Enlarging loft hatch

[jayc89]

We currently have a tiny loft hatch, approx 350 x 350mm (it's tiny), which sits between ceiling joists at 15" centres (380mm). The joists themselves look to be 3x2. 

 

 

I need to make it larger, if only to get the MVHR into the loft!

 

I'm thinking the Dolle hatch + ladders are the best bang for buck - https://www.dolle-uk.co.uk/shop/loft-ladders-clickfix-76g - they come in 55cm and 70cm widths.

 

70cm would be preferred, the distance between 3x joists is 76.5cm which would mean I'd have 6.5cm to pack out between the hatch frame and the next joist, so I think I'd need a 3x2 double header on each end and a 3x2 trimmer to help pack it out, like this;

 

 

Am I on the right tracks here?

 

[JohnMo]

You need to add the strength lost by cutting the joist.

 

You need to double up the joist either side, add noggins, and add a doubled up cross member either end of the hatch across the joist you cut.

 

Others will correct me if I am over egging the cake.

[markc]

When you say ‘joists’ are 3x2, are they really the bottom cord of trusses? If so cutting them isn’t straight forward …well it is but the end result wouldn’t be clever.

post a pic of the loft space. If they really are joists then 3x2 is very small and you need to be careful of loadings but the cutting process would be as @JohnMo answer

[jayc89]

37 minutes ago, markc said:

When you say ‘joists’ are 3x2, are they really the bottom cord of trusses? If so cutting them isn’t straight forward …well it is but the end result wouldn’t be clever.

post a pic of the loft space. If they really are joists then 3x2 is very small and you need to be careful of loadings but the cutting process would be as @JohnMo answer

 

We have a couple of king post trusses, these lengths of 3x2 are attached to the bottom of those, they're basically only holding up the lath/plaster ceilings below. I don't believe they're structural at all. 

 

You can see the bottom cord of the trusses here, and the 3x2s are below them, buried in the insulation. 

 

 

I'll strip all the insulation back and take a proper picture of the area I'd need to cut open over the weekend. 

[jayc89]

1 hour ago, JohnMo said:

You need to add the strength lost by cutting the joist.

 

You need to double up the joist either side, add noggins, and add a doubled up cross member either end of the hatch across the joist you cut.

 

Others will correct me if I am over egging the cake.

 

When doubling up the joists, is that the full length;

 

 

Not just between the new cross-members/headers?

[markc]

This makes it clearer. 
no problem with cutting the 3x2’s. Add a prop underneath to support the member you cut while you get the header or cross member/s in. This is to prevent the ceiling sagging when the members are cut.

I would add additional angle brackets around the support points onto the trusses as these connections will not have been made with additional loft loads in mind. I’ve seen whole room sized areas of battens come away from the underside of trusses due to loft loading.

[jayc89]

The existing joists aren't even 3x2, I've just measured them and they're 2 3/4" x 2 1/2" (or 65mm x 55mm) but I assume 3x2 would be the best to use these days as a match. They can stand slightly proud in the loft area, beneath the loft rolls.