DIY Forum

 

Ultimate Handyman Ultimate Handyman Ultimate Handyman Ultimate HandymanUltimate Handyman on Pinterest

 

DIY Forum/Home improvement advice forum

 

 

A-Z CONTENTS | DISCLAIMER | DIY VIDEO | HOME | SAFETY FIRST | FORUM RULES

It is currently Wed Sep 20, 2017 2:56 am
Visit Thermo worx


Time zone: Europe/London [ DST ]




 

 


Post new topic Reply to topic  [ 19 posts ]  Go to page 1, 2  Next
Author Message
PostPosted: Wed May 31, 2017 1:27 pm 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
Hello,

We have a new roof, with new frames, new tiles and new velux windows (all 1 year old now).

We have done most things right on our project, however one decision that i am REALLY not happy with is the size of our Velux windows.

We currently have 550x1200 top hung Velux windows, which are quite small.

I want to rip out these windows are replace with 940 x 1400 or 940 x 1600 velux.

Our roof trusses are at 600mm centes and so no trusses were cur for the current windows, but a single top section of truss would need to be cut for the new windows.

Here is a plan for what i want to do:

(Diagram shows face on view of slopes of roof trusses with timbers of 47 x 175mm)

Attachment:
18768453_10158688015425133_6048134459796348751_o.jpg
18768453_10158688015425133_6048134459796348751_o.jpg [ 42.49 KiB | Viewed 1294 times ]


I would normally double up the trusses either side of the window. However, we have VERY heavy duty timbers of 47 x 175mm supporting the roof tiles and 47 x 225 across the bottom string (floor). Also, we are using light weight Roof tiles at a weight of 20kg per square metre, but our roof trusses were designed for tiles that are 60+ kg per square metre.

So overall our roof trusses are over engineered and so i do not think that i need to use a double either side of the new velux. As the TOTAL load of roof components (tiles + top of truss + lats) above each window is only 70kg supported across two sections / three timbers.

However i have incorporated double horizontal timbers above and below the new velux to support this 70kg and spread the load side ways to the other trusses.

- My plan is to remove the current window and strip away a large amount of tiles etc.
- Remove the existing flashing and window frame.
- Put up acroprops and bolt in temp timber bracing across three timbers to support the one timber that is going to be cut.
- Cut the middle truss vertical.
- Then to install the new timber horizontal pieces (47 x 175) using structural screws & steel bracing.
- Then put in the new offset timber vertical pieces to the new windows size.
- Then install the new window frame and sort out the lats, felt, flashing and tiles.
- Then sort out the inside again including a bit of OSB and some insulation.
- Then slot the new window into it's frame.
- Remove temp supports and acroprops

But what do people think?

Many Thanks

Marcus


Share on FacebookShare on TwitterShare on DiggShare on DeliciousShare on Google+
Top
 Profile  
Reply with quote  
PostPosted: Wed May 31, 2017 5:30 pm 
Offline
Senior Member
User avatar

Joined: Sun Aug 18, 2013 8:04 pm
Posts: 7430
Location: Fife
Has thanked: 935 times
Been thanked: 1710 times
100% you need to double the rafter and 100% you'll need an engineer to spec the work as it's manufactured trusses you can alter them without an engineers detail/spec.



For this message the author steviejoiner74 has received gratitude : mestizomad
Top
 Profile  
Reply with quote  
PostPosted: Mon Jun 05, 2017 11:58 am 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
steviejoiner74 wrote:
100% you need to double the rafter and 100% you'll need an engineer to spec the work as it's manufactured trusses you can alter them without an engineers detail/spec.


Hi there,

Thanks for your reply. I have all of the engineers specs.

I would normally agree with you about doubling up if the timbers. However we do not have a standard roof and our roof frames are very high spec.

The timbers that we have used were designed for 3 x more load than we are actually using. As we were originally going to use much much heavier tiles (and so got the roof designed for these). But then switched to much lighter tiles after the roof had been made.

