Roof Framing, Traditional Timber Roofs

Many people are confused about the actual marking out of the rafters and roofs in general. After they have a right angled triangle of the main roof from their setting out, how to mark the common rafter? Where to start?

marking the common rafter
Roof framing - marking out the common rafters.

Here I show the two components of the rafter, the main roof triangle, and the eaves triangle. You need them separated (mark them separately), to get your birds mouth line.

If you are fixing timber fascias, leave the bottom of the rafter uncut, and cut it off after the roof is finished to a string line. When using a metal fascia with pressed metal clips that are nailed to the ends of the rafters, cut your fascia cuts on the ground, because the clips can take up any small discrepancies.

In theory the distance that you should subtract for the ridge is the slope angle distance, but in practice, half the ridge is usually OK. On very steep roofs, draw it and measure it.

Many times I have been tempted to say "stuff it, why bother with the birdsmouth? it's of no structural value, (it actually takes strength away from the rafter) and we are bolting the crap out of everything anyway".

In roof framing like this, the birdsmouth is essential for one clear reason. When the guys have a rafter almost in position, the guy at the ridge pulls the rafter towards him and he feels the birdsmouth slide onto the flat and come up hard against the top plate, then his mate at the bottom bangs in a couple of nails, he can ease off and nail to the ridge very quickly. No pushing and pulling, to line up with a pencil line.

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Roof Framing - Marking out the rafters

I always mark out my rafters like in this sketch below. I have even drawn in three marks indicating the jack rafter lengths using the diminish found in the roofing angles page.

marking out rafters.
Roof framing - Marking out the rafters.

I set my rafters up like this on a couple of saw stools, with to birdsmouth area set on top one of the stools to have that area as solid as possible.

  • Bow up. In all carpentry work, not just roof framing you should always be aware that the timber you are working with can be bowed, or bent. Floor joists, beams, ceiling joists and in this case rafters should always be fixed bow up. That is the round side up, so that natural sagging under load, over time tends to straighten the timber.
  • Another aspect of this is that if you don't sight along the timbers first, but just do them any old way, then you will sooner or later get two very bent ones side by side and the difference will be really noticeable.
  • Set your rafters on the stools in this case bow down as they will be turned the other way up when fixed.
  • Mark your two lengths, main less half the ridge plus the overhang.
  • Mark the birdsmouth.
  • If you have a sash clamp put it on near the birdsmouth area, or may just nail a temporary batten on them to hold them together.
  • Set your power saw to depth and angle and run a cut through for the bit of the birdsmouth that touches the vertical face of the wall. If the pitch is 45 deg you can saw both cuts of course.
  • Split em apart, square all your marks round.
  • At this stage if you are a bit lazy, turn them the right way up and mark out on the top edge all the purlins. This will save heaps of time later on but most guys don't bother or forget. I usually do. (forget:-).
rafter template
Roof framing - A home made template for birds mouths and rafter cuts.
  • Cut all the top and fascia cuts with your power saw.
  • With the huge variety of angles in roofing, it is not possible to say cut out the rest of your birdsmouth in a certain way.
  • It is always possible to clamp a stack of rafters and to do one cut through with a power saw.
  • You may be working on a 45deg or close roof, and do the other at the same time
  • For the rest, a big sharp chisel, a sharp hand saw, a power saw or any combination of the three will get the birdsmouth done.
  • The typically longer cuts on hip rafters can be sawn most of the way with a power saw, then chisel the rest.
  • At the side here is a pattern that is used for marking out the rafters and jack rafters.
  • It is simply made and thrown away after the job
  • On the top of it the ply is cut to the jack rafter top cut.
  • On the side it has the rafter side cut and the birdsmouth cut out.
  • Once the rafters are marked this template is used to mark the top and side cuts and the depth of the birdsmouth.
  • Using the template keeps a consistent depth to the rafter at the birdsmouth. This allows you to keep the tops of the rafters in line, even if you are using rough sawn timber with variations in the widths.

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Why use a birdsmouth?

I have had this question from Mark D.
"what is the purpose of the birdsmouth on a rafter"?

Here's what I replied, and what I should have put here in the first place.

Hi Mark
The birdsmouth provides a flat surface for the rafter to sit on when the rafter is first placed into position. It stops the rafter sliding downhill while it is being fixed. In position it helps to support the dead weight of the roof structure and added loads like snow. Later on of course, if the rafter is bolted to the ceiling joists, or fixed firmly in another way, then it just about becomes redundant. That is why we don't make the birdsmouth too deep. Just enough to do a job, but not enough to weaken the rafter.

The vertical face of the birdsmouth is a locating surface. That is, if the walls are straight, then by bumping the rafter to be fixed up into contact with the wall, then we know it is in the right position.

In general with modern stronger fixings they are going out of fashion, they are essential with nails only fixings.

Where I am, (cyclonic or hurricane construction) birds mouths are redundant. Invariably timber rafters are bolted to steel brackets or steel cleats welded to steel beams. In this case each rafter is clamped, drilled and bolted with no need for birdsmouth cuts. We don't waste time or more importantly strength with them.

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Roof Framing - Marking out the ridge

ridge and rafter layout
Roof Framing - Plan layout at the ridge/hip joint. R = ridge, CR = common rafter, H - hip, JR = jack rafter.
Roof framing is a hard enough job, without making it harder than need be, so I try to make it easier by doing as much as I can on the ground.

I cut the ridge on the ground and mark it out. That is I mark it on both sides with the rafter positions.
If it is a hip roof I use the true length on plan, minus the full width, plus half the ridge thickness at each end. You can see it in the sketch on the right.

So the ridge length is Length minus width plus one rafter thickness.

When you pass the first two up for a test, cross your fingers and say under your breath, "hope it fits". When the boys on the roof yell down "bloody perfect" you should modestly say, "crikey we were lucky there"!

When the end rafters are fixed, we string a line between them at the ridge, to fix the rest of them to.

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Roof Framing - Fixing The Roof

Before starting make sure all the walls are straight and plumb, level etc. Also check it for square (that the diagonals are the same length). If there is something a bit out, you need to know about it so that it doesn't come as a surprise.
  • Mark out on the walls your rafter positions. If I was doing a roof like this now, I would be using a lot of timber connectors. Maybe "triple grips" at the wall plate / rafter connection. So I would be thinking about maybe nailing then onto the wall plate first, to make it quicker when the time comes to fix the roof.
  • Cut to length and mark out your ridge. It's a lot easier on the ground, on stools, but most guys do it on top.
erecting a roof
Roof Framing - Fixing the first few pieces.
  • If there are no convenient internal wall, fix temporary supports flush with the top of the walls, to rest the rafters on, lay planks across and later, support the ceiling joists.
  • You are going to need a fair few temporary braces, that is lengths of timber that you can nail across the rafters etc.
  • Lift up your first two rafters, nail fix to the wall plate, lift up one edge of the ridge and drop it in place and just tack with one nail to each rafter. Then get a good brace back to a plate, fixing the rafters plumb.
  • Do the same at the other end. You will have four rafters and the ridge up then.
  • If it is a hip roof you fix the two middle rafters on the hip ends and they should hold it all firm and plumb.
  • For gable roofs at this stage you might be able to get a a good brace down to the ground to stiffen it all up, plumb.

As I said before, where I live we don't do these sorts of roofs any more. Trusses are the way to go. However I get lots of requests from people about framing roofs, so I will be adding more pages from time to time. Check out the menu below, that's where the new ones will appear.

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Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
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