Structural calcs for roof lights

[jesteh]

I am having an extension sized at 7m x 3m with pitched roof with 2x fixed roof lights sized at 2m wide x 1.4m long (triple glazed & each weighing around 150kg).
Does anyone think if this will require any structural calcs due to size and weight or can I get away without doing these?

 

[RonnyRaygun]

150 kg for 2m X 1.4m is less heavy than the roof that would otherwise be there, but with it being glazing you will want to keep deflections to a minimum.
Depending on the spans of the rafters the trimmers might be quite heavily loaded and should be checked.

 

[jesteh]

Thanks. Thats what I thought as well. Architect told me he spoke to structual engineer who was concerned on lateral load pushing on main house (extension is built with pitched roof) due to the weight of the roof lights. I am bit confused as just the tiles on their own are 50kg per sq meter which is the same weight as roof lights. Add the rest of the structure and like you say the roof is heavier.

 

[Static]

The trimming joists for the sky lights will push the load above and below these openings and concentrate them at the side joists.. this may cause issues with the lateral loading.. but not normally significantly high enough to worry anyone.. reducing the width of the openings to only cut a couple of the rafters may help..

 

[geraint]

you should always double up the rafters alongside a rooflight.. standard practice....

 

[RonnyRaygun]

Sometimes, where loads onto trimmers are high, you can end up with treble joists or even steelwork trimming large openings so the standard practice of using doubled up joists isn't always suitable.

 

[jeds]

It's down to how many rafters you cut. The rule is; cut 1 or 2 add 2. Cut 3 or 4 add 4.

That means if you trim up to 2 rafters you should double up the trimming rafters. (i.e. add one each side = two) If you cut 3 or 4 rafters you should triple the trimming rafters. (i.e. add two each side = four)

 

[geraint]

Sometimes, where loads onto trimmers are high, you can end up with treble joists or even steelwork trimming large openings so the standard practice of using doubled up joists isn't always suitable.

standard practice , but never mentioned in the installation sheet, is..... cut a rafter and replace it.... cut two... put two more in..

i cannot see the situation where fitting a rooflight needs triple rafters...

they hold up dormers...... or are we being pedantic....

 

[freddiemercurystwin]

or are we being pedantic

You should know geraint.

 

[geraint]

or are we being pedantic

You should know geraint.

nice to know you are positive again... just like your namesake........

 

[geraint]

Sometimes, where loads onto trimmers are high, you can end up with treble joists or even steelwork trimming large openings so the standard practice of using doubled up joists isn't always suitable.

would love to see why and how with a roof light...

 

[RonnyRaygun]

I agree not likely on a standard Velux in a standard double pitched roof with a standard pitch of between 30 and 60 degrees...

But as the pitch gets shallower and the rooflights larger, it's often the case that trebles will be used, especially where the timbers aren't very deep.

I very recently trimmed rooflights in steel on a domestic job where treble joists weren't satisfactory...admittedly they were very large rooflights but it does happen...

 

[freddiemercurystwin]

or are we being pedantic

You should know geraint.

nice to know you are positive again... just like your namesake........

 

[noseall]

you should always double up the rafters alongside a rooflight.. standard practice....

Not all rooflights.

For those sized roof-lights/windows that will sit betwixt standard spaced rafters then no additional rafters are necessary.

 

[geraint]

you should always double up the rafters alongside a rooflight.. standard practice....

Not all rooflights.

For those sized roof-lights/windows that will sit betwixt standard spaced rafters then no additional rafters are necessary.

then in that instance you are not cutting a rafter.... hence no need to double up.... please read the earlier posts as to why you should double up....