Is there a universal law of screw sizing?

[epsilonGreedy]

Is there a general formula for choosing the right sized screw assuming softwood and a countersunk head?

 

I am hoping someone will offer a formula like:

 

"not less than 2/3rd of the screw length in the larger piece of wood"

"always screw through the thinner piece of wood into the thicker"

"Screw diameter should not me more than 5% of the thinnest piece of wood being jointed". 

 

For example I am currently adding blocks to rafter tails so that I am not fixing a wooden fascia into the end grain of the rafter tails. The rafters and supplementary blocks are both 38mm thick and in this application a 4 x 50mm screw seems about right but that is just a guess.

 

 

[markc]

No universal “table” or formula due to too many variables.

type of timber, thickness, loads to be carried (tension, twisting, rotational, shear etc).

location of fixings within the piece, close to edge, end grain etc.

asyou say, always better To go from thin to thick and around 3/4 of way through the thick bit.  You can always then complicate (or use what you have in your hand) by skew screwing at an angle to prevent going straight through and into your hand

[Russell griffiths]

2x38mm timber needs a longer screw than 50mm 

i would go 60 or 70 

screw one on with your 50mm and see how easy it is to pry it of with a short bar. 

Always try the destruction test. 

[epsilonGreedy]

33 minutes ago, Russell griffiths said:

2x38mm timber needs a longer screw than 50mm 

i would go 60 or 70 

screw one on with your 50mm and see how easy it is to pry it of with a short bar. 

Always try the destruction test. 

 

Think you are right, I suffered a brain malfunction. The 4x50 screws are for fixing the 20mm fascia onto the reinforced rafter tails, I am on 300 centres and so hope the cast aluminium heritage guttering won't stress the fascia fixing. 

 

I have some 5x60 to hand but I don't like driving such a fat screw into a relatively small lump of wood as used on the rafter tail reinforcement. I will get some 4x60 ordered. So far I only used stainless wood screws, would a real pro be ok with the yellow wood screws on a non exterior facing job inside a boxed eave?

 

https://www.screwfix.com/p/spax-yellox-pz-countersunk-woodscrews-4-x-60mm-200-pack/61066

 

Ok off to do some destructive testing.

 

 

[ToughButterCup]

2 hours ago, epsilonGreedy said:

Is there a general formula for choosing the right sized screw ... 

 

Yep: you've never ever got the right width, length, head type, drive type, metal type . Ever. Universal Law (Newtonian discovery) .

Including screws in stock in Builder's Merchants. 

 

So the formula is;

Use what's in the (bottom of your tool bag, + nick your dad's screws ) when he's not looking, * (pick up  every dropped screw)

 

Or more simply :  RSS = (BTB + DS) * (AllDropped Screws)

 

I discovered this just when I started to suffer from Osteo Arthritis - its the (pick up  every dropped screw) that's the hard bit.

[epsilonGreedy]

5 minutes ago, ToughButterCup said:

Yep: you've never ever got the right width, length, head type, drive type, metal type . Ever. Universal Law (Newtonian discovery) .

Including screws in stock in Builder's Merchants.

 

 

I tried to simplify matters by only using Spax head screws. Now I am looking at the Screwfix site thinking full thread or partial. Yellow coated, zinc coated or proper stainless steel for the inside of a wooden boxed eave and that is before considering size.

[TonyT]

I’m sure there is a British standard for fixings, pretty sure i remember downloading it before I left the free access  to all the standards with my last mob

[ToughButterCup]

34 minutes ago, epsilonGreedy said:

I tried to simplify matters by only using Spax [...] screws. .

 

Did you mean Torx Head Spax screws? Because if you did, so have I. Expensive. Thats  why I pick the buggers up.

[Dreadnaught]

It took my hours of reading (and learning) about screws before I settled on which to use to install my windows (DIY-max install of six big-ish IdealCombi windows).

[epsilonGreedy]

1 hour ago, Dreadnaught said:

It took my hours of reading (and learning) about screws before I settled on which to use to install my windows (DIY-max install of six big-ish IdealCombi windows).

 

 

I thought your new build plans had stalled but it reads as though you are watertight now?

[epsilonGreedy]

16 hours ago, ToughButterCup said:

Did you mean Torx Head Spax screws? Because if you did, so have I. Expensive. Thats  why I pick the buggers up.

 

 

Possibly. I thought Torx was a cross manufacturer design and Spax had enhanced it with an extra lug on the bit head? I could be wrong.

[SteamyTea]

1 hour ago, Dreadnaught said:

It took my hours of reading (and learning) about screws before I settled on which to use to install my windows

Hammer and nails, then lots of foam.

[Dreadnaught]

4 minutes ago, epsilonGreedy said:

I thought your new build plans had stalled but it reads as though you are watertight now?

 

Ah yes. Fair comment. I have been lamentably lax with my blog posts. Should get back to them.

 

Foundations are done. Timber frame is up, roof is going on. Windows are on site and I am just about to install them. Roof windows too. Hope I will be superficially weathertight (without external cladding) within a a month.

[TonyT]

[TonyT]

This has some info, not sure if its online, taken 5 minutes to uploads the screenshot , not in a good signal area today 

[bassanclan]

I always go by the rule that if i was fixing something x thick, i would use a screw at least 2x long. E.g. min 36mm screw to fix 18mm osb to a rafter etc. Reality would be 40-50mm screw

[carlb40]

The old rule used to be 2.5 times the thickness of the material being fixed. So if you were fixing a 25mm item, you would use around a 65mm screw. 

 

[joe90]

I have never thought about “rules” fir this, it also depends on the situation, shear, compression etc also depends on wood, oak vs pine vs ply, I just use what I consider appropriate ?‍♂️