RAFT Foundation

[Daniel O'D]

Has anyone an opinion on the use of what is called a RAFT foundation for a signle storey house extension. Concrete is poured to 7 inches on a metal grid, left to set overnight and block work begins the following day. A 20 foot by 20 foot structure with apex roof can be built in a week. Is this as good as it seems. Common sense dictates that there must be shortcomings or pitfalls, otherwise it would be common practice with todays construction demands.

[emailtojohn]

Have built several garages and extensions using a Raft. I would suggest leaving the concrete harden for a few days before building blocks. I assume that the raft will also be the finished floor which will save you having to pour concrete a second time. Be careful with your levels and pipes as you will not have a second chace ro correct any problems.

You will hear lots of horror stories of rafts, but for a small single story structure, especially if the ground is suspect, they are ideal. Use plenty of steel, especially around the edge,(where the walls will sit) and you will have no problems.

[Bren88]

Raft foundations are slightly harder than "start mixing and do it yourself". In the right situation they are ideal. Ans as for using the raft as a finished floor, you can no problem. But a better finish can is advisable for certain floor finishes, should you wish to change in the future, so a small screed wouldn't hurt. Or you can power-float it.