Flat Roof

[JGB]

Types of Flat Roof Construction

 

Hello, I’m thinking of having an extension and wanted to know the construction details of flat roofs for my own knowledge as much as anything.

I’ve scoured the Internet for as many examples of flat roof construction details as I possibly could and have found the following. Please see attached.

I will go through each in order, I’m only really interested in how the joists connect to the outer wall.

1.       Construction one - appears to show the joist sitting on a wall plate on the inner leaf with the firrings on top. The joist appears to be cut at the end and is therefore at a lower depth when it sits on the outer wall. Based on the depth of the outer leaf brick it looks like the joist has been cut by approximately 65mm. Is this an acceptable method of joining a joist to the external wall.

 

2.       Construction two - It appears that the architect has simply placed the joist directly on both the inner and outer leaf of the external wall, though appears to have missed out the firrings. They appear to have then just placed one wall plate on the inner leaf. Then they appear to have a facia only covering the joist end, not the insulation board above it. Is this an acceptable method of joining a joist to the external wall.

 

3.       Construction 3 - construction three appears to be very similar to construction two, however the fascia board covers both the joist and the insulation board above it, it also appears to have some form of batten just below the joist overhang, against the outer leaf wall. Are you able to get fascia boards that actually cover the depth required to cover both say a 200mm joist, plyboard and a say 100mm insulation board above? It would have to be a very wide fascia board I would have thought. Is this an acceptable method of joining a joist to the external wall.

 

4.       Construction four - again appears to simply show the joist sitting directly on top of both the inner and outer leaf, however interestingly there are no wall plates in this construction detail. Can a joist simply sit directly on the outer walls without any wall plate? Again the fascia appears to cover both the joist and insulation boards. There also appears to be a batten at the end of the insulation board which is interesting, I don’t really know how that would work? Is this an acceptable method of joining a joist to the external wall.

 

5.       Construction five - it appears to show the joist simply connecting to the inner leaf via may be a joist hanger or something of that nature. Then the firrings seems to go above it and onto the top of the inner leaf. Then I think there is plywood above it. Is this an acceptable method of joining a joist to the external wall.

 

6.       Construction six – very similar to construction five however the do not appear to be any battens under the facia board? Is this an acceptable method of joining a joist to the external wall.

 

7.       Construction seven - already covered, but there appears to be a batten on the outside of the fascia board above the gutter, and the finishing membrane seems to go over said batten. Is this an acceptable method of joining a joist to the external wall.

 

It appears to me that there may be multiple ways that one could join the joists of a flat roof to the outer walls, could you please tell me if there is a preferred method these days, and one that is used more commonly than the rest.

 

As I said, I am somewhat of an amateur at this, so would really appreciate some expert opinions.

 

If anyone has any pictures of common flat roof construction details like those I have attached, that would be greatly appreciated as well

 

Kind Regards 

 

Judith 

Combined_Flat_Roof_Examples.pdf

[Gone West]

Hi Judith, welcome to the forum. I'm afraid I can't help with your question but I'm sure someone more knowledgeable will be along soon.

[ToughButterCup]

A very warm welcome.

It is particularly pleasing to see contributions based on research gathered by the person posting.

 

I bet there is no 'common' or preferred way: but I hope to be proved wrong by those here that have built more roofs than  we have had hot dinners.

There might well be a more efficient way in terms of cost, and then again in terms of cost of ownership.

 

Now that will be interesting to read about because I have to build a flat roof in the next couple of weeks, so I will read this thread with interest.

Thanks for the question.

Ian

[MAB]

The outer ends of the roof joists on my flat roof are skew nailed (or more often screwed these days) to the timber wall plate which usually sits on the inner leaf of a cavity wall. The timber wall plate must be tied to the walls with L shaped galvanised metal restraint straps screwed to the inner leaf to prevent the roof lifting in high winds - see building regs. The fascia boards are nailed/screwed to the joist ends and the depth of fascia board often increases when you have a 'warm' roof construction with insulation above the joists. A 'cold' roof has insulation between the joists so usually less increase in roof height.

 

If you are designing your own roof or doing the Building Reg drawings yourself then this website in the link below can be useful for the details, some can be accessed for free but you would have to register for full information:-

https://www.buildingregs4plans.co.uk/index.php

Edited February 3, 2018 by MAB