rafter size and spacing , lean to.

[robgil]

see thats what i was wondering , do i need purlins. ive got it into my head now to use green oak purlins and leave them exposed but its just a thought.
i guess because the roof already has purlins then i am simply putting it all back the way it was.
whats wbp ply?

 

[christianb]

WBP = weather and boil proof ply. Most common thicknesses are 4,6,9 and 12mm.

Out of interest, to what would you be fixing the upper ends of the rafters to? I only ask as for a lean-to extension I built I used a 8x2 trimmer bar propped up by 5x5" C24 posts, each of these fixed into the wall with resin fixings. Worked a treat.

C.

 

[robgil]

i was wondering about that also , presently its fixed with a puny little bit of wood and it really doesn't look too strong. but i suppose if there are purlins involved it doesn't have to be very strong anyway? the walls it is fixed to are timber framed with wattle/daub infill's so i am assuming its fixed to the timber frame. i am off up there tomorrow to measure up and see what's what.

 

[Jon Maine]

A 6 x 2 will be approx 50% stronger than a 4 x 3 even though they both have the same amount of wood in them.

Depends on which way around the same argument applies to the 4 x 3 the other way than you intend.

 

[Jon Maine]

i was wondering about that also , presently its fixed with a puny little bit of wood and it really doesn't look too strong. but i suppose if there are purlins involved it doesn't have to be very strong anyway? the walls it is fixed to are timber framed with wattle/daub infill's so i am assuming its fixed to the timber frame. i am off up there tomorrow to measure up and see what's what.

You will just need a plate coach bolted ( or bolted right thro ) to the studs of the frame. my favourite method is to use one the same depth as the rafters and glue and screw a 2 x 2 to its lower edge and then notch the rafters over the 2 x 2 butting the plate above then skew nail the rafter from both sides to the plate.

 

[Penners]

WBP = weather and boil proof ply.

Well, thanks for that information, Christian. Although I've used the WPB nomenclature to specify exterior grade ply on hundreds of occasions in the past, I've never actually known what it stands for!

Any idea what the interior grade, MR, stands for? (Just to complete my education. :wink: )

 

[David Dyer]

Come on Penners you worked in the furniture industry.
Moisture Resistant

 

[Penners]

Thanks for exacerbating my embarrassment, David!

It's just one of those terms that I used so often that it came to be a "word" in its own right. It never occurred to me to wonder what it stood for.

I could bore for Britain on the subject of softwood grading, though. :roll:

 

[middi]

You could use just four rafters for the whole roof as long as they are 300mm square and the roof battens 150mm square :wink:
The 400 spacing of rafters and studs is a convenient distance to avoid plasterboard or shuttering plywood (etc.)waste

 

[robgil]

now that is interesting midi , huge bits of wood though and probably expensive?

 

[middi]

Sorry robgil, the sarcastic in me manifested itself.

I was just pointing out (as already pointed out by others) there are many ways to build a roof, if you have to satisfy building reg criteria you must have approved drawings.

 

[Penners]

Now then Signore - shouldn't you be working on that cottage? Just because it's Saturday, there's no excuse to be slacking around here.

 

[middi]

Well Mr Penners, I looked out of the window this morning and I saw 20 feet of tarpauling flapping around in the wind and because I proud myself for being a thinking man I immediately thought there is no point in progressing with any repairs as the wind may demolish the cottage therefore rendering any further repairs futile, hence the reason for being lazy :wink:

I shall wait for a calm sunny day to resume.( note "day" written in the singular !! )

 

[robgil]

ok , ive got it all sorted now except for the wall plate's size , any ideas guys? also , do i bed the exterior wall plate in a bed of mortar? do i use hydraulic or none hydraulic?

 

[middi]

There is no point in using hydraulic lime.

Be carefull about building with lime mortars this time of the year, damp and breezy weather is perfect, damp and frosts are killers the mortar will not set and crumbles into powder,

Any doubt make sure you protect your work with thick covers etc..

 

[robgil]

thanks midi , i have heard tales of adding portland cement to lime mortar to help it go off quicker , ever tried this?

 

[MdB]

Err, you might want to remove that post Rob before the SPAB police get you. I believe it is a complete no no.

 

[middi]

thanks midi , i have heard tales of adding portland cement to lime mortar to help it go off quicker , ever tried this?

Yes, I have used a 4-5-3-1 mix on occasions where I needed for the mortar to go off a bit quicker when I needed to math some old pointing mortar.The joints were lightly wire brushed the day after and even the CO had trouble telling which was which.

This mix is reonably weak in that when cured you can easily drive a nail in the joint as you would normal lime mortar. This is no excuse not to use pure lime mortar and it still turns to powder if frost gets to it when still wet.(i.e- not cured)

The mix:-

4 building sand ( not washed)
5 sharp sand
3 hydrated lime
1 WHITE cement

 

[robgil]

interesting , but pure lime mortar it is then. hopefuly we wont have any frost anytime soon , lots of rain though.

 

[Jon Maine]

interesting , but pure lime mortar it is then. hopefuly we wont have any frost anytime soon , lots of rain though.

Good decision, you can protect it with bubble wrap and possibly even introduce some heat under your little bubblewrap tent, just be careful if it is a fan heater that it does not dry the joints before they cure. But am sure a bit of lateral thinking on your part will go a long way :wink: