Interpreting deflection numbers in a 1st floor joist design.

[Russell griffiths]

Cost verses value. 

 

My main philosophy is that if I feel getting a better product is beneficial to the long term quality of the house then the cost is immaterial. 

 

Lets say you can spec a better floor and the price goes up £500, so what that is a tiny price to pay. 

 

I would rather spec the fabric of the building as best I can and buy a cheaper kitchen, you can upgrade kitchens and bathrooms, but that bouncy floor will annoy you till the day you move. 

[epsilonGreedy]

6 hours ago, Russell griffiths said:

Cost verses value. 

 

My main philosophy is that if I feel getting a better product is beneficial to the long term quality of the house then the cost is immaterial. 

 

Lets say you can spec a better floor and the price goes up £500, so what that is a tiny price to pay. 

 

I would rather spec the fabric of the building as best I can and buy a cheaper kitchen, you can upgrade kitchens and bathrooms, but that bouncy floor will annoy you till the day you move. 

 

 

Yes though there are limits.

 

Avoiding floor bounce is high up my list of priorities based on the experience of my previous mass builder house of 15 years. The master bedroom floor (14' x 12') was manifestly under specified with its solid timber joists (thanks Bryant Homes). The adjacent trance of houses built by McClean to a similar price point had wooden I- beam joists and felt a lot more solid.

 

Conversely this week I ordered 100m2 of R37 cavity batts rather than higher performing R32, this equates to a whole house build cost saving of £600. After running the numbers through the@Jeremy Harrisspread sheet I found the annual heating cost difference was £27 using £0.15 per kWH.

 

Edit: Also I purchased a pack of the R32 and R37 batts and trial fitted them into the cavity with the lead brickie, he was a bit concerned the denser R32 batts would push out against the freshly laid brick or block courses.

 

Edited February 16, 2020 by epsilonGreedy

[Mr Punter]

5 hours ago, epsilonGreedy said:

Presumably deflection numbers are calculated with an industry standard reference load? 

 

Yes they have a standard for housing, flats and commercial.  You could also add your own loadings if they were extreme.  Normal stuff like water tanks are easily dealt with locally but you can adjust for thick screeds, heavy equipment etc.  JJI do Joistmaster software you can download.  You can adjust joist size, deflection etc and it gives you acceptable joists and even price per m2.

 

Make sure you let a "proper" joist designer do the final design as you may have omitted something that could come back and bite you.

[epsilonGreedy]

Update:

 

I got a revised quote back this morning for increasing the chord size in 35 joists. The cost increase is about 10% or £200.

 

10 joists were upped from 97mm to 122mm. Another 10 4m span joists upped from 72mm chords to 122mm and the remainder 3.6m span joists from 72mm chords to 97mm.

[Moonshine]

22 minutes ago, epsilonGreedy said:

Update:

 

I got a revised quote back this morning for increasing the chord size in 35 joists. The cost increase is about 10% or £200.

 

10 joists were upped from 97mm to 122mm. Another 10 4m span joists upped from 72mm chords to 122mm and the remainder 3.6m span joists from 72mm chords to 97mm.

 

Did you go for the 8mm deflection criteria?

[epsilonGreedy]

17 minutes ago, Moonshine said:

Did you go for the 8mm deflection criteria?

 

 

Not yet. I will have that conversation prior to placing an order, for the moment I am waiting on the other main supplier in my area to produce the initial quote.

 

If the larger size chords do not perform to the 8mm deflection criteria I am not sure there are further improvement options available having upped the joists chords one or two sizes at 400mm centres. My plot is subject to a ridge height limit so going up one joist height would lead to a problematic ceiling design on the first floor.

[epsilonGreedy]

7 hours ago, Moonshine said:

Did you go for the 8mm deflection criteria?

 

 

I am pleased you prompted me on this... the floor designer has all the deflection numbers to hand per joist size.

 

He says relative to 100% deflection (which I take to mean the max allowable by some industry standard) my original and improved defections numbers are:

 

Quote

 

J4 (5.4m clear span)   was at 78.7% deflection (17.52mm) now it’s 65.7% (14.63mm)

J10 (4.0m clear span) was at 49.4% deflection (8.12mm) now it’s 35% (5.74mm)

J9  (3.6m clear span) was at 40.3% deflection (6mm) now it’s 33.4% (4.97mm)

 

 

So that 14.63mm deflection seems significant though where most important on the landing there is a double J4 along the stairwell and a 3rd J4 less than 400mm away.

Edited February 17, 2020 by epsilonGreedy

[Mr Punter]

Sometimes you have to double up or use a glulam or two.  The 5.4 is a fair span.

[ToughButterCup]

31 minutes ago, Mr Punter said:

Sometimes you have to double up or use a glulam or two.  The 5.4 is a fair span.

 

And that's how we got our mezannine .... doubled the POSIs using the clips provided

[Visti]

You can see what we did to strengthen existing joists, though additional static load bearing capacity isn't the same as deflection under dynamic loads such as walking:

 

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