Cut Roof Construction

[CHarr]

I know this is a naive question, but I'm puzzling over one aspect of my roof design (meeting the chippie on Thursday, and would rather not still be puzzling, by then!)...

It's a hipped roof, to a side extension, having front, side and rear planes, but the rear plane extends across the back of the house as a mono-pitch. The main rafters are specified as 175x50mm @ 450 ctrs, but the hip rafters are 225x75mm. Does that mean that the bird mouth cuts to the hip rafters where they bear on the wallplate are deeper, so that the upper surface of the differently sized timbers are at the same level?

The rafters specified for the mono-pitch section are reduced to 150x50mm, which would seem to call for some very deep cuts into the 225mm hip rafters to make that plane flat. The mono pitch section only extends 3m, and I wondered whether it would just be simpler to use 175x50mm across the whole of the rear section?

 

[noseall]

The main rafters are specified as 175x50mm @ 450 ctrs, but the hip rafters are 225x75mm. Does that mean that the bird mouth cuts to the hip rafters where they bear on the wallplate are deeper, so that the upper surface of the differently sized timbers are at the same level?

Correct.

What you need to understand also is that the hip blade is at a totally different pitch to the rest of the commons or jacks.

What this means is that you have no pattern for either the plumb cuts or seat cuts on the hip blade.

We have a method for marking the hip blade cuts by offering the blade in place and physically marking the wall plate and ridge cuts in-situ, using a short level etc.

The rafters specified for the mono-pitch section are reduced to 150x50mm, which would seem to call for some very deep cuts into the 225mm hip rafters to make that plane flat. The mono pitch section only extends 3m, and I wondered whether it would just be simpler to use 175x50mm across the whole of the rear section?

If it makes life easier yes, use 175mm x 50mm.

Or raise the wall plate for this section or add a knock on to the top of the wall plate. This will inevitably raise the ceiling joists also but you could then add a batten underneath the ceiling joists to bring the ceilings level. Unless it is vaulted of course.

Which ever is easier.

 

[CHarr]

... but you could then add a batten underneath the ceiling joists to bring the ceilings level. Unless it is vaulted of course.

It is.

 

[noseall]

Raise the plate or add a knock on to the top of the plate. Simples.

 

[CHarr]

Two different builders merchants have looked at me as though I just farted when I asked for C24 timber, suggesting that almost no one uses it for roofing. They want to sell me C16. I know that strength is the reason why C24 is often specified by designers, but that the actual difference is modest (and probably only critical over large spans). What do most people actually use?

Also, the finished height of the blockwork calls for a 100mm wallplate, and there is a disproportonate cost difference between 100mm timber compared with 2x50mm. Is there any reason not to use two 50mm timbers together? I can see a potential advantage in that joins could be lapped, and stronger than single timbers butted (are they butted?).

 

[noseall]

The wall plate WIDTH will be 100mm.

The depth of the plate can vary but a typical plate would be 75mm deep and is sold as such - usually tanolised.

We have fitted 50mm deep plates where practicality and marrying up to an existing plate, dictates.

I have never heard of a 100mm deep plate but have ended up with this depth in instances where the already bedded plate needed a knock on. This is usually when a truss company drops a bo llack.

A 75mm x 100mm plate is a good solid stout piece of timber to fix a b/m'd cut timber roof to. 100mm x 50mm is adequate also but ideal for trusses.

 

[noseall]

Two different builders merchants have looked at me as though I just farted when I asked for C24 timber, suggesting that almost no one uses it for roofing. They want to sell me C16. I know that strength is the reason why C24 is often specified by designers, but that the actual difference is modest (and probably only critical over large spans). What do most people actually use?

It's not often that we are specified to use C24 timber, unless it is an important bearing timber element doing a lot of work.

If there is no explicit request, then C16 it is. Likewise, building control will be more interested in the section sizes making sure these comply with the spans.

 

[CHarr]

The wall plate WIDTH will be 100mm.

The depth of the plate can vary but a typical plate would be 75mm deep and is sold as such - usually tanolised.

We have fitted 50mm deep plates where practicality and marrying up to an existing plate, dictates.

I have never heard of a 100mm deep plate but have ended up with this depth in instances where the already bedded plate needed a knock on. This is usually when a truss company drops a bo llack.

A 75mm x 100mm plate is a good solid stout piece of timber to fix a b/m'd cut timber roof to. 100mm x 50mm is adequate also but ideal for trusses.

100mm is the height (so 100x100mm) required from the finished blockwork, so that we can match existing internal ceilings when we break through. The original finished height on the plans was incorrect (!) as it didn't allow for matching the actual existing height, but assumed a normal 'new build' standard height, and while I would have preferred the last course of blocks to be trimmed so that 100x75 would have fitted, the brickes cut them differently while I had turned my back for ten minutes, so I have to make the best of it.

I'm still hoping to be able to use 2 x 100x50mm for the cost saving, and for ready availability.

 

[noseall]

Doubling up on your 50mm timbers will be fine and will afford you a simple way to lap the joins without having to chop 'n' chisel.

You may end up with some lateral 'curling' so try and use opposing natural kinks in the timber. What I mean by this is the bedded plate may try and leave the bed when everything dries out.

It is important (with a cut roof) that the plate is fitted straight parallel to whatever it is opposing so that your pattern rafters can be fitted without worry of any dipping. Not quite so critical with a hipped roof but make sure that the any changes of direction are square, i.e 90 degrees.

 

[CHarr]

Untitled

• CHarr
• 11 Nov 2013
• 1

Here's the 2x50mm wallplate, lapped at a corner. Walls were almost perfectly square and were perfectly level. Chippie very complimentary. Now I just need to find a roofer.

My only local interest is from a builder who wants to sub the job, take a fat margin and charge me VAT. There's no point in a labour-only job unless you engage an unregistered tradesman (I'm paying VAT on the materials already), and the middleman is adding nothing but cost. He very openly offered to accept cash for no VAT in our very first conversation - does he know I'm an HMRC employee?