How to build a porch roof Build It

How to build a porch roof Build It

The carpenters steel framing square is probably the best invention ever made for marking out rafters, its more than just a square, it’s a complete set of rafter tables as well and is capable of working out any cut and angle on any roof, something a book of rafter tables will not do as they tend to be filled with the most common pitches, and a proper roofing calculator will set you back at least £100 and you still need something to mark out the angles with.

The rafter square is made up of a tongue 16 inches long and 1 ½ inches wide and a body 24 inches long and 2 inches wide and both sections are marked out in inches.

Down the centre of the tongue is a brace table and this table is used to cut braces in order to level up posts or get frames square, if you look at the first set of numbers on the brace table you will see it is the number 24 on top of the number 24 followed by the number 33.94

What this means is if we need to level up a post or square up a frame you get a piece of timber and put a 45’ cut on one end, measure from the point down the timber 33.94” and put another 45’ angle coming back the opposite way, if we then get our post or frame and measure 24” from the corner each way and put a mark we then fix our brace to these marks, the post will then be level or the frame square.

If you know hold the square in front of you with the tongue hanging down to the right hand side and look at the let hand side of the body you will see six sentences –

Length of common rafter

Length of hip/valley

Length of jacks at 16” centres

Length of jacks at 24” centres

Side cut of jacks

Side cut of hip/valley rafters

This is your rafter tables and more, with these few numbers you can work out the length of any rafter on any roof and also all of the angles and side cuts without the use of rafter table books which get wet and damaged, without fancy expensive calculators that you will be scared to take out in case they get damaged, the only thing that would be an advantage for speed more than anything would be a cheap calculator out the £ shop.

How to build a porch roof Build It

So after studying our run and rise section we have worked out the pitch of our roof, for this example we will use a 40 degree pitch as it is the most common for bay roofs, we know a 40 degree pitch roof is equivalent to a 10/12, so we get a piece of our rafter material which is usually 32 or 42 for a bay roof

We lie our timber down flat on it’s side and place our square on it with the short side (tongue) to the right and the long side (the body) to the left, as we have established calculations for a roof are worked by foot per run so it is logical that run or horizontal part of our will be sat on our timber on the number 12 mark we always use the 12 when we are working out common rafters and jack rafters as these are just commons that reduce in size, as we are doing a 10/12 on the short/plumb side of our square we put the number 10 on our square level with the edge, make sure that you always use both numbers on the outside of the square or on the inside and try to always try to stick to the same way to save confusion if you use an inside and an outside mark your angles will not be correct.

Draw a line down the number 10 side and this will be your top cut and it will be 40’

If you look on your square where it says length of common rafters and follow it along until you get to the number 10 on the outside of the square you will see the number 15.62, this is how long your rafter will be for every 12” it travels, so if we are doing a bay roof or lean to roof to need to measure from the front of your roof to the wall and subtract the thickness of timber that you are going to put on the wall to secure your rafters to (ridge board ), to give us our run, if you are doing a up and over roof you need to measure the distance from wall plate to wall plate subtract the thickness of the ridge board the divide by 2 to give us our run, in the case of our bay roof lets assume that from the front of our bay to the wall is 26inches which is 2ft 2inches and our ridge board is 2” thick, subtract the ridge board 2” and that will give us 24” or 2ft, multiply the 2ft by the 15.62 and that will give you 31.24 inches this is the length of your rafter from the point where it touches your ridge board at the top to where it fits on your wall plate at the bottom excluding any overhang. NOTE when working out your rafter your final rafter length will always be in inches.

That was an easy one but lets say your distance from the wall is 2ft 4inches (28 inches) for the purpose of finding our length the 4 inches needs to be turned into a decimal, as decimals work in 10’s and there are 12” in a foot we need to divide 10 by 12 giving us 0.83 so that means each inch represents 0.83 as a decimal so 4 inches multiply 0.833 = 3.33 so we would have our 2ft. 33 ( 2.33) multiply 15.62 = 36.44 for the sake of 0.06 we can call it 36 ½ “

So from the top cut that we just made we measure down the rafter 36 ½ “ and put a mark, put our square back on and place the number 12 on the body this mark then on the tongue put the number 10 level with the top, we can now draw down our number 12 side this will be our seat cut (bottom) and will be 50’ ( as roofs are made up of right angled triangles and there are 180’ in right angled triangle we already know one of them as to be 90’ we worked our top out to 40’ so 90 + 40 =130 and180 – 130 = 50


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