Our timbers are 7" x 2" at 600mm centres, but for the tiles that we are using, we could have used 4" x 2" timbers.

Also, We have neighbours (who have done the same roof work) who have used the same tiles, but have sued thinner 6" x 2" roof timbers as spacings of 1.5m with cross peices. We are one of the only people who have actually put in frames at 600mm centres and we are the only people that have used such heavy duty 7" x 2" on the slopes and 9" x 2" timber for the bottom string.

The entire load above each window is 40kg over each roof timber (roof frame from pinacle to window on one side + lats + felt + tiles).

So if the load was diverted sideways via my horizontal bracing, the additional load to the timbers Left + Right would only be approx 20kg. Yet these timbers are Underloaded (loaded to less than designed) by approx 75kg each side or 150kg each frame.

I am not trying to argue and i do appreciate all comments. As it is indeed possible that i am missing something.

Many Thanks


Top
 Profile  
Reply with quote  
PostPosted: Mon Jun 05, 2017 4:54 pm 
Offline
Senior Member
User avatar

Joined: Sun Aug 18, 2013 8:04 pm
Posts: 7430
Location: Fife
Has thanked: 935 times
Been thanked: 1710 times
Ask the Truss manufacturer you bought them from and you'll get the same answer I've given you.
If you think it'll be fine then bash on.


Top
 Profile  
Reply with quote  
PostPosted: Mon Jun 05, 2017 7:00 pm 
Offline
Senior Member

Joined: Wed Apr 16, 2008 7:49 pm
Posts: 6084
Location: south tyneside
Has thanked: 41 times
Been thanked: 487 times
100% agree with stevie. i wouldnt be cutting a truss unless it was approved by an se.

if it were a traditional style cut roof then its a different story.

your calculations are only taking into account the weight of the roofing materials also. not wind and snow loading etc

as far as i know there has never been a failure of a roof truss. but there has been when theyve been cut and altered.



For this message the author fin has received gratitude : mestizomad
Top
 Profile  
Reply with quote  
PostPosted: Mon Jun 05, 2017 9:31 pm 
Offline
Senior Member
User avatar

Joined: Thu Mar 09, 2017 4:47 pm
Posts: 528
Location: East Riding of Yorkshire
Has thanked: 11 times
Been thanked: 148 times
:withstupid:

with you both Stevie & fin

davyp1


Top
 Profile  
Reply with quote  
PostPosted: Mon Jun 05, 2017 10:08 pm 
Offline
Senior Member
User avatar

Joined: Wed Nov 17, 2010 5:51 pm
Posts: 6951
Has thanked: 344 times
Been thanked: 1202 times
mestizomad wrote:

The entire load above each window is 40kg over each roof timber (roof frame from pinacle to window on one side + lats + felt + tiles).

So if the load was diverted sideways via my horizontal bracing, the additional load to the timbers Left + Right would only be approx 20kg. Yet these timbers are Underloaded (loaded to less than designed) by approx 75kg each side or 150kg each frame.

I am not trying to argue and i do appreciate all comments. As it is indeed possible that i am missing something.


When you're dealing with manufactured trusses it's not just the load that has to be taken into consideration.

The truss is designed as a complete unit with the triangulation factor being very important: cut a truss, anywhere, and you destroy the triangulation.

You really do need to discuss it with the truss manufacturer. I'd be surprised if anyone on a public forum will tell you it's OK to cut a truss, if they do, then you can be assured that they have very little in the way of experience as a carpenter in the UK.

_________________
One day it will all be firewood.


Top
 Profile  
Reply with quote  
PostPosted: Thu Jun 08, 2017 3:46 pm 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
HI there,

Thanks for all of your replies. I have already contacted the Truss Manufacturer and they are getting back to me.

However, to be honest i don't fully agree. I do appreciate wind loading and snow loading and i am doing those calcs now also.

