up a hotel... my builder was more than surprised... alone my founds are costing 25K thats including the garage founds also.

Then ask the architect why, if the builder has said it seem excessive.

kadman

Hi Yop,

If you're doing a raft foundation with finished floor, does this put your ch pipe runs further up program, if it does you need to organise it early.

In the case of your timberframe , you would need to position your runs in accordance with the sole plate layout of your tf kit plans, and not your original architects drawings. When tf designers are drawing panel plans, they may sometimes move wall panels to accommodate disabled access regs, and to fit in stairs to regulations, sometimes omitted on architects plans. Although this may only be 100mm, its enough to cause major headaches.

So its safer to design pipe runs in accordance with the tf kit soleplate layout as this is the datum set by the tf company.

Some info on founds,
http://www.selfbuildland.co.uk/self-build-uk.html

kadman

slayer92

Be very careful with the costing on the raft. Raft include lots of mild steel rods. The price of mild steel has jumped over 30% in the past couple of months. I know from bitter experience.

Regards

kadman

Hi Yop,

Another quick thought. What was the spec for the sole plate fixing, and who is fixing it
Normally it was just strapped down onto the block course with something similar to a wall plate strap. Or the more adventurous direct fixed with a rawl fixing or similar, necessitating boring.

On a raft with finished floors in the one go, leaves the sole plate needing direct fixing, so you need to check with the tf company.

Normally when the soleplate is fixed to blocks, the floor screed more or less comes to this level, so the panel heights start from the top of the soleplate.
Now that you may be fixing soleplate onto finished floor, this increases floor to ceiling height by the thickness of the sole plate.
So a 2400 panel onto a 75 mm soleplate will give a floor to ceiling height of 2475. Makes no odds really in the scheme of things

kadman

yop

You have me lost there Kadam,
What you are saying is that the raft is the finished floor?? That could not be correct?

The sole plate or soap bars will go directly on top of the raft, then the frame strapped to that

Victor

The raft is the structural floor slab. You can add a screed however, just make sure it all fits if the doors come with the timber frame.

kadman

kadman wrote:

On a raft with finished floors in the one go, leaves the sole plate needing direct fixing, so you need to check with the tf company.

QUOTE]

Hi Yop,

You are correct, sorry for any confusion.

To clarify my earlier post where I said "On a raft with finished floors in the one go", I should have said On a raft foundation, that then has the finished floors prepared before the soleplate is fitted. Then the sole plate would need direct fitting.

It does happen this way sometimes, the slab is fully prepped with all ch pipe runs, water domestic supply, floors finished , then the tf soleplate is fitted and tf kit installed.

As you correctly say Yop, your arrangement is spot on, if you use the soap bars. If you dont you're back to direct fitting the sole plate to the raft, or you could strap it to it, but it would be harder to level and position strapped.

Sorry for the confusion

kadman

YAPP

Hi,

Has any builders here ever done a direct cost comparison between a
Raft and a Strip Footing?


Taking into account steel cost and steel fixers rates,
cost of concrete and the volume diffrence per footing,
labour in prep of raft and that of strip footing?

I think a strip works out cheaper, but i havent actually done a direct
comparison.

Usually, the design engineer (signing off the build) will specify the
footings reqd based on structural loading, type of soil on site and area
in country.

Theres no problem with putting a tf on a raft; same structural effect/transfer.
And if the base is powerfloated to a good finish, you can always fix the kit to the concrete (less labour & materials) but the base has to be perfect!

As sean says, ensure that the u/f heating and assoc plumbing is fitted to the dimensions as the tf company's base plan, and make sure the guy fitting the base works EXACTLY in accordance with those dimensions, (EXACTLY!!!)
You wouldnt want the kit comming out at 13.150m and your base fitted at 13.000m would you;... it happens i'm sure...
\
YAPP

kadman

Hi Yop,

As you probably already know, your soleplate upper face is normally regarded as finish floor level. So you would need to organise your insulation and floor screed to achieve this.

Your wall panels are fitted down onto the soleplate, and then the bottom chord of the door ope is cut away, to allow for the door frame fitting. So your soleplate upper face needs to be at f.f.l, or slightly below.

Given this arrangement, your soapbars+soleplate is going to give you a depth of approx 200 mm, for insulation and screed.

You could always finish the slab , as I said before, and then direct fix the kit onto the slab. No soap bars, no soleplate. There are benefits in this approach.

But considering that there is a change in foundation requirements at this stage, a meeting with your tf provider would be beneficial for you to decide on the appropriate method of soleplate fixing for you both.

Also in soleplate fixing, it is not acceptable to pack under a soleplate, where the base is low with timber or ply. Steel shimpacks should be used, but I have seen bits of ply and red deal.

I assume you have been given soleplate fixing details by your tf company.

kadman