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Phil, Mechanical Engineer
Category: Home Improvement
Satisfied Customers: 7227
Experience:  Retired contractor, 51 years experience
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Hi We have a patio on the pilbara coast (cat D cyclone rating)

Resolved Question:

We have a patio on the pilbara coast (cat D cyclone rating) which at present has metal purlins, we are wanting to change to a wood purlin due to corrosion issues can you tell me the size required and spacing to allow us to meet the above catagory for cyclone rating of the building codes.
Submitted: 2 years ago.
Category: Home Improvement
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Expert:  Phil replied 2 years ago.

Welcome to Just Answer!

The entire structure and all of the fasteners and roofing material has to be rated for category D, the best way to do that is to make a sketch of the patio, along with the wall structure members and roof, and with the purlins you want to use, and take that to the local building authority and ask for a permit.

If I were building to meet that standard, I would have 4 inch posts set into the concrete slab at least 24 inches, every four feet on center for the walls. I would use 1 x 6" diagonal cross bracing,... with double 2 x 8"'s around the perimeter atop the 4 inch posts, and then I would fit 2 x 8" purlins, if they do not have to be more than 15' long on 18" centers, and cover those with 3/4" plywood decking screwed into the purlins every 10 inches with 3" deck screws. the apply roofing of my choice over the top, Use only structural grade wood, and use treated wood for the posts.

Let me know if you need those USA lumber sizes translated to metric lumber sizes.

That should pass. Let me know what you think, we can go from there.


Expert:  Phil replied 2 years ago.
ooops! I should have typed, the purlins should be on 16" centers. (the width of a sheet of plywood, divided by 3 when converting to metric)

If you care to rate my advice so far *positively, i will hold the question open for 3 weeks as you may need more follow up.

Customer: replied 2 years ago.

Hi Phil

I already have a steel patio which meets the standards and is in ok condition.The purlins are at present 2" x 2" box section not sure of spacings .

I just wish to replace the box section purlins with wood.

Is it right to say they need to be 2 x8 with 16" spacings.

Expert:  Phil replied 2 years ago.

Hello again,

How the steel is attached to the patio is crucial... a lot of people forget that.. then their patio blows away in a storm. Anything you put above poorly attached support posts will blow away in high winds.

The thickness of the metal in the existing 2 x 2" purlins defines their strength... most are very thin metal and not storm rated to any degree..

It is the entirety of the structure that determines if it will hold up in a storm or not.
The easiest thing to do would be to remove one of the steel purlins, and take it to a local steel supply wholesaler and ask them to measure the wall thickness and cut you some replacements in a galvanized steel version if they have it.....if not, then buy the bare steel box sections and paint them with two coats of rustoleum primer.... and be sure to attach them to the steel frame the same way the originals were attached.


If you change to wood, all of the attachment has to change, and that will not be officially rated for storms. It would be the local building officials who have to approve the design in that case (as a custom built patio)


Beyond that are issues related to the classification system for storms.

Winds must be sustained for more than 15 minutes at any given speed to meet the various storm ratings...yet in any typical storm there are wind speed surges 20 or 30% beyond that level for 5 or 10 seconds... it only takes one of those gusts to relocate a persons patio to the nearest vacant lot, tree or ocean.

If you are a contractor and must meet legal limits to fulfill your contract, then the storm ratings are very important.
.. if you are a home owner and do not want your patio to blow away, then a sound structure is limitlessly more important than any rating system, which is arbitrary by nature, since storms beyond any rating are common in any area.... and wind gusts beyond that level even more common.


That is the basis if my advice to you, and why I recommend that you make a sketch and take it to the local building authorities for approval... but even that fails in many cases as evidenced by all the damage we see after storms...

Most of those destroyed homes etc were built to code and still blew away.... that is why I am recommending long deck screws be used instead of nails and the 2 x 8" wood purlins on 16" centers. (any grade works that way)


There are other factors as well related to the patio's position in relation to the slope of the local terrain and homes, and any attachment to the home...

For instance, if your patio is on the side of hill with the wind blowing up slope, it will see much more destructive wind forces in any category storm than a patio set on flat ground between houses or on the down wind side of a house.

It is these considerations that are limitlessly more important than designing to the minimum standards that the state necessarily has to set out...

Again, these define *minimums and are not at all the basis of any good design, that is stated on the preface pages of most of those code books.... also stated is that the local authorities have the responsibility to approve only *safe construction, regardless what the minimums are in the code book.

This why I have advised you as I have so far.

Let me know if this makes any sense on your end or not... we can go from there.


Phil, Mechanical Engineer
Category: Home Improvement
Satisfied Customers: 7227
Experience: Retired contractor, 51 years experience
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