How do you explain the fact that My neighbour two doors down, has an identical property also with a new roof. They used Heavier tiles that our (2.5 times heavier) and they used thinner 6" x 2" timbers at centres of 1.5m??? And none of these roof frames have been doubled even with centres of 1.5m.

The previous roof that was on our building also had frames at 1.8m centres with lightweight asbestos tiles (that were only slightly lighter than the tiles that we are using now).

So even with my adjustments our roof would still be significantly more substantial than our neighbours property.

The way that i cut the truss and box in timber would also mean that the original cut truss would still take approx 50% of the load over it's section (it would take all load below windows and some load from above window, with 25% of it's load being supported by the Left truss and 25% supported by the Right truss.

And even with these increased loads of 25%, the Left and Right trusses would be stressed to absolutely nowhere near their max capacity. As previously mentioned they were original designed for a much heavier roof than is actually being used.

The roof trusses that we have were designed for 4 tonnes more weight (or approx 200kg per truss) MORE than is currently loaded.

Setting aside wind loading and snow loading, my proposal would only add approx 40kg of deadload to frames that are underloaded by 200kg. So they would still be underloaded by 160kg.

However yes you are correct. I do need to do snow load calcs and wind loading calcs and i do need to hear back from the manufacturer.

But they are usually quite slow and charge for calcs. But i will pay for the calcs before doing any work.

Many Thanks


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 10, 2017 7:22 am 
Offline
Newly registered Member

Joined: Sun Apr 20, 2014 6:43 pm
Posts: 65
Has thanked: 41 times
Been thanked: 1 times
If he did cut into the trusses would he have to support them with purlins


Top
 Profile  
Reply with quote  
PostPosted: Tue Sep 12, 2017 1:40 pm 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
Ferny1 wrote:
If he did cut into the trusses would he have to support them with purlins


Hi, thanks, Yes i do plan to install double thickness (4" x 7") purlins to support the cut out timber above and below.

But i don't want to have to double up the 7" x 2" roof frame timbers either side of the new window. As i think that would be excessive.


Top
 Profile  
Reply with quote  
PostPosted: Tue Sep 12, 2017 10:52 pm 
Offline
Senior Member
User avatar

Joined: Mon Sep 12, 2011 4:27 pm
Posts: 5034
Location: Lancashire
Has thanked: 566 times
Been thanked: 1182 times
Sorry to say this, but I'm in complete agreement with Stevie, Fin, Davyp1 and AyJay. When you cut into trusses like you want to it's standard practice to double-up - and get an S-E calc done. It isn't just about supporting the weight of the roof, it's also about the roof being able to carry snow loads, withstand high winds, control wall spread, wall loadings, etc. So now you have five experienced time sereved joiners all saying the same sort of thing, so maybe there's a reason for that

_________________
"The person who never made a mistake, never made anything"
Albert Einstein



For this message the author Job and Knock has received gratitude : mestizomad
Top
 Profile  
Reply with quote  
PostPosted: Tue Sep 12, 2017 11:30 pm 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
Job and Knock wrote:
Sorry to say this, but I'm in complete agreement with Stevie, Fin, Davyp1 and AyJay. When you cut into trusses like you want to it's standard practice to double-up - and get an S-E calc done. It isn't just about supporting the weight of the roof, it's also about the
roof being able to carry snow loads, withstand high winds, control wall spread, wall loadings, etc. So now you have five experienced time sereved joiners all saying the same sort of thing, so maybe there's a reason for that


Hi, thank you for the reply, I do appreciate it.

Yes I agree with you that it is standard practice to double up.

But what I have been saying throughout the thread is that our current frames are far from standard.

To say that our current frames are over engineered is a serious understatement.

Every complete roof frame was designed to take 2-300kg more dead weight (tiles) than is currently loaded. Or 5-6 tonnes total. And even with the heavier tile, they where still over engineered. When we reduced the weight of the tile, we should have had thinner frames installed, but we had already had them made.

