Morning_Glory
New Member
I must fix drywall onto my ceiling and I have two problems that are related.
First, there are 50 mm thick foam insulation boards fixed on my ceiling already and this means that the drywall must be on top of them (well, below actually). This, in turn, means that the screws must be very long.
I could not find screws that are both long and thin. The thinnest screw that can go 25 mm into the timber is at least 5 mm thick. It is 90 mm long as the insulation is 50 mm and the drywall is 15 mm: 50 plus 15 plus 25 equals 90.
Why 15 mm thick drywall?
That is the second problem: the joists are 600 mm (0,6m) apart and putting extra wood between them is not an option (because the foam boards cannot be removed and I don’t have access to the loft space). As far as I know, the drywall must be at least 15 mm thick in order not to sag when the span is 600 mm.
Given that I need about 10 screws per square meter of drywall and that the joists are 600 mm apart, I must drive the screws about 170 mm apart; that makes 7 screws per 1200 mm (1,2m) of joist instead of 3 to 4.
Putting 5 mm thick screws so close to each other may split the joists. Thicker screws also transfer more heat.
One solution I consider is to use shorter and thinner screws, say 4,5 mm by 85 mm. Believe it or not, half a mm makes a difference re splitting a joist. (I experimented.) However, 4,5 by 85mm will go no more than 20 mm into the timber (if that).
So, I will end up with extra heavy drywall fixed with extra shallow screws. This doesn’t sound like a good combination, but when I experimented driving a screw less than 20 mm into the timber, it turned out that a single one can hold my weight (100 kg). That seems more than sufficient given that the whole 2,4 by 1,2m drywall weighs no more than 25 kg and will be fixed with almost 30 screws.
Does this mean that I can proceed using the 85 mm screws, placing them 170 mm apart to fix 15 mm thick drywall?
First, there are 50 mm thick foam insulation boards fixed on my ceiling already and this means that the drywall must be on top of them (well, below actually). This, in turn, means that the screws must be very long.
I could not find screws that are both long and thin. The thinnest screw that can go 25 mm into the timber is at least 5 mm thick. It is 90 mm long as the insulation is 50 mm and the drywall is 15 mm: 50 plus 15 plus 25 equals 90.
Why 15 mm thick drywall?
That is the second problem: the joists are 600 mm (0,6m) apart and putting extra wood between them is not an option (because the foam boards cannot be removed and I don’t have access to the loft space). As far as I know, the drywall must be at least 15 mm thick in order not to sag when the span is 600 mm.
Given that I need about 10 screws per square meter of drywall and that the joists are 600 mm apart, I must drive the screws about 170 mm apart; that makes 7 screws per 1200 mm (1,2m) of joist instead of 3 to 4.
Putting 5 mm thick screws so close to each other may split the joists. Thicker screws also transfer more heat.
One solution I consider is to use shorter and thinner screws, say 4,5 mm by 85 mm. Believe it or not, half a mm makes a difference re splitting a joist. (I experimented.) However, 4,5 by 85mm will go no more than 20 mm into the timber (if that).
So, I will end up with extra heavy drywall fixed with extra shallow screws. This doesn’t sound like a good combination, but when I experimented driving a screw less than 20 mm into the timber, it turned out that a single one can hold my weight (100 kg). That seems more than sufficient given that the whole 2,4 by 1,2m drywall weighs no more than 25 kg and will be fixed with almost 30 screws.
Does this mean that I can proceed using the 85 mm screws, placing them 170 mm apart to fix 15 mm thick drywall?
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