Timber frame foundation layout

[pauldoc]

Hi, just like to start by saying thanks to all of you who got this site going, it's much appreciated!

Onto my question.....

I have just ordered my timberframe and got the line and point loads back from the supplier. Will they also supply me with the foundation layout for my main contractor to do the work from, or is that the job of my structural engineer? Is this a silly question? I just don't want to be in a position where the structural engineer will do his calcs for the foundation design and the ground workers turn up with nothing to work off as the frame company might say that's not down to us? if that makes sense 

[pauldoc]

maybe I should also add that it is not a passive slab, but regular trench fill foundation

[Alex C]

It's up to your structural engineer, although they may not give you a dimensioned drawn layout unless you specifically ask for it.

[Jeremy Harris]

As above, but be aware that you will need to make sure that there is no chance of interstitial condensation at the sole plate.  It seems clear that with increasing levels of insulation and airtightness not all SEs are taking full account of the need to change the way that the sole plate is both sealed and insulated from the cold foundation. 

This wasn't really a problem when we were building thermally poor houses, but is very definitely a major issue now, and one that has to be addressed by design, as it is challenging to retrofit a solution.  There are still a lot of standard details for timber frame to foundation junctions that are badly outdated and need review.

[IanR]

You may not require SE calcs at all for a strip foundation. Are there any unusual features within your build that are producing abnormal loadings, or do you have "difficult" ground conditions?

If not, your Main Contractor should be able to agree depths with Building Control during the excavation, there are some pretty standard rules applied.

 

However, what's stopped you going the insulated raft route. Is there a technical reason for your specific site conditions? If not, a passive raft may well be cheaper, with a lot more benefits beside.

[pauldoc]

Thanks for the replies gents. The one slightly unusual thing is that due to the sloping site, the front entrance is at a lower level. Its not a 'basement' but split level really.

This is below the dpc of the timber frame, and has a 'retaining element' although once the site is cleared I'm not sure how much retaining will be needed. It's hard to explain but I hope that makes sense?!

[ProDave]

I would say a SE is needed if there is a retaining element.

I am building my house on strip foundations. One of my design briefs to the SE and architectural technician was I wanted "simplified" foundations. A lot of timber frame companies will design a house that uses thin joists on the ground floor and then a lot of intermediate sleeper walls to support them. I took the view that I need thick joists anyway to get enough insulation in (I am using 300mm JJI joist) and the first floor joists (posi joists in my case) are capable of spanning the distances in one go, so there is no reason the ground floor ones can't also span the same distance without intermediate sleeper walls.

One thing others will warn you about is what happens if the timber frame arrives and it does not fit? Who is responsible?  I solved that problem by contracting one building firm to build the foundations, and construct and errect the timber frame. If it didn't fit, it was entirely their problem to solve. As it happened, the joiners came to measure the foundations before building the frame just to make sure the ground workers had got it right.

[Alex C]

To ensure the foundations and coursing blocks fit the frame you could  get a surveyor to set out them out and then measure the level of the laid coursing blocks. A specification of tolerance given to the groundworker is then easy to enforce. A timber frame company will give you a sole plate layout that you can work to.

[pauldoc]

The one thing I find difficult is that I am the one in the middle and they all communicate through me, and I don't understand everything that is asked or said...although it is a steep learning curve! I have told them talk to each other but they don't seem keen or don't reply as quickly!

Dave, I have gone the same route as you. The same company is responsible for the foundations and erecting the frame. I have also put a clause in the contract stipulating their responsibility if any errors....we shall see!!

 

[pauldoc]

Alex,

I agree. it will be professionally set out to the tolerances given....what can possibly go wrong!