Travelling around the country in recent weeks since storm Éowyn, the impact that it’s had on farm buildings is evident, particularly older round roof sheds that were common-place before lean-to and portal frame became more popular from the 80s onwards. Many of these buildings are in excess of 50 years old, so storm or no storm, they were in need of a revamp.
Replacing round sheeting
Round roof sheeting can still be bought from most manufacturers, though due to the extra labour cost of cutting and curving sheets to the buyer’s size, you will see a fairly significant cost increase on a per m² basis, compared to straight sheeting.
To ascertain the length and curving of sheeting, the round roof trusses will have to be measured. If the trusses are on the ground, then it is simple to measure the length of the truss and the height from the lowest point to the highest, with these figures then supplied to the sheeting manufacturer.
The number of cleats per truss (the bracket which holds the roof timber) should also be noted, and where there is uneven spacing between cleats this should also be measured and passed on to the manufacturer.
Where the trusses are still in situ in the roof, then a string line can be pulled from the bottom of one truss to the other. Using a cherry picker or safety cage, you can then measure the highest point of the truss down to the string line to ascertain the height.
Replacing timbers
While replacing a handful of sheets on a shed with timber round roof trusses is fine, putting all new sheeting over such is not wise, as the sheeting will far outlast the timber trusses.
Either new or second-hand steel round roof trusses should be used, or a portal frame roof can be created.
Inspect the older roof timbers as well before installing new sheeting. Wet rot will be obvious, but dry rot or woodworm is less so, and any affected timbers should be replaced with new kiln-dried, treated timber.
Many older hay sheds used 4in by 3in timbers, and these should suffice again. Stapling on a DPC layer between the new timber and sheeting is highly recommended.
Steel trusses
Where there has been storm damage, steel roof trusses should be carefully inspected for any twisting or warping that may have happened due to stress. If they are seen to be okay, then corrosion is the next major issue, especially where the trusses have been used in animal housing, as the warm, damp air accelerates the corrosion process.
It may be worth taking down the trusses to carry out any necessary repair and to have them sandblasted, primed and painted before re-using.
Sandblasting will likely expose more corrosion than is visible to the naked eye as well. Galvanising may be an option for shorter length trusses as well, but the length of galvanising baths mean that many larger span trusses will not fit.
Clear lights
Clear roof lights are likely the first thing to go in a storm on a round roof shed, and similar to metal sheeting, manufacturers will be able to supply you with replacements once the radius of the truss has been calculated.
It is worth considering placing some rebar underneath this roof sheeting, running from one timber purlin to the other. Using a blow torch and some brute force, the steel can be bent to suit the curvature of the roof.
Replacing with a portal frame
Where the trusses are in too bad a shape to repair an option may be to create a portal frame out using the existing upright RSJs. New RSJ trusses can be bolted on to the side of the upright RSJs to avoid cutting the tops off to ensure all is level. With older buildings, it is important to measure each bay as there could be up to several inches of a difference between the first and last bays.
In short
When replacing round roof sheeting, the radius of the trusses needs to be calculated, along with the length and spacing and number of cleats.Steel trusses need to be inspected for warping and twisting, and ideally removed for sandblasting, priming and painting.Clear roof lights should ideally be reinforced underneath using rebar steel.Replacing a round roof with a portal frame may be an option, where the upright RSJs are in good health.
Travelling around the country in recent weeks since storm Éowyn, the impact that it’s had on farm buildings is evident, particularly older round roof sheds that were common-place before lean-to and portal frame became more popular from the 80s onwards. Many of these buildings are in excess of 50 years old, so storm or no storm, they were in need of a revamp.
Replacing round sheeting
Round roof sheeting can still be bought from most manufacturers, though due to the extra labour cost of cutting and curving sheets to the buyer’s size, you will see a fairly significant cost increase on a per m² basis, compared to straight sheeting.
To ascertain the length and curving of sheeting, the round roof trusses will have to be measured. If the trusses are on the ground, then it is simple to measure the length of the truss and the height from the lowest point to the highest, with these figures then supplied to the sheeting manufacturer.
The number of cleats per truss (the bracket which holds the roof timber) should also be noted, and where there is uneven spacing between cleats this should also be measured and passed on to the manufacturer.
Where the trusses are still in situ in the roof, then a string line can be pulled from the bottom of one truss to the other. Using a cherry picker or safety cage, you can then measure the highest point of the truss down to the string line to ascertain the height.
Replacing timbers
While replacing a handful of sheets on a shed with timber round roof trusses is fine, putting all new sheeting over such is not wise, as the sheeting will far outlast the timber trusses.
Either new or second-hand steel round roof trusses should be used, or a portal frame roof can be created.
Inspect the older roof timbers as well before installing new sheeting. Wet rot will be obvious, but dry rot or woodworm is less so, and any affected timbers should be replaced with new kiln-dried, treated timber.
Many older hay sheds used 4in by 3in timbers, and these should suffice again. Stapling on a DPC layer between the new timber and sheeting is highly recommended.
Steel trusses
Where there has been storm damage, steel roof trusses should be carefully inspected for any twisting or warping that may have happened due to stress. If they are seen to be okay, then corrosion is the next major issue, especially where the trusses have been used in animal housing, as the warm, damp air accelerates the corrosion process.
It may be worth taking down the trusses to carry out any necessary repair and to have them sandblasted, primed and painted before re-using.
Sandblasting will likely expose more corrosion than is visible to the naked eye as well. Galvanising may be an option for shorter length trusses as well, but the length of galvanising baths mean that many larger span trusses will not fit.
Clear lights
Clear roof lights are likely the first thing to go in a storm on a round roof shed, and similar to metal sheeting, manufacturers will be able to supply you with replacements once the radius of the truss has been calculated.
It is worth considering placing some rebar underneath this roof sheeting, running from one timber purlin to the other. Using a blow torch and some brute force, the steel can be bent to suit the curvature of the roof.
Replacing with a portal frame
Where the trusses are in too bad a shape to repair an option may be to create a portal frame out using the existing upright RSJs. New RSJ trusses can be bolted on to the side of the upright RSJs to avoid cutting the tops off to ensure all is level. With older buildings, it is important to measure each bay as there could be up to several inches of a difference between the first and last bays.
In short
When replacing round roof sheeting, the radius of the trusses needs to be calculated, along with the length and spacing and number of cleats.Steel trusses need to be inspected for warping and twisting, and ideally removed for sandblasting, priming and painting.Clear roof lights should ideally be reinforced underneath using rebar steel.Replacing a round roof with a portal frame may be an option, where the upright RSJs are in good health. 
This is a subscriber-only article
SHARING OPTIONS: