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Raw plug spec for wallplate strap fixing.


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I have a boring question about selection of wall plugs for fixing roof wallplate straps into the wall. The NHBC advise:

 

HOLDING DOWN STRAPS

If the design specifies holding down straps to prevent the roof being lifted off the supporting structure, they should be at 2.0m centres (maximum).

Where straps are fixed to masonry, hardened nails 4mm in diameter x 75mm long or No 12 wood screws x 50mm long into plugs should be used.

The number of fixings should be in accordance with design requirements and the lowest fixing should be within 150mm of the bottom of the vertical strap

 

The NHBC diagram also states at least 4 fixings per strap. 

http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2011/Part7/section2/sitework.htm

 

If I take the No 12 x 50mm long wood screw option what rawplug should I buy? Given that these screws effectively prevent the roof lifting off in a storm I am thinking hole size and rawplug type is more critical than fixing a curtain pole.

 

I am fixing to Plasmore fibolites which are classed a lightweight block but at 8kg a block they have a bit more meat than the lighter aerated blocks.

 

This following page is useful. A No12 wood screw is 5.5mm metric and the reference table suggest a 7mm hole with a brown wall plug.

 

https://handycrowd.com/screws-explained-gauge-size-pilot-holes-and-wall-plugs/

 

If this info is correct I reckon my only remaining question is, do I need some special high load rawplug?

 

Actually one more question. Should the raw plug be longer or shorter than the screw length?

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Nails or concrete screws here. Depending on the blocks. Don’t like screws for this sort of stuff as most (read all) chippies seem to just use a countersunk head screw and they look crap .. should be a pan head but who has those on the van ..? So it’s whatever is to hand usually ..!

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2 minutes ago, joe90 said:

I tend to drill half a mm less than should be , especially in lightweight blocks but your straps I would nail with hardened nails, so much easier and quicker.

 

Just need to clarify. If say the recommended wall plug hole diameter is 7mm you would drill using a 6.5mm bit?

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3 minutes ago, joe90 said:

but your straps I would nail with hardened nails, so much easier and quicker.

 

 

I have an irrational mental block about nailing metal into masonry, I just feel it is better to torque up a fixing with an intermediate material (rawplug) progressively taking up the strain.

 

If I nailed in these wallplate straps then during every winter gale I would imagine the masonry nails gradually opening up their holes in each storm force gust and then working loose. 

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35 minutes ago, joe90 said:

I tend to drill half a mm less than should be , especially in lightweight blocks but your straps I would nail with hardened nails, so much easier and quicker.


yes, 6.5 fir brown 5.5 fir red

 

15 minutes ago, Declan52 said:

Wall plate straps have been held in place for years and years without any issue. Use the right nail for the job and it will outlast you. 


yes again, I know what you mean above pulling tight with screws but these fixings are in shear force and screws are not designed fir that re the previous thread about not using screws in joist hangers, these are the nails to use.  https://www.toolstation.com/masonry-nail/p77559?store

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32 minutes ago, Declan52 said:

Wall plate straps have been held in place for years and years without any issue. Use the right nail for the job and it will outlast you. 

 

 

From your previous posts I get the impression that in Northern Ireland medium and heavy blocks are used. Do you think masonry nails into light blocks works just a well for a wallplate strap? 

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6 minutes ago, epsilonGreedy said:

 

From your previous posts I get the impression that in Northern Ireland medium and heavy blocks are used. Do you think masonry nails into light blocks works just a well for a wallplate strap? 

But your blocks arent the crumbly mess type blocks. Nail a strap on using the correct nails and try and pull it out.

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19 minutes ago, Declan52 said:

But your blocks arent the crumbly mess type blocks. Nail a strap on using the correct nails and try and pull it out.


+1 to this - and try pulling up perpendicular to the nail not outward. You’ll bend the strap end first before you get the nails to move. 

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7 minutes ago, PeterW said:

+1 to this - and try pulling up perpendicular to the nail not outward. You’ll bend the strap end first before you get the nails to move. 

 

 

Hmm yes. I suppose my thinking has been driven by a concern over cyclical loading and the thought of the metal nail opening up a wobbly hole in the block over the years.

 

Given the strong trade preference for nailing in straps I wonder why NHBC and the likes of Simson even bother to specify rawplugs and wood screws as a solution.

 

Ok so now my option is 70mm long regular masonry nails as specified in the NHBC extract above or the Hilti nails with a washer as mentioned by @Declan52

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If 10 plus tonnes of roof is moving upward enough to lift the wall plates then you have bigger issues .....

 

For ease I tend to use 150mm concrete screws through the tops of wall plates into blocks as you can use them to level or twist any irregularities out of the wall plate timber.  

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I would just use brown plugs and 5mm screws.  You will need to pilot a smaller hole through the strap, then bend out the way and go with the 6.5.  I would not go any higher than I can reach without a hop up.  It can be easy to displace the top block if it has no load above.

 

The wind uplift is a bigger deal with lightweight flat roofs than pitched and tiled.

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3 hours ago, PeterW said:

If 10 plus tonnes of roof is moving upward enough to lift the wall plates then you have bigger issues .....

 

 

I decided to calculate the weight of my main roof which is 105 m2 including the eave overhang.

 

The trusses are about 1.2 tons and the slate is 4.4 tons, so call it 6 tons including the battens.

 

There are 58 truss and jack rafter seating points on the wall plate = 103 kg per seat. I expect the central trusses will place a larger load and the minor corner hip end jacks much less than 103kg. Pleased now that I fitted heavy window lintels.

 

I am unable to calculate the lift force on the on the lee side of a 30 degree hipped roof but I know that a 40 mph wind striking 60m2 of sail will pin a 6 ton yacht on its side

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Isn’t that because a 6 ton yacht only has a 1 ton keel and you’re talking about an 18m lever acting on 4m centre of rotational axis ..? Therefore most of the force is quite low..? 
 

Pretty sure the dual angle of a pitch roof cancels a lot of the negative pressure and lift issues that occur with flat roof structures 

 

@Gus Potter one you can help with .?


 

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