9. Flat Roof Structure
This was my favourite part of the build. It’s so satisfying to see the structure complete and the wall braces come off. Much of this video focuses on warm roof vs cold roof and sloped joists vs firring strips. It’s really the last piece of the puzzle to comprehend before you start your floor because the choices made here will feed into the overall height of your garden room.
Roof joist sizing
If you have a timber floor you’ll be familiar with joist spans. You can find my span tables in the Resources page or use this website. In most cases you’ll be okay using the 0.75kN/m2 imposed loads but if you live up north with potential for higher snow loads then consider the 1kN/m2 span tables.
Can you undercook the joist size?
A question I get asked from time to time. I wouldn’t recommend it exactly but if you’re on the threshold of going up a size then you could probably lose 1 inch (25mm) without having problems. The span tables are after all somewhat conservative. I’m sure I could have gotten away with 8×2″s rather than the 9×2″ I used. However I have seen photos where the joists were significantly undersized and this ended up with sagging joists and a pond forming on top worsening the issue. If you can stick to the span tables and see if upgrading to C24 grade timber helps.
What about doubling up joists instead?
Without going into all the maths, the strength of a joist is predominantly in its depth, not its width. So doubling up your roof joists will not compensate for a lack of depth. You also add more weight to roof. This is also why reducing your spacings between joists to less than 400mm doesn’t help greatly either. If you’re really struggling to get the height of your roof down go for planning permission to achieve an overall taller building. An alternative would be to run an RSJ down the middle of your building and have your joists run perpendicular, which will reduce the span. To do this you’ll need appropriate foundations and multiple wall studs directly under the RSJ.
Other options instead of joists?
You can use SIPS in some circumstances, LVL or I-joists for wider spans or insulated metal sheet panels which have both the insulation and roof finish in one.
Warm roof vs cold roof
As a reminder here are the advantages of constructing a cold roof with the insulation between the joists vs a warm roof with the insulation above the joists.
In most building circumstances a warm roof wins (an extension for example) but with the restricted height of a garden room under permitted development within 2m of a boundary a cold roof is a viable option. In the main series I go into detail on how I construct my warm roof but here’s a video I made about how I’d go about constructing a cold roof.
What's a hybrid roof and is it viable?
There was misinformation about what a hybrid roof is on youtube for a while which confused a lot of people. Let’s clear this up. The picture shows what a proper hybrid roof looks like.
The insulation is both above and below the sub-deck with that between the joists pushed up to the deck. The advantage of a hybrid roof is you don’t have ventilation requirements as you would with a cold roof yet you reduce the thickness of the roof while keeping thermal bridging in check.
The problem is you have to keep the between-joist insulation reasonably thin otherwise you could bring the condensation point below the deck but without having the ventilation to clear the moisture. It’s a lot of work cutting insulation to fit between joists and it’s not really worth it for just a thin slice of insulation. Plus you could still get moisture issues so my opinion is hybrid roofs should be avoided for most.
Sloped joists vs flat roof
We have to create a slope for rain water run off – a flat roof does not mean a level roof! Here’s the summary of the advantages to going for firring strips vs sloped joists.
It’s a tough call with no ‘right’ choice. In general I would recommend sloped joists for larger spans (wider garden rooms) as the greater depth of firring strips at the front of the building will be add to the already thick joists. If you go for a warm roof as well, that means a very thick facia. For smaller spans firring strips will be a good option and will save you from building rake walls.
Birds mouths for sloped joists
Let’s stick with sloped joists. Do you need to cut a wedge out of the joists so they sit firmly on the walls’ top plates?
I like to think my garden room is well engineered. Over-engineered possibly but cutting birds mouths was something I didn’t bother with. The reasons I had for not doing so was the angle was so slight it wasn’t worth it and the angle brackets do a lot of the work of transferring the weight of the roof to the walls.
If you do decide to cut birds mouth, perhaps because your roof gradient is greater, they’re not difficult to do. Once you’ve worked it out for one joist you can replicate it across the rest.
Plumb cut the joists?
Again, this pertains to sloped joists only. In order to have a fascia which is perpendicular to the ground you need to ‘plumb cut’ each end of your joists as shown in the dotted line.
Again, because my gradient is relatively shallow I didn’t do this – it’s not noticeable once the facias are on. To do plumb simply cut at the same degree as the gradient of the joists.
Angle brackets vs Joist hangers
It didn’t occur to me at the time but instead of attaching the joists to the walls’ top plates with an L-shaped angle bracket either side, you can use joist hangers.
By turning them upside down with the lip over the top of the joists and the flaps secured to your walls they’re a good alternative.
How to work out the roof gradient
I went for a 1:60 roof gradient which means for every 60 units (cm, inches etc) horizontally the roof drops in height by 1 unit. So my span is 5m or 500cm. 500cm divided by 60 = 8.33cm which is the difference in height between my front and back wall.
If using firring strips on top of the joists you have to account for the overhangs as well. So if you have 20cm overhang at the back and a 60cm overhang at the front you would add this to your wall span = 580cm. 580 divided by 60 = 9.66, so your firring strips would need to be 10cm thick at the front tapering to 0cm at the back.
What gradient is best?
It will depend on your roof finish. I used rubber EPDM which allows for a roof gradient as shallow as 1:80 though because there can be dips in the roof it’s recommended to go for a minimum of 1:40. While in 5 years I’ve not had any issues, if I were doing it again I would opt for this steeper gradient. If going steeper still doesn’t affect other issues such as ceiling height, that can only be a good thing in my book.
If you are looking to work out the equivalent angle of a roof gradient for cutting studs on a rake wall or a plumb cut here are some common gradients converted to angles for you:
1:10 = 5.71°
1:20 = 2.86°
1:30 = 1.90°
1:40 = 1.43°
1:50 = 1.15°
1:60 = 0.95°
Are roof overhangs necessary?
There’s two things consider. Aesthetics and functionality. Personally I think overhangs on all four sides looks better, more robust and less shed like. They also help prolong the life of the cladding by shielding it from rain – you wouldn’t see a roof of a house without overhangs after all. Each to their own though. You can stop the joists at the edge of the walls and clad all the way up if you like. Alternatively as we discussed under ‘Hang your joists‘ you may want to hang your joists to the inside of your walls in which case there will be no overhangs.
What about an apex roof?
Flat or ‘pent’ roofs did have a bad rep for a long time. The enemy of any building structure is water and flat roofs are more susceptible to leaks than an apex roof – it’s why it’s asked what percentage of your house is flat roofed on buildings insurance quotes. However, the finishes have improved a lot and have long guarantees – such as the rubber EPDM I’ll be installing on mine.
An apex roof could be considered superior though and if it’s the look you want, go for it. Tiling it will last a very long time and you could use the roof space for storage, but do take into account eaves height (go back to Part 3.). Being at such as steep angle for water runoff, felt would be reasonable finish to go for if looking to save on costs. They’re a little trickier to build, though not outside the limits of the DIYer but they’re outside the scope of this series.
Help! My walls are still swaying!
I’ve had this reported to me a few times and though it wasn’t something I experienced with mine I suspect it’s due to not having OSB on the walls. It almost always resolves itself once the deck has been secured to the top of the roof joists so don’t worry.
That’s the structure finished, now to get it watertight.
Task: Cut your joists to length, rest one side on one wall and walk the other side up a ladder. Fill in your noggins and any side overhangs, attach the walls and you’re done!