I have done some snow loading calcs. And even with Very extreme snow loading such as would never actually happen in the U.K. (More like Antarctica). The total load across each section of cut timber (above window) is only 120kg.

This load will be supported partly by the original timber and the box that I will form around the window, but mostly by the timbers to left and right.

So even with very very extreme snow loading, the total additional weight to each uncut roof frame left and right is approx 60kg.

I would estimate with basic calcs that each one frame can take a weight of 750+ kg. But is currently loaded with approximately only 160kg basic load (not inc snow or wind).

Our frames also do nothing for the walls of the house as we have an unusual construction steel and concrete house with steel cross / tension bars that cross the entire width of the house and hold the wall together.

We did have a structural engineer around last week to look as something else. But whilst he was here he commented on how massively massively overengineered our roof was and he didn't understand why we had timbers that were so heavy duty.

Anyways. I am not going to do any work at all until I have spoken to atleast a couple of engineers and paid for some calcs (which I can't afford right now!).

And if they also do full calcs and still say I need to double up. Then I will double up.

Thanks


Top
 Profile  
Reply with quote  
PostPosted: Wed Sep 13, 2017 9:12 am 
Offline
Senior Member

Joined: Fri Oct 02, 2015 12:43 pm
Posts: 148
Location: France
Has thanked: 0 times
Been thanked: 20 times
When we had our house built the truss company new about the large velux, the trusses came doubled up on both sides and with extra braces from top to bottom, so Iam with the others, wait and see what the manufacturer says, they may come up with a quarter truss to screw either side of your existing ones. Nos



For this message the author Nos has received gratitude : mestizomad
Top
 Profile  
Reply with quote  
PostPosted: Wed Sep 13, 2017 3:41 pm 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
Nos wrote:
When we had our house built the truss company new about the large velux, the trusses came doubled up on both sides and with extra braces from top to bottom, so Iam with the others, wait and see what the manufacturer says, they may come up with a quarter truss to screw either side of your existing ones. Nos


Hi, thanks for your reply.

I am waiting to here back from the truss manufacturer now. But they also want to charge £300 just for calcs!!.

I was also thinking of trying to double up the timbers between the wall plate at the window, but NOT up to the ridge.

OR doing this:

https://ibb.co/jwpiTa

Thanks


Attachments:
velux diagram.jpg
velux diagram.jpg [ 44.37 KiB | Viewed 135 times ]
Top
 Profile  
Reply with quote  
PostPosted: Wed Sep 13, 2017 6:44 pm 
Offline
Newly registered Member

Joined: Wed May 31, 2017 1:23 pm
Posts: 8
Has thanked: 5 times
Been thanked: 0 times
I have just found the Original manufacturers drawings with FULL snow loading, wind loading and dead load calcs for each truss.

It turns out that my estimations were actually too high for the loads. The Basic dead load above each window is only 30kg per truss / section.

And even with Building Regs max snow loading, the load above the total window is still only 101kg.

This means that by chopping a truss and then getting the adjacent trusses to take the weight, the additional load for each 7" x 2" truss will be a basic dead load of only 15kg OR 51kg if SNOW loading is included.

I am still trying to figure out wind loading, But if i assume that it is equal to snow loading (it is usually a lot less), then the total load for each section under maximum combined Dead load, Snow Load and Wind Load is only 170kg. This 170kg would then be equally supported roughly 50/50, delivering 85kg on additional load to each adjacent timber under the most extreme snow and wind conditions.

There is only 0.87 square metres of roof above each window (0.6 m x 1.4m).

Thanks


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 19 posts ]  Go to page 1, 2  Next


Similar topics
   

Time zone: Europe/London [ DST ]


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Visit Hilti


 

 

 

News News Site map Site map SitemapIndex SitemapIndex RSS Feed RSS Feed Channel list Channel list
ultimatehandyman privacy policy

Contact

 

Powered by phpBB® Forum Software © phpBB Group

phpBB SEO