Decking roof/Lean-to roofing advice

[PThomas]

Hi All

I'm building a 4.9m long and 3m deep lean-to over the decking on the back of my house. Planning on constructing it from 4x2 timber with a stronger 6x2 along the front supported by a pair or 6x6" posts.

Initially I was planning on using 10mm twinwall with white rafting glazing bars and guttering.

However I've since seen some nice photos on the net of lean-to's with clear Perspex/acrylic roofs which looks much more like glass and gives a very cleaner modern finish.

I'm struggling to find suppliers at anywhere approaching sensible prices for the sheet material.

Has anyone experience of doing this and can recommend the right materials, suppliers etc?

 

[tony1851]

You should look again at the sizes of your rafters and main beam, and also the spacing of the posts.

 

[PThomas]

You should look again at the sizes of your rafters and main beam, and also the spacing of the posts.

Why? As I understand it plastic sheets are relatively light?

If I double up 2 x 6x2" joists for the front surely they will be sufficient?

Are 4x2" not suitable for 3m spans with noggins in between?

 

[tony1851]

It's not the dead load of the sheeting that's the problem - it's the live load (ie snow load) that you can be subject to.

This is usually taken at about 15lbs/sq ft, and on that basis, your front beam would be carrying approx. 2/3 ton, which is way too much for a 2x6 spanning 6m. Your rafters should be at least 5x2, possibly 6x2 if they are 2ft apart. Also remember that you need to securely tie the roof down to the main beam because of wind uplift.

Having said this, your work would not require approval under building regulations, so you could pretty much build it as you want, but in your own interest, you should allow for the worst loading case.

 

[PThomas]

It's not the dead load of the sheeting that's the problem - it's the live load (ie snow load) that you can be subject to.

This is usually taken at about 15lbs/sq ft, and on that basis, your front beam would be carrying approx. 2/3 ton, which is way too much for a 2x6 spanning 6m. Your rafters should be at least 5x2, possibly 6x2 if they are 2ft apart. Also remember that you need to securely tie the roof down to the main beam because of wind uplift.

Having said this, your work would not require approval under building regulations, so you could pretty much build it as you want, but in your own interest, you should allow for the worst loading case.

Thanks for your advice

 

[^woody^]

4x2 will be fine for that use

However, single sheet perspex or acrylic will not, as it will sag under its own weight when used horizontally and will pop out of the fixing bars unless you have unfeasibly narrow widths less than 600mm, or lots of noggins

 

[tony1851]

4x2 will be fine for that use

On a 3m span? What table are you reading from?

 

[^woody^]

Tables don't apply for this use. Calculate the load per rafter, and remember it is not a load sharing situation, and the glazing bars act as stiffeners

However, under any compromising load the sheets will give way before the rafters in any case

 

[tony1851]

Might I suggest this - though it gets more difficult after the contents page

Untitled

• tony1851
• 4 Aug 2013
• 1

 

[^woody^]

May I suggest common sense?

 

[tony1851]

Assuming his rafters are @ 600 c/s, spanning 3m, with snow load of 0.75
kn/m^2, the max. deflection of 4x2 will be more than 20mm

Maximum normally permitted = span x 0.003

IMO he needs at least 5x2. That depth will also enable him to have a better fixing to the main beam.

 

[^woody^]

Assuming his rafters are @ 600 c/s, spanning 3m, with snow load of 0.75
kn/m^2, the max. deflection of 4x2 will be more than 20mm
.

1. Will that load assumption work for single rafters when not a load sharing situation and stiffened by glazing bars? Can the thin plastic sheets hold that much?

2. What plaster finishes will be cracked by deflection in any case?

The OP's situation is not a tiled roof. Don't design as one

 

[tony1851]

The figures assumed no load-sharing between the rafters.

Obviously he has no plaster ceiling, but the maximum deflection will still be > twice the recommended maximum. It wasn't calculated as a tiled roof - I did it on the bais of only live load and no dead load.

Still, whatever he uses for the rafters, OP still needs to re-assess the main beam at the front.

 

[^woody^]

A deflection check is only for the purposes of finishes. Don't cloud things with irrelevant details