How do I select a screed thickness for UFH in real life with buildability cost in mind?

[Gus Potter]

Hiya all. 

 

This post is intended to provide insight into UFH screeds, the build process and build tolerances.. which drives overall cost. The first part sets the narrative. The last sets out and describes what drives screed thickness. 

 

At the end of the day if you are building a new house or just extending then it is vital that you understand how each bit of the design works, how you are going to build it and the tolerances you have to apply to each element so you don't cause a later problem. On self builds few folk have the ability  / time to ensure that the contractor is delivering what they say they will!

 

To get round this we need to simplify the design so self builders / extenders can see that they are getting what they paying for, compare builders quotes. We need to build in tolerances that the builder can work to when the rain is pissing down their neck. This is real life in the building trade!

 

With my SE hat on I want to make sure that the slab is thick enough, but buildable, so the slab could be 10-15mm high unless the Client wants to pay extra for the tighter tolerance!

 

"But the Architect told us it would work!"  It might but they may not have told you how much extra that would cost to work to a tighter tolerance!

 

Oh no, if the slab is high that can't be as my UFH pipes won't fit!

 

As an SE I want to control the level of the sub base, if sub base is too high the slab is too thin. Now in a stricty controlled enviroment with a traditional Clerk of works we may get this down to 10mm high on the top side, not an SE issue in the main. SE washes their hands at this point, but if they are nice guys like me they will alert you to the problem. 

 

Any more tighter tolerances than that and the Contractor will add cost.

 

But the whole idea of self building is to cut out these middle men! The only way you do that is to keep it simple stupid!

 

That statement is based on the what I've learnt from being in this business for 40 years! This is the art of design! See where you can save time on site, make it easily buildable, reduced Contractor risk ( so they don't inflate the price) then you can use the savings to deliver what you actually see as the end result. 

 

Many self builders just "hope for the best".. but this is the building trade and there are no friends in the desert. 

 

Once you get into this mind set then you are in a good position to "sell yourself " to builders. If you make it simple then the risk to them is less and the price will likely go down. If you take the time to understand what you want to build and how it is going to be executed then you are in a strong position. Your reward will be that you open up the field for the more builders to compete for the work not least. At some point you will find a builder that says, I want this job and can see how I'm going to make a profit as I can see that is is not diffucult and carries less risk. 

 

Now I've had a few Clients over the years. If you are smart enough to be able to think about extending / self building and have some funds then you are not daft. My role is to give you the information that you need to make an informed descision. If your advisors are not doing this then you need to ask why. 

 

Yes, this post is about screed thickness but it's actually about what is below that drives the equation. I design insulated rafts, a passive raft is just a nuance!  have done so for decades, but always having in mind what we need to put on top of the structural slab. The tolerance on a structural slab or just the supporting floor is,  having done lots of insulated build raft designs, is about 10mm  plus or minus on a good day!

 

I shite you not this is a good day! If you have beam and block then these have a pre camber and that inroduces another aspect.

 

If you assume a flat screed at nomimal 40mm thick then you are likely on a hiding to nothing. DO NOT DO THIS SCREED AT 40mm. Its complete bonkers!

 

Now I know that there are many folk on BH that advocate thin screeds, but my own view is that they may have only done one or two projects at most, some may have done more but have never checked or just put their heads in the sand and never admit they cocked it up!

 

In summary my main gripe is:

 

1/ That there is a total lack of design coordination and this results in extra cost as many contractors and designers as covering their arses. There is this perception that what you are putting the screed on is going to be flat and level.. that is cloud cuchoo land! Any Architect that wants to dish out advice needs to have practical experience and an understanding of what the SE is doing, the complete process and the tolerances every trade is normally expected to work within...otherwise they are a potential liability.  

 

2/ There are far too many folk poncing about on screed thickness without understanding what they are laying it on, structural slabs have a wide tolerance as do beam and block. 

 

3/ You can use the likes of "Loop cad" as much as you like but in the heat of battle some UFH pipes may have to cross over one another. There is an old saying you can plan for war, but as soon as the war starts the plans go out the window, this happens from time to time when building stuff. That is why I'm advocating for the simple stupid,it at least give you a fighting chance. 

 

Please folk try and think about cutting yourself some slack, the tighter the tolerances you ask for the higher the cost.. in reality, I see it so many times unless you make it simple you'll end up paying for something that never gets delivered.

 

Best to pay for something that is achievable, even if you need to make compromises? 

 

Good design always involves compromising to get the best overall outcome. This is often missed by novice self builders, its not a bad thing to have that enthusiasm.. what is bad is if they then chuck their money down the drain by not beaing able to recognise that design is about balance / the trade off. 

 

 

 

 

 

 

 

 

 

 

Edited 4 hours ago by Gus Potter